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Chlorhexidine mouthwash is known to have minor adverse effects. Chlorhexidine binds to tannins, meaning that prolonged use in persons who consume coffee, tea or red wine is associated with extrinsic staining (i.e. removable staining) of teeth. A systematic review of commercial chlorhexidine products with anti-discoloration systems (ADSs) found that the ADSs were able to reduce tooth staining without affecting the beneficial effects of chlorhexidine. Chlorhexidine mouthwash can also cause taste disturbance or alteration. Chlorhexidine is rarely associated with other issues like overgrowth of enterobacteria in persons with leukemia, desquamation, irritation, and stomatitis of oral mucosa, salivary gland pain and swelling, and hypersensitivity reactions including anaphylaxis.
Hexetidine also has anti-plaque, analgesic, astringent and anti-malodor properties, but is considered an inferior alternative to chlorhexidine.
Chlorine dioxide
In dilute concentrations, chlorine dioxide is an ingredient that acts as an antiseptic agent in some mouthwashes.
Edible oils
In traditional Ayurvedic medicine, the use of oil mouthwashes is called "Kavala" ("oil swishing") or "Gandusha", and this practice has more recently been re-marketed by the complementary and alternative medicine industry as "oil pulling". Its promoters claim it works by "pulling out" "toxins", which are known as ama in Ayurvedic medicine, and thereby reducing inflammation. Ayurvedic literature claims that oil pulling is capable of improving oral and systemic health, including a benefit in conditions such as headaches, migraines, diabetes mellitus, asthma, and acne, as well as whitening teeth.
Oil pulling has received little study and there is little evidence to support claims made by the technique's advocates. When compared with chlorhexidine in one small study, it was found to be less effective at reducing oral bacterial load, and the other health claims of oil pulling have failed scientific verification or have not been investigated. There is a report of lipid pneumonia caused by accidental inhalation of the oil during oil pulling. | Mouthwash | Wikipedia | 486 | 782 | https://en.wikipedia.org/wiki/Mouthwash | Biology and health sciences | Hygiene products | Health |
The mouth is rinsed with approximately one tablespoon of oil for 10–20 minutes then spat out. Sesame oil, coconut oil and ghee are traditionally used, but newer oils such as sunflower oil are also used.
Essential oils
Phenolic compounds and monoterpenes include essential oil constituents that have some antibacterial properties, such as eucalyptol, eugenol, hinokitiol, menthol, phenol, or thymol.
Essential oils are oils which have been extracted from plants. Mouthwashes based on essential oils could be more effective than traditional mouthcare as anti-gingival treatments. They have been found effective in reducing halitosis, and are being used in several commercial mouthwashes.
Fluoride (anticavity)
Anti-cavity mouthwashes contain fluoride compounds (such as sodium fluoride, stannous fluoride, or sodium monofluorophosphate) to protect against tooth decay. Fluoride-containing mouthwashes are used as prevention for dental caries for individuals who are considered at higher risk for tooth decay, whether due to xerostomia related to salivary dysfunction or side effects of medication, to not drinking fluoridated water, or to being physically unable to care for their oral needs (brushing and flossing), and as treatment for those with dentinal hypersensitivity, gingival recession/ root exposure.
Flavoring agents and xylitol
Flavoring agents include sweeteners such as sorbitol, sucralose, sodium saccharin, and xylitol, which stimulate salivary function due to their sweetness and taste and helps restore the mouth to a neutral level of acidity.
Xylitol rinses double as a bacterial inhibitor, and have been used as substitute for alcohol to avoid dryness of mouth associated with alcohol.
Hydrogen peroxide
Hydrogen peroxide can be used as an oxidizing mouthwash (e.g. Peroxyl, 1.5%). It kills anaerobic bacteria, and also has a mechanical cleansing action when it froths as it comes into contact with debris in mouth. It is often used in the short term to treat acute necrotising ulcerative gingivitis. Side effects can occur with prolonged use, including hypertrophy of the lingual papillae. | Mouthwash | Wikipedia | 494 | 782 | https://en.wikipedia.org/wiki/Mouthwash | Biology and health sciences | Hygiene products | Health |
Lactoperoxidase (saliva substitute)
Enzymes and non-enzymatic proteins, such as lactoperoxidase, lysozyme, and lactoferrin, have been used in mouthwashes (e.g., Biotene) to reduce levels of oral bacteria, and, hence, of the acids produced by these bacteria.
Lidocaine/xylocaine
Oral lidocaine is useful for the treatment of mucositis symptoms (inflammation of mucous membranes) induced by radiation or chemotherapy. There is evidence that lidocaine anesthetic mouthwash has the potential to be systemically absorbed, when it was tested in patients with oral mucositis who underwent a bone marrow transplant.
Methyl salicylate
Methyl salicylate functions as an antiseptic, antiinflammatory, and analgesic agent, a flavoring, and a fragrance. Methyl salicylate has some anti-plaque action, but less than chlorhexidine. Methyl salicylate does not stain teeth.
Nystatin
Nystatin suspension is an antifungal ingredient used for the treatment of oral candidiasis.
Potassium oxalate
A randomized clinical trial found promising results in controlling and reducing dentine hypersensitivity when potassium oxalate mouthwash was used in conjugation with toothbrushing.
Povidone/iodine (PVP-I)
A 2005 study found that gargling three times a day with simple water or with a povidone-iodine solution was effective in preventing upper respiratory infection and decreasing the severity of symptoms if contracted. Other sources attribute the benefit to a simple placebo effect.
PVP-I in general covers "a wider virucidal spectrum, covering both enveloped and nonenveloped viruses, than the other commercially available antiseptics", which also includes the novel SARS-CoV-2 virus. | Mouthwash | Wikipedia | 398 | 782 | https://en.wikipedia.org/wiki/Mouthwash | Biology and health sciences | Hygiene products | Health |
Sanguinarine
Sanguinarine-containing mouthwashes are marketed as anti-plaque and anti-malodor treatments. Sanguinarine is a toxic alkaloid herbal extract, obtained from plants such as Sanguinaria canadensis (bloodroot), Argemone mexicana (Mexican prickly poppy), and others. However, its use is strongly associated with the development of leukoplakia (a white patch in the mouth), usually in the buccal sulcus. This type of leukoplakia has been termed "sanguinaria-associated keratosis", and more than 80% of people with leukoplakia in the vestibule of the mouth have used this substance. Upon stopping contact with the causative substance, the lesions may persist for years. Although this type of leukoplakia may show dysplasia, the potential for malignant transformation is unknown. Ironically, elements within the complementary and alternative medicine industry promote the use of sanguinaria as a therapy for cancer.
Sodium bicarbonate (baking soda)
Sodium bicarbonate is sometimes combined with salt to make a simple homemade mouthwash, indicated for any of the reasons that a saltwater mouthwash might be used. Pre-mixed mouthwashes of 1% sodium bicarbonate and 1.5% sodium chloride in aqueous solution are marketed, although pharmacists will easily be able to produce such a formulation from the base ingredients when required. Sodium bicarbonate mouthwash is sometimes used to remove viscous saliva and to aid visualization of the oral tissues during examination of the mouth.
Sodium chloride (salt)
Saline has a mechanical cleansing action and an antiseptic action, as it is a hypertonic solution in relation to bacteria, which undergo lysis. The heat of the solution produces a therapeutic increase in blood flow (hyperemia) to the surgical site, promoting healing. Hot saltwater mouthwashes also encourage the draining of pus from dental abscesses. In contrast, if heat is applied on the side of the face (e.g., hot water bottle) rather than inside the mouth, it may cause a dental abscess to drain extra-orally, which is later associated with an area of fibrosis on the face . | Mouthwash | Wikipedia | 476 | 782 | https://en.wikipedia.org/wiki/Mouthwash | Biology and health sciences | Hygiene products | Health |
Saltwater mouthwashes are also routinely used after oral surgery, to keep food debris out of healing wounds and to prevent infection. Some oral surgeons consider saltwater mouthwashes the mainstay of wound cleanliness after surgery. In dental extractions, hot saltwater mouthbaths should start about 24 hours after a dental extraction. The term mouth bath implies that the liquid is passively held in the mouth, rather than vigorously swilled around (which could dislodge a blood clot). Once the blood clot has stabilized, the mouthwash can be used more vigorously. These mouthwashes tend to be advised for use about 6 times per day, especially after meals (to remove food from the socket).
Sodium lauryl sulfate (foaming agent)
Sodium lauryl sulfate (SLS) is used as a foaming agent in many oral hygiene products, including many mouthwashes. Some may suggest that it is probably advisable to use mouthwash at least an hour after brushing with toothpaste when the toothpaste contains SLS, since the anionic compounds in the SLS toothpaste can deactivate cationic agents present in the mouthwash.
Sucralfate
Sucralfate is a mucosal coating agent, composed of an aluminum salt of sulfated sucrose. It is not recommended for use in the prevention of oral mucositis in head and neck cancer patients receiving radiotherapy or chemoradiation, due to a lack of efficacy found in a well-designed, randomized controlled trial.
Tetracycline (antibiotic)
Tetracycline is an antibiotic which may sometimes be used as a mouthwash in adults (it causes red staining of teeth in children). It is sometimes use for herpetiforme ulceration (an uncommon type of aphthous stomatitis), but prolonged use may lead to oral candidiasis, as the fungal population of the mouth overgrows in the absence of enough competing bacteria. Similarly, minocycline mouthwashes of 0.5% concentrations can relieve symptoms of recurrent aphthous stomatitis. Erythromycin is similar. | Mouthwash | Wikipedia | 457 | 782 | https://en.wikipedia.org/wiki/Mouthwash | Biology and health sciences | Hygiene products | Health |
Tranexamic acid
A 4.8% tranexamic acid solution is sometimes used as an antifibrinolytic mouthwash to prevent bleeding during and after oral surgery in persons with coagulopathies (clotting disorders) or who are taking anticoagulants (blood thinners such as warfarin).
Triclosan
Triclosan is a non-ionic chlorinate bisphenol antiseptic found in some mouthwashes. When used in mouthwash (e.g. 0.03%), there is moderate substantivity, broad spectrum anti-bacterial action, some anti-fungal action, and significant anti-plaque effect, especially when combined with a copolymer or zinc citrate. Triclosan does not cause staining of the teeth. The safety of triclosan has been questioned.
Zinc
Astringents like zinc chloride provide a pleasant-tasting sensation and shrink tissues. Zinc, when used in combination with other antiseptic agents, can limit the buildup of tartar. | Mouthwash | Wikipedia | 224 | 782 | https://en.wikipedia.org/wiki/Mouthwash | Biology and health sciences | Hygiene products | Health |
Asparagales (asparagoid lilies) are a diverse order of flowering plants in the monocots. Under the APG IV system of flowering plant classification, Asparagales are the largest order of monocots with 14 families, 1,122 genera, and about 36,000 species, with members as varied as asparagus, orchids, yuccas, irises, onions, garlic, leeks, and other Alliums, daffodils, snowdrops, amaryllis, agaves, butcher's broom, Agapanthus, Solomon's seal, hyacinths, bluebells, spider plants, grasstrees, aloe, freesias, gladioli, crocuses, and saffron.
Most species of Asparagales are herbaceous perennials, although some are climbers and some are trees or shrubs. The order also contains many geophytes (bulbs, corms, and various kinds of tuber). The leaves of almost all species form a tight rosette, either at the base of the plant or at the end of the stem, but occasionally along the stem. The flowers are not particularly distinctive, being 'lily type', with six tepals and up to six stamina. One of the defining characteristics (synapomorphies) of the order is the presence of phytomelanin, a black pigment present in the seed coat, creating a dark crust. Phytomelanin is found in most families of the Asparagales (although not in Orchidaceae, thought to be the sister-group of the rest of the order). | Asparagales | Wikipedia | 347 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
The order Asparagales takes its name from the type family Asparagaceae and has only recently been recognized in classification systems. The order is clearly circumscribed on the basis of molecular phylogenetics, but it is difficult to define morphologically since its members are structurally diverse. The order was first put forward by Huber in 1977 and later taken up in the Dahlgren system of 1985 and then the Angiosperm Phylogeny Group systems. Before this, many of its families were assigned to the old order Liliales, which was redistributed over three orders, Liliales, Asparagales, and Dioscoreales, based on molecular phylogenetics. The boundaries of the Asparagales and of its families have undergone a series of changes in recent years; future research may lead to further changes and ultimately greater stability.
The order is thought to have first diverged from other related monocots some 120–130 million years ago (early in the Cretaceous period), although given the difficulty in classifying the families involved, estimates are likely to be uncertain.
From an economic point of view, the order Asparagales is second in importance within the monocots only to the order Poales (which includes grasses and cereals). Species are used as food and flavourings (e.g. onion, garlic, leek, asparagus, vanilla, saffron), in medicinal or cosmetic applications (Aloe), as cut flowers (e.g. freesia, gladiolus, iris, orchids), and as garden ornamentals (e.g. day lilies, lily of the valley, Agapanthus).
Description
Although most species in the order are herbaceous, some no more than 15 cm high, there are a number of climbers (e.g., some species of Asparagus), as well as several genera forming trees (e.g. Agave, Cordyline, Yucca, Dracaena, Aloidendron ), which can exceed 10 m in height. Succulent genera occur in several families (e.g. Aloe). | Asparagales | Wikipedia | 448 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
Almost all species have a tight cluster of leaves (a rosette), either at the base of the plant or at the end of a more-or-less woody stem as with Yucca. In some cases, the leaves are produced along the stem. The flowers are in the main not particularly distinctive, being of a general 'lily type', with six tepals, either free or fused from the base and up to six stamina. They are frequently clustered at the end of the plant stem.
The Asparagales are generally distinguished from the Liliales by the lack of markings on the tepals, the presence of septal nectaries in the ovaries, rather than the bases of the tepals or stamen filaments, and the presence of secondary growth. They are generally geophytes, but with linear leaves, and a lack of fine reticular venation.
The seeds characteristically have the external epidermis either obliterated (in most species bearing fleshy fruit), or if present, have a layer of black carbonaceous phytomelanin in species with dry fruits (nuts). The inner part of the seed coat is generally collapsed, in contrast to Liliales whose seeds have a well developed outer epidermis, lack phytomelanin, and usually display a cellular inner layer. | Asparagales | Wikipedia | 278 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
The orders which have been separated from the old Liliales are difficult to characterize. No single morphological character appears to be diagnostic of the order Asparagales.
The flowers of Asparagales are of a general type among the lilioid monocots. Compared to Liliales, they usually have plain tepals without markings in the form of dots. If nectaries are present, they are in the septa of the ovaries rather than at the base of the tepals or stamens.
Those species which have relatively large dry seeds have a dark, crust-like (crustose) outer layer containing the pigment phytomelan. However, some species with hairy seeds (e.g. Eriospermum, family Asparagaceae s.l.), berries (e.g. Maianthemum, family Asparagaceae s.l.), or highly reduced seeds (e.g. orchids) lack this dark pigment in their seed coats. Phytomelan is not unique to Asparagales (i.e. it is not a synapomorphy) but it is common within the order and rare outside it. The inner portion of the seed coat is usually completely collapsed. In contrast, the morphologically similar seeds of Liliales have no phytomelan, and usually retain a cellular structure in the inner portion of the seed coat.
Most monocots are unable to thicken their stems once they have formed, since they lack the cylindrical meristem present in other angiosperm groups. Asparagales have a method of secondary thickening which is otherwise only found in Dioscorea (in the monocot order Disoscoreales). In a process called 'anomalous secondary growth', they are able to create new vascular bundles around which thickening growth occurs. Agave, Yucca, Aloidendron, Dracaena, Nolina and Cordyline can become massive trees, albeit not of the height of the tallest dicots, and with less branching. Other genera in the order, such as Lomandra and Aphyllanthes, have the same type of secondary growth but confined to their underground stems. | Asparagales | Wikipedia | 460 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
Microsporogenesis (part of pollen formation) distinguishes some members of Asparagales from Liliales. Microsporogenesis involves a cell dividing twice (meiotically) to form four daughter cells. There are two kinds of microsporogenesis: successive and simultaneous (although intermediates exist). In successive microsporogenesis, walls are laid down separating the daughter cells after each division. In simultaneous microsporogenesis, there is no wall formation until all four cell nuclei are present. Liliales all have successive microsporogenesis, which is thought to be the primitive condition in monocots. It seems that when the Asparagales first diverged they developed simultaneous microsporogenesis, which the 'lower' Asparagales families retain. However, the 'core' Asparagales (see Phylogenetics ) have reverted to successive microsporogenesis.
The Asparagales appear to be unified by a mutation affecting their telomeres (a region of repetitive DNA at the end of a chromosome). The typical 'Arabidopsis-type' sequence of bases has been fully or partially replaced by other sequences, with the 'human-type' predominating.
Other apomorphic characters of the order according to Stevens are: the presence of chelidonic acid, anthers longer than wide, tapetal cells bi- to tetra-nuclear, tegmen not persistent, endosperm helobial, and loss of mitochondrial gene sdh3. | Asparagales | Wikipedia | 306 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
According to telomere sequence, at least two evolutionary switch-points happened within the order. The basal sequence is formed by TTTAGGG like in the majority of higher plants. Basal motif was changed to vertebrate-like TTAGGG and finally, the most divergent motif CTCGGTTATGGG appears in Allium.
Taxonomy
As circumscribed within the Angiosperm Phylogeny Group system Asparagales is the largest order within the monocotyledons, with 14 families, 1,122 genera and about 25,000–42,000 species, thus accounting for about 50% of all monocots and 10–15% of the flowering plants (angiosperms). The attribution of botanical authority for the name Asparagales belongs to Johann Heinrich Friedrich Link (1767–1851) who coined the word 'Asparaginae' in 1829 for a higher order taxon that included Asparagus although Adanson and Jussieau had also done so earlier (see History). Earlier circumscriptions of Asparagales attributed the name to Bromhead (1838), who had been the first to use the term 'Asparagales'.
History | Asparagales | Wikipedia | 254 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
The type genus, Asparagus, from which the name of the order is derived, was described by Carl Linnaeus in 1753, with ten species. He placed Asparagus within the Hexandria Monogynia (six stamens, one carpel) in his sexual classification in the Species Plantarum. The majority of taxa now considered to constitute Asparagales have historically been placed within the very large and diverse family, Liliaceae. The family Liliaceae was first described by Michel Adanson in 1763, and in his taxonomic scheme he created eight sections within it, including the Asparagi with Asparagus and three other genera. The system of organising genera into families is generally credited to Antoine Laurent de Jussieu who formally described both the Liliaceae and the type family of Asparagales, the Asparagaceae, as Lilia and Asparagi, respectively, in 1789. Jussieu established the hierarchical system of taxonomy (phylogeny), placing Asparagus and related genera within a division of Monocotyledons, a class (III) of Stamina Perigynia and 'order' Asparagi, divided into three subfamilies. The use of the term Ordo (order) at that time was closer to what we now understand as Family, rather than Order. In creating his scheme he used a modified form of Linnaeus' sexual classification but using the respective topography of stamens to carpels rather than just their numbers. While De Jussieu's Stamina Perigynia also included a number of 'orders' that would eventually form families within the Asparagales such as the Asphodeli (Asphodelaceae), Narcissi (Amaryllidaceae) and Irides (Iridaceae), the remainder are now allocated to other orders. Jussieu's Asparagi soon came to be referred to as Asparagacées in the French literature (Latin: Asparagaceae). Meanwhile, the 'Narcissi' had been renamed as the 'Amaryllidées' (Amaryllideae) in 1805, by Jean Henri Jaume Saint-Hilaire, using Amaryllis as the type species rather than Narcissus, and thus has the authority attribution for Amaryllidaceae | Asparagales | Wikipedia | 481 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
In 1810, Brown proposed that a subgroup of Liliaceae be distinguished on the basis of the position of the ovaries and be referred to as Amaryllideae and in 1813 de Candolle described Liliacées Juss. and Amaryllidées Brown as two quite separate families | Asparagales | Wikipedia | 58 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
The literature on the organisation of genera into families and higher ranks became available in the English language with Samuel Frederick Gray's A natural arrangement of British plants (1821). Gray used a combination of Linnaeus' sexual classification and Jussieu's natural classification to group together a number of families having in common six equal stamens, a single style and a perianth that was simple and petaloid, but did not use formal names for these higher ranks. Within the grouping he separated families by the characteristics of their fruit and seed. He treated groups of genera with these characteristics as separate families, such as Amaryllideae, Liliaceae, Asphodeleae and Asparageae.
The circumscription of Asparagales has been a source of difficulty for many botanists from the time of John Lindley (1846), the other important British taxonomist of the early nineteenth century. In his first taxonomic work, An Introduction to the Natural System of Botany (1830) he partly followed Jussieu by describing a subclass he called Endogenae, or Monocotyledonous Plants (preserving de Candolle's Endogenæ phanerogamæ) divided into two tribes, the Petaloidea and Glumaceae. He divided the former, often referred to as petaloid monocots, into 32 orders, including the Liliaceae (defined narrowly), but also most of the families considered to make up the Asparagales today, including the Amaryllideae. | Asparagales | Wikipedia | 313 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
By 1846, in his final scheme Lindley had greatly expanded and refined the treatment of the monocots, introducing both an intermediate ranking (Alliances) and tribes within orders (i.e. families). Lindley placed the Liliaceae within the Liliales, but saw it as a paraphyletic ("catch-all") family, being all Liliales not included in the other orders, but hoped that the future would reveal some characteristic that would group them better. The order Liliales was very large and included almost all monocotyledons with colourful tepals and without starch in their endosperm (the lilioid monocots). The Liliales was difficult to divide into families because morphological characters were not present in patterns that clearly demarcated groups. This kept the Liliaceae separate from the Amaryllidaceae (Narcissales). Of these, Liliaceae was divided into eleven tribes (with 133 genera) and Amaryllidaceae into four tribes (with 68 genera), yet both contained many genera that would eventually segregate to each other's contemporary orders (Liliales and Asparagales respectively). The Liliaceae would be reduced to a small 'core' represented by the tribe Tulipae, while large groups such Scilleae and Asparagae would become part of Asparagales either as part of the Amaryllidaceae or as separate families. While of the Amaryllidaceae, the Agaveae would be part of Asparagaceae but the Alstroemeriae would become a family within the Liliales. | Asparagales | Wikipedia | 335 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
The number of known genera (and species) continued to grow and by the time of the next major British classification, that of the Bentham & Hooker system in 1883 (published in Latin) several of Lindley's other families had been absorbed into the Liliaceae. They used the term 'series' to indicate suprafamilial rank, with seven series of monocotyledons (including Glumaceae), but did not use Lindley's terms for these. However, they did place the Liliaceous and Amaryllidaceous genera into separate series. The Liliaceae were placed in series Coronariae, while the Amaryllideae were placed in series Epigynae. The Liliaceae now consisted of twenty tribes (including Tulipeae, Scilleae and Asparageae), and the Amaryllideae of five (including Agaveae and Alstroemerieae). An important addition to the treatment of the Liliaceae was the recognition of the Allieae as a distinct tribe that would eventually find its way to the Asparagales as the subfamily Allioideae of the Amaryllidaceae.
Post-Darwinian
The appearance of Charles Darwin's Origin of Species in 1859 changed the way that taxonomists considered plant classification, incorporating evolutionary information into their schemata. The Darwinian approach led to the concept of phylogeny (tree-like structure) in assembling classification systems, starting with Eichler. Eichler, having established a hierarchical system in which the flowering plants (angiosperms) were divided into monocotyledons and dicotyledons, further divided into former into seven orders. Within the Liliiflorae were seven families, including Liliaceae and Amaryllidaceae. Liliaceae included Allium and Ornithogalum (modern Allioideae) and Asparagus. | Asparagales | Wikipedia | 391 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
Engler, in his system developed Eichler's ideas into a much more elaborate scheme which he treated in a number of works including Die Natürlichen Pflanzenfamilien (Engler and Prantl 1888) and Syllabus der Pflanzenfamilien (1892–1924). In his treatment of Liliiflorae the Liliineae were a suborder which included both families Liliaceae and Amaryllidaceae. The Liliaceae had eight subfamilies and the Amaryllidaceae four. In this rearrangement of Liliaceae, with fewer subdivisions, the core Liliales were represented as subfamily Lilioideae (with Tulipae and Scilleae as tribes), the Asparagae were represented as Asparagoideae and the Allioideae was preserved, representing the alliaceous genera. Allieae, Agapantheae and Gilliesieae were the three tribes within this subfamily. In the Amaryllidaceae, there was little change from the Bentham & Hooker. A similar approach was adopted by Wettstein.
Twentieth century
In the twentieth century the Wettstein system (1901–1935) placed many of the taxa in an order called 'Liliiflorae'. Next Johannes Paulus Lotsy (1911) proposed dividing the Liliiflorae into a number of smaller families including Asparagaceae. Then Herbert Huber (1969, 1977), following Lotsy's example, proposed that the Liliiflorae be split into four groups including the 'Asparagoid' Liliiflorae.
The widely used Cronquist system (1968–1988) used the very broadly defined order Liliales. | Asparagales | Wikipedia | 362 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
These various proposals to separate small groups of genera into more homogeneous families made little impact till that of Dahlgren (1985) incorporating new information including synapomorphy. Dahlgren developed Huber's ideas further and popularised them, with a major deconstruction of existing families into smaller units. They created a new order, calling it Asparagales. This was one of five orders within the superorder Liliiflorae. Where Cronquist saw one family, Dahlgren saw forty distributed over three orders (predominantly Liliales and Asparagales). Over the 1980s, in the context of a more general review of the classification of angiosperms, the Liliaceae were subjected to more intense scrutiny. By the end of that decade, the Royal Botanic Gardens at Kew, the British Museum of Natural History and the Edinburgh Botanical Gardens formed a committee to examine the possibility of separating the family at least for the organization of their herbaria. That committee finally recommended that 24 new families be created in the place of the original broad Liliaceae, largely by elevating subfamilies to the rank of separate families.
Phylogenetics
The order Asparagales as currently circumscribed has only recently been recognized in classification systems, through the advent of phylogenetics. The 1990s saw considerable progress in plant phylogeny and phylogenetic theory, enabling a phylogenetic tree to be constructed for all of the flowering plants. The establishment of major new clades necessitated a departure from the older but widely used classifications such as Cronquist and Thorne based largely on morphology rather than genetic data. This complicated the discussion about plant evolution and necessitated a major restructuring. rbcL gene sequencing and cladistic analysis of monocots had redefined the Liliales in 1995. from four morphological orders sensu Dahlgren. The largest clade representing the Liliaceae, all previously included in Liliales, but including both the Calochortaceae and Liliaceae sensu Tamura. This redefined family, that became referred to as core Liliales, but corresponded to the emerging circumscription of the Angiosperm Phylogeny Group (1998).
Phylogeny and APG system
The 2009 revision of the Angiosperm Phylogeny Group system, APG III, places the order in the clade monocots. | Asparagales | Wikipedia | 482 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
From the Dahlgren system of 1985 onwards, studies based mainly on morphology had identified the Asparagales as a distinct group, but had also included groups now located in Liliales, Pandanales and Zingiberales. Research in the 21st century has supported the monophyly of Asparagales, based on morphology, 18S rDNA, and other DNA sequences, although some phylogenetic reconstructions based on molecular data have suggested that Asparagales may be paraphyletic, with Orchidaceae separated from the rest. Within the monocots, Asparagales is the sister group of the commelinid clade.
This cladogram shows the placement of Asparagales within the orders of Lilianae sensu Chase & Reveal (monocots) based on molecular phylogenetic evidence. The lilioid monocot orders are bracketed, namely Petrosaviales, Dioscoreales, Pandanales, Liliales and Asparagales. These constitute a paraphyletic assemblage, that is groups with a common ancestor that do not include all direct descendants (in this case commelinids as the sister group to Asparagales); to form a clade, all the groups joined by thick lines would need to be included. While Acorales and Alismatales have been collectively referred to as "alismatid monocots" (basal or early branching monocots), the remaining clades (lilioid and commelinid monocots) have been referred to as the "core monocots". The relationship between the orders (with the exception of the two sister orders) is pectinate, that is diverging in succession from the line that leads to the commelinids. Numbers indicate crown group (most recent common ancestor of the sampled species of the clade of interest) divergence times in mya (million years ago).
Subdivision
A phylogenetic tree for the Asparagales, generally to family level, but including groups which were recently and widely treated as families but which are now reduced to subfamily rank, is shown below. | Asparagales | Wikipedia | 438 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
The tree shown above can be divided into a basal paraphyletic group, the 'lower Asparagales (asparagoids)', from Orchidaceae to Asphodelaceae, and a well-supported monophyletic group of 'core Asparagales' (higher asparagoids), comprising the two largest families, Amaryllidaceae sensu lato and Asparagaceae sensu lato.
Two differences between these two groups (although with exceptions) are: the mode of microsporogenesis and the position of the ovary. The 'lower Asparagales' typically have simultaneous microsporogenesis (i.e. cell walls develop only after both meiotic divisions), which appears to be an apomorphy within the monocots, whereas the 'core Asparagales' have reverted to successive microsporogenesis (i.e. cell walls develop after each division). The 'lower Asparagales' typically have an inferior ovary, whereas the 'core Asparagales' have reverted to a superior ovary. A 2002 morphological study by Rudall treated possessing an inferior ovary as a synapomorphy of the Asparagales, stating that reversions to a superior ovary in the 'core Asparagales' could be associated with the presence of nectaries below the ovaries. However, Stevens notes that superior ovaries are distributed among the 'lower Asparagales' in such a way that it is not clear where to place the evolution of different ovary morphologies. The position of the ovary seems a much more flexible character (here and in other angiosperms) than previously thought.
Changes to family structure in APG III
The APG III system when it was published in 2009, greatly expanded the families Xanthorrhoeaceae, Amaryllidaceae, and Asparagaceae. Thirteen of the families of the earlier APG II system were thereby reduced to subfamilies within these three families. The expanded Xanthorrhoeaceae is now called "Asphodelaceae". The APG II families (left) and their equivalent APG III subfamilies (right) are as follows:
Structure of Asparagales | Asparagales | Wikipedia | 473 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
Orchid clade
Orchidaceae is possibly the largest family of all angiosperms (only Asteraceae might – or might not – be more speciose) and hence by far the largest in the order. The Dahlgren system recognized three families of orchids, but DNA sequence analysis later showed that these families are polyphyletic and so should be combined. Several studies suggest (with high bootstrap support) that Orchidaceae is the sister of the rest of the Asparagales. Other studies have placed the orchids differently in the phylogenetic tree, generally among the Boryaceae-Hypoxidaceae clade. The position of Orchidaceae shown above seems the best current hypothesis, but cannot be taken as confirmed.
Orchids have simultaneous microsporogenesis and inferior ovaries, two characters that are typical of the 'lower Asparagales'. However, their nectaries are rarely in the septa of the ovaries, and most orchids have dust-like seeds, atypical of the rest of the order. (Some members of Vanilloideae and Cypripedioideae have crustose seeds, probably associated with dispersal by birds and mammals that are attracted by fermenting fleshy fruit releasing fragrant compounds, e.g. vanilla.)
In terms of the number of species, Orchidaceae diversification is remarkable, with recent estimations suggesting that despite the old origin of the family dating back to the late cretaceous, modern orchid diversity originated mostly during the last 5 million years. However, although the other Asparagales may be less rich in species, they are more variable morphologically, including tree-like forms.
Boryaceae to Hypoxidaceae
The four families excluding Boryaceae form a well-supported clade in studies based on DNA sequence analysis. All four contain relatively few species, and it has been suggested that they be combined into one family under the name Hypoxidaceae sensu lato. The relationship between Boryaceae (which includes only two genera, Borya and Alania), and other Asparagales has remained unclear for a long time. The Boryaceae are mycorrhizal, but not in the same way as orchids. Morphological studies have suggested a close relationship between Boryaceae and Blandfordiaceae. There is relatively low support for the position of Boryaceae in the tree shown above. | Asparagales | Wikipedia | 490 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
Ixioliriaceae to Xeronemataceae
The relationship shown between Ixioliriaceae and Tecophilaeaceae is still unclear. Some studies have supported a clade of these two families, others have not. The position of Doryanthaceae has also varied, with support for the position shown above, but also support for other positions.
The clade from Iridaceae upwards appears to have stronger support. All have some genetic characteristics in common, having lost Arabidopsis-type telomeres. Iridaceae is distinctive among the Asparagales in the unique structure of the inflorescence (a rhipidium), the combination of an inferior ovary and three stamens, and the common occurrence of unifacial leaves whereas bifacial leaves are the norm in other Asparagales.
Members of the clade from Iridaceae upwards have infra-locular septal nectaries, which Rudall interpreted as a driver towards secondarily superior ovaries.
Asphodelaceae + 'core Asparagales'
The next node in the tree (Xanthorrhoeaceae sensu lato + the 'core Asparagales') has strong support. 'Anomalous' secondary thickening occurs among this clade, e.g. in Xanthorrhoea (family Asphodelaceae) and Dracaena (family Asparagaceae sensu lato), with species reaching tree-like proportions.
The 'core Asparagales', comprising Amaryllidaceae sensu lato and Asparagaceae sensu lato, are a strongly supported clade, as are clades for each of the families. Relationships within these broadly defined families appear less clear, particularly within the Asparagaceae sensu lato. Stevens notes that most of its subfamilies are difficult to recognize, and that significantly different divisions have been used in the past, so that the use of a broadly defined family to refer to the entire clade is justified. Thus the relationships among subfamilies shown above, based on APWeb , is somewhat uncertain.
Evolution
Several studies have attempted to date the evolution of the Asparagales, based on phylogenetic evidence. Earlier studies generally give younger dates than more recent studies, which have been preferred in the table below. | Asparagales | Wikipedia | 477 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
A 2009 study suggests that the Asparagales have the highest diversification rate in the monocots, about the same as the order Poales, although in both orders the rate is little over half that of the eudicot order Lamiales, the clade with the highest rate.
Comparison of family structures
The taxonomic diversity of the monocotyledons is described in detail by Kubitzki. Up-to-date information on the Asparagales can be found on the Angiosperm Phylogeny Website.
The APG III system's family circumscriptions are being used as the basis of the Kew-hosted World Checklist of Selected Plant Families. With this circumscription, the order consists of 14 families (Dahlgren had 31) with approximately 1120 genera and 26000 species.
Order Asparagales Link
Family Amaryllidaceae J.St.-Hil. (including Agapanthaceae F.Voigt, Alliaceae Borkh.)
Family Asparagaceae Juss. (including Agavaceae Dumort. [which includes Anemarrhenaceae, Anthericaceae, Behniaceae and Herreriaceae], Aphyllanthaceae Burnett, Hesperocallidaceae Traub, Hyacinthaceae Batsch ex Borkh., Laxmanniaceae Bubani, Ruscaceae M.Roem. [which includes Convallariaceae] and Themidaceae Salisb.)
Family Asteliaceae Dumort.
Family Blandfordiaceae R.Dahlgren & Clifford
Family Boryaceae M.W. Chase, Rudall & Conran
Family Doryanthaceae R.Dahlgren & Clifford
Family Hypoxidaceae R.Br.
Family Iridaceae Juss.
Family Ixioliriaceae Nakai
Family Lanariaceae R.Dahlgren & A.E.van Wyk
Family Orchidaceae Juss.
Family Tecophilaeaceae Leyb.
Family Xanthorrhoeaceae Dumort. (including Asphodelaceae Juss. and Hemerocallidaceae R.Br.), now Asphodelaceae Juss.
Family Xeronemataceae M.W.Chase, Rudall & M.F.Fay | Asparagales | Wikipedia | 478 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
The earlier 2003 version, APG II, allowed 'bracketed' families, i.e. families which could either be segregated from more comprehensive families or could be included in them. These are the families given under "including" in the list above. APG III does not allow bracketed families, requiring the use of the more comprehensive family; otherwise the circumscription of the Asparagales is unchanged. A separate paper accompanying the publication of the 2009 APG III system provided subfamilies to accommodate the families which were discontinued. The first APG system of 1998 contained some extra families, included in square brackets in the list above.
Two older systems which use the order Asparagales are the Dahlgren system and the Kubitzki system. The families included in the circumscriptions of the order in these two systems are shown in the first and second columns of the table below. The equivalent family in the modern APG III system (see below) is shown in the third column. Note that although these systems may use the same name for a family, the genera which it includes may be different, so the equivalence between systems is only approximate in some cases.
Uses
The Asparagales include many important crop plants and ornamental plants. Crops include Allium, Asparagus and Vanilla, while ornamentals include irises, hyacinths and orchids. | Asparagales | Wikipedia | 284 | 786 | https://en.wikipedia.org/wiki/Asparagales | Biology and health sciences | Asparagales | Plants |
Asterales ( ) is an order of dicotyledonous flowering plants that includes the large family Asteraceae (or Compositae) known for composite flowers made of florets, and ten families related to the Asteraceae. While asterids in general are characterized by fused petals, composite flowers consisting of many florets create the false appearance of separate petals (as found in the rosids).
The order is cosmopolitan (plants found throughout most of the world including desert and frigid zones), and includes mostly herbaceous species, although a small number of trees (such as the Lobelia deckenii, the giant lobelia, and Dendrosenecio, giant groundsels) and shrubs are also present.
Asterales are organisms that seem to have evolved from one common ancestor. Asterales share characteristics on morphological and biochemical levels. Synapomorphies (a character that is shared by two or more groups through evolutionary development) include the presence in the plants of oligosaccharide inulin, a nutrient storage molecule used instead of starch; and unique stamen morphology. The stamens are usually found around the style, either aggregated densely or fused into a tube, probably an adaptation in association with the plunger (brush; or secondary) pollination that is common among the families of the order, wherein pollen is collected and stored on the length of the pistil.
Taxonomy
The name and order Asterales is botanically venerable, dating back to at least 1926 in the Hutchinson system of plant taxonomy when it contained only five families, of which only two are retained in the APG III classification. Under the Cronquist system of taxonomic classification of flowering plants, Asteraceae was the only family in the group, but newer systems (such as APG II and APG III) have expanded it to 11. In the classification system of Rolf Dahlgren the Asterales were in the superorder Asteriflorae (also called Asteranae).
The order Asterales currently includes 11 families, the largest of which are the Asteraceae, with about 25,000 species, and the Campanulaceae (bellflowers), with about 2,000 species. The remaining families count together for less than 1500 species. The two large families are cosmopolitan, with many of their species found in the Northern Hemisphere, and the smaller families are usually confined to Australia and the adjacent areas, or sometimes South America. | Asterales | Wikipedia | 512 | 789 | https://en.wikipedia.org/wiki/Asterales | Biology and health sciences | Asterales | Plants |
Only the Asteraceae have composite flower heads; the other families do not, but share other characteristics such as storage of inulin that define the 11 families as more closely related to each other than to other plant families or orders such as the rosids.
The phylogenetic tree according to APG III for the Campanulid clade is as below.
Phylogeny
Although most extant species of Asteraceae are herbaceous, the examination of the basal members in the family suggests that the common ancestor of the family was an arborescent plant, a tree or shrub, perhaps adapted to dry conditions, radiating from South America. Less can be said about the Asterales themselves with certainty, although since several families in Asterales contain trees, the ancestral member is most likely to have been a tree or shrub.
Because all clades are represented in the Southern Hemisphere but many not in the Northern Hemisphere, it is natural to conjecture that there is a common southern origin to them. Asterales belong to angiosperms or flowering plants, a clade that appeared about 140 million years ago. The Asterales order probably originated in the Cretaceous (145 – 66 Mya) on the supercontinent Gondwana which broke up from 184 – 80 Mya, forming the area that is now Australia, South America, Africa, India and Antarctica.
Asterales contain about 14% of eudicot diversity. From an analysis of relationships and diversities within the Asterales and with their superorders, estimates of the age of the beginning of the Asterales have been made, which range from 116 Mya to 82Mya. However few fossils have been found, of the Menyanthaceae-Asteraceae clade in the Oligocene, about 29 Mya.
Fossil evidence of the Asterales is rare and belongs to rather recent epochs, so the precise estimation of the order's age is quite difficult. An Oligocene (34 – 23 Mya) pollen is known for Asteraceae and Goodeniaceae, and seeds from Oligocene and Miocene (23 – 5.3 Mya) are known for Menyanthaceae and Campanulaceae respectively. According to molecular clock calculations, the lineage that led to Asterales split from other plants about 112 million years ago or 94 million years ago. | Asterales | Wikipedia | 476 | 789 | https://en.wikipedia.org/wiki/Asterales | Biology and health sciences | Asterales | Plants |
Biogeography
The core Asterales are Stylidiaceae (six genera), APA clade (Alseuosmiaceae, Phellinaceae and Argophyllaceae, together seven genera), MGCA clade (Menyanthaceae, Goodeniaceae, Calyceraceae, in total twenty genera), and Asteraceae (about sixteen hundred genera). Other Asterales are Rousseaceae (four genera), Campanulaceae (eighty-four genera) and Pentaphragmataceae (one genus).
All Asterales families are represented in the Southern Hemisphere; however, Asteraceae and Campanulaceae are cosmopolitan and Menyanthaceae nearly so.
Uses
The Asterales, by dint of being a super-set of the family Asteraceae, include some species grown for food, including the sunflower (Helianthus annuus), lettuce (Lactuca sativa) and chicory (Cichorium). Many are also used as spices and traditional medicines.
Asterales are common plants and have many known uses. For example, pyrethrum (derived from Old World members of the genus Chrysanthemum) is a natural insecticide with minimal environmental impact. Wormwood, derived from a genus that includes the sagebrush, is used as a source of flavoring for absinthe, a bitter classical liquor of European origin. | Asterales | Wikipedia | 297 | 789 | https://en.wikipedia.org/wiki/Asterales | Biology and health sciences | Asterales | Plants |
An asteroid is a minor planet—an object that is neither a true planet nor an identified comet— that orbits within the inner Solar System. They are rocky, metallic, or icy bodies with no atmosphere, classified as C-type (carbonaceous), M-type (metallic), or S-type (silicaceous). The size and shape of asteroids vary significantly, ranging from small rubble piles under a kilometer across and larger than meteoroids, to Ceres, a dwarf planet almost 1000 km in diameter. A body is classified as a comet, not an asteroid, if it shows a coma (tail) when warmed by solar radiation, although recent observations suggest a continuum between these types of bodies.
Of the roughly one million known asteroids, the greatest number are located between the orbits of Mars and Jupiter, approximately 2 to 4 AU from the Sun, in a region known as the main asteroid belt. The total mass of all the asteroids combined is only 3% that of Earth's Moon. The majority of main belt asteroids follow slightly elliptical, stable orbits, revolving in the same direction as the Earth and taking from three to six years to complete a full circuit of the Sun.
Asteroids have historically been observed from Earth. The first close-up observation of an asteroid was made by the Galileo spacecraft. Several dedicated missions to asteroids were subsequently launched by NASA and JAXA, with plans for other missions in progress. NASA's NEAR Shoemaker studied Eros, and Dawn observed Vesta and Ceres. JAXA's missions Hayabusa and Hayabusa2 studied and returned samples of Itokawa and Ryugu, respectively. OSIRIS-REx studied Bennu, collecting a sample in 2020 which was delivered back to Earth in 2023. NASA's Lucy, launched in 2021, is tasked with studying ten different asteroids, two from the main belt and eight Jupiter trojans. Psyche, launched October 2023, aims to study the metallic asteroid Psyche.
Near-Earth asteroids have the potential for catastrophic consequences if they strike Earth, with a notable example being the Chicxulub impact, widely thought to have induced the Cretaceous–Paleogene mass extinction. As an experiment to meet this danger, in September 2022 the Double Asteroid Redirection Test spacecraft successfully altered the orbit of the non-threatening asteroid Dimorphos by crashing into it.
Terminology | Asteroid | Wikipedia | 480 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
In 2006, the International Astronomical Union (IAU) introduced the currently preferred broad term small Solar System body, defined as an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite; this includes asteroids, comets, and more recently discovered classes. According to IAU, "the term 'minor planet' may still be used, but generally, 'Small Solar System Body' will be preferred."
Historically, the first discovered asteroid, Ceres, was at first considered a new planet. It was followed by the discovery of other similar bodies, which with the equipment of the time appeared to be points of light like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term asteroid, coined in Greek as ἀστεροειδής, or asteroeidēs, meaning 'star-like, star-shaped', and derived from the Ancient Greek astēr 'star, planet'. In the early second half of the 19th century, the terms asteroid and planet (not always qualified as "minor") were still used interchangeably.
Traditionally, small bodies orbiting the Sun were classified as comets, asteroids, or meteoroids, with anything smaller than one meter across being called a meteoroid. The term asteroid, never officially defined, can be informally used to mean "an irregularly shaped rocky body orbiting the Sun that does not qualify as a planet or a dwarf planet under the IAU definitions". The main difference between an asteroid and a comet is that a comet shows a coma (tail) due to sublimation of its near-surface ices by solar radiation. A few objects were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and become asteroid-like. A further distinction is that comets typically have more eccentric orbits than most asteroids; highly eccentric asteroids are probably dormant or extinct comets.
The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations. However, it is becoming increasingly common for the term asteroid to be restricted to minor planets of the inner Solar System. Therefore, this article will restrict itself for the most part to the classical asteroids: objects of the asteroid belt, Jupiter trojans, and near-Earth objects. | Asteroid | Wikipedia | 492 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
For almost two centuries after the discovery of Ceres in 1801, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few, such as 944 Hidalgo, ventured farther for part of their orbit. Starting in 1977 with 2060 Chiron, astronomers discovered small bodies that permanently resided further out than Jupiter, now called centaurs. In 1992, 15760 Albion was discovered, the first object beyond the orbit of Neptune (other than Pluto); soon large numbers of similar objects were observed, now called trans-Neptunian object. Further out are Kuiper-belt objects, scattered-disc objects, and the much more distant Oort cloud, hypothesized to be the main reservoir of dormant comets. They inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies exhibit little cometary activity; if centaurs or trans-Neptunian objects were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets.
The Kuiper-belt bodies are called "objects" partly to avoid the need to classify them as asteroids or comets. They are thought to be predominantly comet-like in composition, though some may be more akin to asteroids. Most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional comet nuclei. Other recent observations, such as the analysis of the cometary dust collected by the Stardust probe, are increasingly blurring the distinction between comets and asteroids, suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.
In 2006, the IAU created the class of dwarf planets for the largest minor planets—those massive enough to have become ellipsoidal under their own gravity. Only the largest object in the asteroid belt has been placed in this category: Ceres, at about across.
History of observations
Despite their large numbers, asteroids are a relatively recent discovery, with the first one—Ceres—only being identified in 1801. Only one asteroid, 4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye in dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be briefly visible to the naked eye. , the Minor Planet Center had data on 1,199,224 minor planets in the inner and outer Solar System, of which about 614,690 had enough information to be given numbered designations. | Asteroid | Wikipedia | 507 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Discovery of Ceres
In 1772, German astronomer Johann Elert Bode, citing Johann Daniel Titius, published a numerical procession known as the Titius–Bode law (now discredited). Except for an unexplained gap between Mars and Jupiter, Bode's formula seemed to predict the orbits of the known planets. He wrote the following explanation for the existence of a "missing planet":
This latter point seems in particular to follow from the astonishing relation which the known six planets observe in their distances from the Sun. Let the distance from the Sun to Saturn be taken as 100, then Mercury is separated by 4 such parts from the Sun. Venus is 4 + 3 = 7. The Earth 4 + 6 = 10. Mars 4 + 12 = 16. Now comes a gap in this so orderly progression. After Mars there follows a space of 4 + 24 = 28 parts, in which no planet has yet been seen. Can one believe that the Founder of the universe had left this space empty? Certainly not. From here we come to the distance of Jupiter by 4 + 48 = 52 parts, and finally to that of Saturn by 4 + 96 = 100 parts.
Bode's formula predicted another planet would be found with an orbital radius near 2.8 astronomical units (AU), or 420 million km, from the Sun. The Titius–Bode law got a boost with William Herschel's discovery of Uranus near the predicted distance for a planet beyond Saturn. In 1800, a group headed by Franz Xaver von Zach, editor of the German astronomical journal Monatliche Correspondenz (Monthly Correspondence), sent requests to 24 experienced astronomers (whom he dubbed the "celestial police"), asking that they combine their efforts and begin a methodical search for the expected planet. Although they did not discover Ceres, they later found the asteroids 2 Pallas, 3 Juno and 4 Vesta.
One of the astronomers selected for the search was Giuseppe Piazzi, a Catholic priest at the Academy of Palermo, Sicily. Before receiving his invitation to join the group, Piazzi discovered Ceres on 1 January 1801. He was searching for "the 87th [star] of the Catalogue of the Zodiacal stars of Mr la Caille", but found that "it was preceded by another". Instead of a star, Piazzi had found a moving star-like object, which he first thought was a comet: | Asteroid | Wikipedia | 496 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
The light was a little faint, and of the colour of Jupiter, but similar to many others which generally are reckoned of the eighth magnitude. Therefore I had no doubt of its being any other than a fixed star. [...] The evening of the third, my suspicion was converted into certainty, being assured it was not a fixed star. Nevertheless before I made it known, I waited till the evening of the fourth, when I had the satisfaction to see it had moved at the same rate as on the preceding days.
Piazzi observed Ceres a total of 24 times, the final time on 11 February 1801, when illness interrupted his work. He announced his discovery on 24 January 1801 in letters to only two fellow astronomers, his compatriot Barnaba Oriani of Milan and Bode in Berlin. He reported it as a comet but "since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet". In April, Piazzi sent his complete observations to Oriani, Bode, and French astronomer Jérôme Lalande. The information was published in the September 1801 issue of the Monatliche Correspondenz.
By this time, the apparent position of Ceres had changed (mostly due to Earth's motion around the Sun), and was too close to the Sun's glare for other astronomers to confirm Piazzi's observations. Toward the end of the year, Ceres should have been visible again, but after such a long time it was difficult to predict its exact position. To recover Ceres, mathematician Carl Friedrich Gauss, then 24 years old, developed an efficient method of orbit determination. In a few weeks, he predicted the path of Ceres and sent his results to von Zach. On 31 December 1801, von Zach and fellow celestial policeman Heinrich W. M. Olbers found Ceres near the predicted position and thus recovered it. At 2.8 AU from the Sun, Ceres appeared to fit the Titius–Bode law almost perfectly; however, Neptune, once discovered in 1846, was 8 AU closer than predicted, leading most astronomers to conclude that the law was a coincidence. Piazzi named the newly discovered object Ceres Ferdinandea, "in honor of the patron goddess of Sicily and of King Ferdinand of Bourbon".
Further search | Asteroid | Wikipedia | 476 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Three other asteroids (2 Pallas, 3 Juno, and 4 Vesta) were discovered by von Zach's group over the next few years, with Vesta found in 1807. No new asteroids were discovered until 1845. Amateur astronomer Karl Ludwig Hencke started his searches of new asteroids in 1830, and fifteen years later, while looking for Vesta, he found the asteroid later named 5 Astraea. It was the first new asteroid discovery in 38 years. Carl Friedrich Gauss was given the honor of naming the asteroid. After this, other astronomers joined; 15 asteroids were found by the end of 1851. In 1868, when James Craig Watson discovered the 100th asteroid, the French Academy of Sciences engraved the faces of Karl Theodor Robert Luther, John Russell Hind, and Hermann Goldschmidt, the three most successful asteroid-hunters at that time, on a commemorative medallion marking the event.
In 1891, Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This dramatically increased the rate of detection compared with earlier visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had been discovered up to that point. It was known that there were many more, but most astronomers did not bother with them, some calling them "vermin of the skies", a phrase variously attributed to Eduard Suess and Edmund Weiss. Even a century later, only a few thousand asteroids were identified, numbered and named.
19th and 20th centuries
In the past, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope or astrograph. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films or plates of the same region were viewed under a stereoscope. A body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would seem to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations. | Asteroid | Wikipedia | 452 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a provisional designation, made up of the year of discovery, a letter representing the half-month of discovery, and finally a letter and a number indicating the discovery's sequential number (example: ). The last step is sending the locations and time of observations to the Minor Planet Center, where computer programs determine whether an apparition ties together earlier apparitions into a single orbit. If so, the object receives a catalogue number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honor of naming the object subject to the approval of the International Astronomical Union.
Naming
By 1851, the Royal Astronomical Society decided that asteroids were being discovered at such a rapid rate that a different system was needed to categorize or name asteroids. In 1852, when de Gasparis discovered the twentieth asteroid, Benjamin Valz gave it a name and a number designating its rank among asteroid discoveries, 20 Massalia. Sometimes asteroids were discovered and not seen again. So, starting in 1892, new asteroids were listed by the year and a capital letter indicating the order in which the asteroid's orbit was calculated and registered within that specific year. For example, the first two asteroids discovered in 1892 were labeled 1892A and 1892B. However, there were not enough letters in the alphabet for all of the asteroids discovered in 1893, so 1893Z was followed by 1893AA. A number of variations of these methods were tried, including designations that included year plus a Greek letter in 1914. A simple chronological numbering system was established in 1925.
Currently all newly discovered asteroids receive a provisional designation (such as ) consisting of the year of discovery and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name (e.g. ). The formal naming convention uses parentheses around the number—e.g. (433) Eros—but dropping the parentheses is quite common. Informally, it is also common to drop the number altogether, or to drop it after the first mention when a name is repeated in running text. In addition, names can be proposed by the asteroid's discoverer, within guidelines established by the International Astronomical Union.
Symbols | Asteroid | Wikipedia | 491 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
The first asteroids to be discovered were assigned iconic symbols like the ones traditionally used to designate the planets. By 1852 there were two dozen asteroid symbols, which often occurred in multiple variants.
In 1851, after the fifteenth asteroid, Eunomia, had been discovered, Johann Franz Encke made a major change in the upcoming 1854 edition of the Berliner Astronomisches Jahrbuch (BAJ, Berlin Astronomical Yearbook). He introduced a disk (circle), a traditional symbol for a star, as the generic symbol for an asteroid. The circle was then numbered in order of discovery to indicate a specific asteroid. The numbered-circle convention was quickly adopted by astronomers, and the next asteroid to be discovered (16 Psyche, in 1852) was the first to be designated in that way at the time of its discovery. However, Psyche was given an iconic symbol as well, as were a few other asteroids discovered over the next few years. 20 Massalia was the first asteroid that was not assigned an iconic symbol, and no iconic symbols were created after the 1855 discovery of 37 Fides.
Formation
Many asteroids are the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets. It is thought that planetesimals in the asteroid belt evolved much like the rest of objects in the solar nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption. Ceres and Vesta grew large enough to melt and differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.
In the Nice model, many Kuiper-belt objects are captured in the outer asteroid belt, at distances greater than 2.6 AU. Most were later ejected by Jupiter, but those that remained may be the D-type asteroids, and possibly include Ceres.
Distribution within the Solar System
Various dynamical groups of asteroids have been discovered orbiting in the inner Solar System. Their orbits are perturbed by the gravity of other bodies in the Solar System and by the Yarkovsky effect. Significant populations include:
Asteroid belt | Asteroid | Wikipedia | 484 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
The majority of known asteroids orbit within the asteroid belt between the orbits of Mars and Jupiter, generally in relatively low-eccentricity (i.e. not very elongated) orbits. This belt is estimated to contain between 1.1 and 1.9 million asteroids larger than in diameter, and millions of smaller ones. These asteroids may be remnants of the protoplanetary disk, and in this region the accretion of planetesimals into planets during the formative period of the Solar System was prevented by large gravitational perturbations by Jupiter.
Contrary to popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that reaching an asteroid without aiming carefully would be improbable. Nonetheless, hundreds of thousands of asteroids are currently known, and the total number ranges in the millions or more, depending on the lower size cutoff. Over 200 asteroids are known to be larger than 100 km, and a survey in the infrared wavelengths has shown that the asteroid belt has between 700,000 and 1.7 million asteroids with a diameter of 1 km or more. The absolute magnitudes of most of the known asteroids are between 11 and 19, with the median at about 16.
The total mass of the asteroid belt is estimated to be kg, which is just 3% of the mass of the Moon; the mass of the Kuiper Belt and Scattered Disk is over 100 times as large. The four largest objects, Ceres, Vesta, Pallas, and Hygiea, account for maybe 62% of the belt's total mass, with 39% accounted for by Ceres alone.
Trojans
Trojans are populations that share an orbit with a larger planet or moon, but do not collide with it because they orbit in one of the two Lagrangian points of stability, and , which lie 60° ahead of and behind the larger body. | Asteroid | Wikipedia | 384 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
In the Solar System, most known trojans share the orbit of Jupiter. They are divided into the Greek camp at (ahead of Jupiter) and the Trojan camp at (trailing Jupiter). More than a million Jupiter trojans larger than one kilometer are thought to exist, of which more than 7,000 are currently catalogued. In other planetary orbits only nine Mars trojans, 28 Neptune trojans, two Uranus trojans, and two Earth trojans, have been found to date. A temporary Venus trojan is also known. Numerical orbital dynamics stability simulations indicate that Saturn and Uranus probably do not have any primordial trojans.
Near-Earth asteroids
Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross Earth's orbital path are known as Earth-crossers. , a total of 28,772 near-Earth asteroids were known; 878 have a diameter of one kilometer or larger.
A small number of NEAs are extinct comets that have lost their volatile surface materials, although having a faint or intermittent comet-like tail does not necessarily result in a classification as a near-Earth comet, making the boundaries somewhat fuzzy. The rest of the near-Earth asteroids are driven out of the asteroid belt by gravitational interactions with Jupiter.
Many asteroids have natural satellites (minor-planet moons). , there were 85 NEAs known to have at least one moon, including three known to have two moons. The asteroid 3122 Florence, one of the largest potentially hazardous asteroids with a diameter of , has two moons measuring across, which were discovered by radar imaging during the asteroid's 2017 approach to Earth. | Asteroid | Wikipedia | 340 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Near-Earth asteroids are divided into groups based on their semi-major axis (a), perihelion distance (q), and aphelion distance (Q):
The Atiras or Apoheles have orbits strictly inside Earth's orbit: an Atira asteroid's aphelion distance (Q) is smaller than Earth's perihelion distance (0.983 AU). That is, , which implies that the asteroid's semi-major axis is also less than 0.983 AU.
The Atens have a semi-major axis of less than 1 AU and cross Earth's orbit. Mathematically, and . (0.983 AU is Earth's perihelion distance.)
The Apollos have a semi-major axis of more than 1 AU and cross Earth's orbit. Mathematically, and . (1.017 AU is Earth's aphelion distance.)
The Amors have orbits strictly outside Earth's orbit: an Amor asteroid's perihelion distance (q) is greater than Earth's aphelion distance (1.017 AU). Amor asteroids are also near-earth objects so . In summary, . (This implies that the asteroid's semi-major axis (a) is also larger than 1.017 AU.) Some Amor asteroid orbits cross the orbit of Mars.
Martian moons
It is unclear whether Martian moons Phobos and Deimos are captured asteroids or were formed due to impact event on Mars. Phobos and Deimos both have much in common with carbonaceous C-type asteroids, with spectra, albedo, and density very similar to those of C- or D-type asteroids. Based on their similarity, one hypothesis is that both moons may be captured main-belt asteroids. Both moons have very circular orbits which lie almost exactly in Mars's equatorial plane, and hence a capture origin requires a mechanism for circularizing the initially highly eccentric orbit, and adjusting its inclination into the equatorial plane, most probably by a combination of atmospheric drag and tidal forces, although it is not clear whether sufficient time was available for this to occur for Deimos. Capture also requires dissipation of energy. The current Martian atmosphere is too thin to capture a Phobos-sized object by atmospheric braking. Geoffrey A. Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated under tidal forces. | Asteroid | Wikipedia | 506 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Phobos could be a second-generation Solar System object that coalesced in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars.
Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large planetesimal. The high porosity of the interior of Phobos (based on the density of 1.88 g/cm3, voids are estimated to comprise 25 to 35 percent of Phobos's volume) is inconsistent with an asteroidal origin. Observations of Phobos in the thermal infrared suggest a composition containing mainly phyllosilicates, which are well known from the surface of Mars. The spectra are distinct from those of all classes of chondrite meteorites, again pointing away from an asteroidal origin. Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit, similar to the prevailing theory for the origin of Earth's moon.
Characteristics
Size distribution
Asteroids vary greatly in size, from almost for the largest down to rocks just 1 meter across, below which an object is classified as a meteoroid. The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors, and are thought to be surviving protoplanets. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either battered planetesimals or fragments of larger bodies.
The dwarf planet Ceres is by far the largest asteroid, with a diameter of . The next largest are 4 Vesta and 2 Pallas, both with diameters of just over . Vesta is the brightest of the four main-belt asteroids that can, on occasion, be visible to the naked eye. On some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis. | Asteroid | Wikipedia | 406 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
The mass of all the objects of the asteroid belt, lying between the orbits of Mars and Jupiter, is estimated to be , ≈ 3.25% of the mass of the Moon. Of this, Ceres comprises , about 40% of the total. Adding in the next three most massive objects, Vesta (11%), Pallas (8.5%), and Hygiea (3–4%), brings this figure up to a bit over 60%, whereas the next seven most-massive asteroids bring the total up to 70%. The number of asteroids increases rapidly as their individual masses decrease.
The number of asteroids decreases markedly with increasing size. Although the size distribution generally follows a power law, there are 'bumps' at about and , where more asteroids than expected from such a curve are found. Most asteroids larger than approximately 120 km in diameter are primordial (surviving from the accretion epoch), whereas most smaller asteroids are products of fragmentation of primordial asteroids. The primordial population of the main belt was probably 200 times what it is today.
Largest asteroids
Three largest objects in the asteroid belt, Ceres, Vesta, and Pallas, are intact protoplanets that share many characteristics common to planets, and are atypical compared to the majority of irregularly shaped asteroids. The fourth-largest asteroid, Hygiea, appears nearly spherical although it may have an undifferentiated interior, like the majority of asteroids. The four largest asteroids constitute half the mass of the asteroid belt.
Ceres is the only asteroid that appears to have a plastic shape under its own gravity and hence the only one that is a dwarf planet. It has a much higher absolute magnitude than the other asteroids, of around 3.32, and may possess a surface layer of ice. Like the planets, Ceres is differentiated: it has a crust, a mantle and a core. No meteorites from Ceres have been found on Earth. | Asteroid | Wikipedia | 403 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Vesta, too, has a differentiated interior, though it formed inside the Solar System's frost line, and so is devoid of water; its composition is mainly of basaltic rock with minerals such as olivine. Aside from the large crater at its southern pole, Rheasilvia, Vesta also has an ellipsoidal shape. Vesta is the parent body of the Vestian family and other V-type asteroids, and is the source of the HED meteorites, which constitute 5% of all meteorites on Earth.
Pallas is unusual in that, like Uranus, it rotates on its side, with its axis of rotation tilted at high angles to its orbital plane. Its composition is similar to that of Ceres: high in carbon and silicon, and perhaps partially differentiated. Pallas is the parent body of the Palladian family of asteroids.
Hygiea is the largest carbonaceous asteroid and, unlike the other largest asteroids, lies relatively close to the plane of the ecliptic. It is the largest member and presumed parent body of the Hygiean family of asteroids. Because there is no sufficiently large crater on the surface to be the source of that family, as there is on Vesta, it is thought that Hygiea may have been completely disrupted in the collision that formed the Hygiean family and recoalesced after losing a bit less than 2% of its mass. Observations taken with the Very Large Telescope's SPHERE imager in 2017 and 2018, revealed that Hygiea has a nearly spherical shape, which is consistent both with it being in hydrostatic equilibrium, or formerly being in hydrostatic equilibrium, or with being disrupted and recoalescing.
Internal differentiation of large asteroids is possibly related to their lack of natural satellites, as satellites of main belt asteroids are mostly believed to form from collisional disruption, creating a rubble pile structure.
Rotation
Measurements of the rotation rates of large asteroids in the asteroid belt show that there is an upper limit. Very few asteroids with a diameter larger than 100 meters have a rotation period less than 2.2 hours. For asteroids rotating faster than approximately this rate, the inertial force at the surface is greater than the gravitational force, so any loose surface material would be flung out. However, a solid object should be able to rotate much more rapidly. This suggests that most asteroids with a diameter over 100 meters are rubble piles formed through the accumulation of debris after collisions between asteroids. | Asteroid | Wikipedia | 504 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Color
Asteroids become darker and redder with age due to space weathering. However evidence suggests most of the color change occurs rapidly, in the first hundred thousand years, limiting the usefulness of spectral measurement for determining the age of asteroids.
Surface features
Except for the "big four" (Ceres, Pallas, Vesta, and Hygiea), asteroids are likely to be broadly similar in appearance, if irregular in shape. 253 Mathilde is a rubble pile saturated with craters with diameters the size of the asteroid's radius. Earth-based observations of 511 Davida, one of the largest asteroids after the big four, reveal a similarly angular profile, suggesting it is also saturated with radius-size craters. Medium-sized asteroids such as Mathilde and 243 Ida, that have been observed up close, also reveal a deep regolith covering the surface. Of the big four, Pallas and Hygiea are practically unknown. Vesta has compression fractures encircling a radius-size crater at its south pole but is otherwise a spheroid.
Dawn spacecraft revealed that Ceres has a heavily cratered surface, but with fewer large craters than expected. Models based on the formation of the current asteroid belt had suggested Ceres should possess 10 to 15 craters larger than in diameter. The largest confirmed crater on Ceres, Kerwan Basin, is across. The most likely reason for this is viscous relaxation of the crust slowly flattening out larger impacts. | Asteroid | Wikipedia | 302 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Composition
Asteroids are classified by their characteristic emission spectra, with the majority falling into three main groups: C-type, M-type, and S-type. These describe carbonaceous (carbon-rich), metallic, and silicaceous (stony) compositions, respectively. The physical composition of asteroids is varied and in most cases poorly understood. Ceres appears to be composed of a rocky core covered by an icy mantle; Vesta is thought to have a nickel-iron core, olivine mantle, and basaltic crust. Thought to be the largest undifferentiated asteroid, 10 Hygiea seems to have a uniformly primitive composition of carbonaceous chondrite, but it may actually be a differentiated asteroid that was globally disrupted by an impact and then reassembled. Other asteroids appear to be the remnant cores or mantles of proto-planets, high in rock and metal. Most small asteroids are believed to be piles of rubble held together loosely by gravity, although the largest are probably solid. Some asteroids have moons or are co-orbiting binaries: rubble piles, moons, binaries, and scattered asteroid families are thought to be the results of collisions that disrupted a parent asteroid, or possibly a planet.
In the main asteroid belt, there appear to be two primary populations of asteroid: a dark, volatile-rich population, consisting of the C-type and P-type asteroids, with albedos less than 0.10 and densities under , and a dense, volatile-poor population, consisting of the S-type and M-type asteroids, with albedos over 0.15 and densities greater than 2.7. Within these populations, larger asteroids are denser, presumably due to compression. There appears to be minimal macro-porosity (interstitial vacuum) in the score of asteroids with masses greater than . | Asteroid | Wikipedia | 376 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Composition is calculated from three primary sources: albedo, surface spectrum, and density. The last can only be determined accurately by observing the orbits of moons the asteroid might have. So far, every asteroid with moons has turned out to be a rubble pile, a loose conglomeration of rock and metal that may be half empty space by volume. The investigated asteroids are as large as 280 km in diameter, and include 121 Hermione (268×186×183 km), and 87 Sylvia (384×262×232 km). Few asteroids are larger than 87 Sylvia, none of them have moons. The fact that such large asteroids as Sylvia may be rubble piles, presumably due to disruptive impacts, has important consequences for the formation of the Solar System: computer simulations of collisions involving solid bodies show them destroying each other as often as merging, but colliding rubble piles are more likely to merge. This means that the cores of the planets could have formed relatively quickly.
Water
Scientists hypothesize that some of the first water brought to Earth was delivered by asteroid impacts after the collision that produced the Moon. In 2009, the presence of water ice was confirmed on the surface of 24 Themis using NASA's Infrared Telescope Facility. The surface of the asteroid appears completely covered in ice. As this ice layer is sublimating, it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface. The presence of ice on 24 Themis makes the initial theory plausible.
In October 2013, water was detected on an extrasolar body for the first time, on an asteroid orbiting the white dwarf GD 61. On 22 January 2014, European Space Agency (ESA) scientists reported the detection, for the first definitive time, of water vapor on Ceres, the largest object in the asteroid belt. The detection was made by using the far-infrared abilities of the Herschel Space Observatory. The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids."
Findings have shown that solar winds can react with the oxygen in the upper layer of the asteroids and create water. It has been estimated that "every cubic metre of irradiated rock could contain up to 20 litres"; study was conducted using an atom probe tomography, numbers are given for the Itokawa S-type asteroid. | Asteroid | Wikipedia | 509 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Acfer 049, a meteorite discovered in Algeria in 1990, was shown in 2019 to have an ultraporous lithology (UPL): porous texture that could be formed by removal of ice that filled these pores, this suggests that UPL "represent fossils of primordial ice".
Organic compounds
Asteroids contain traces of amino acids and other organic compounds, and some speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth (an event called "panspermia"). In August 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine and related organic molecules) may have been formed on asteroids and comets in outer space.
In November 2019, scientists reported detecting, for the first time, sugar molecules, including ribose, in meteorites, suggesting that chemical processes on asteroids can produce some fundamentally essential bio-ingredients important to life, and supporting the notion of an RNA world prior to a DNA-based origin of life on Earth, and possibly, as well, the notion of panspermia.
Classification
Asteroids are commonly categorized according to two criteria: the characteristics of their orbits, and features of their reflectance spectrum.
Orbital classification
Many asteroids have been placed in groups and families based on their orbital characteristics. Apart from the broadest divisions, it is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past. Families are more common and easier to identify within the main asteroid belt, but several small families have been reported among the Jupiter trojans. Main belt families were first recognized by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor.
About 30–35% of the bodies in the asteroid belt belong to dynamical families, each thought to have a common origin in a past collision between asteroids. A family has also been associated with the plutoid dwarf planet . | Asteroid | Wikipedia | 440 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Some asteroids have unusual horseshoe orbits that are co-orbital with Earth or another planet. Examples are 3753 Cruithne and . The first instance of this type of orbital arrangement was discovered between Saturn's moons Epimetheus and Janus. Sometimes these horseshoe objects temporarily become quasi-satellites for a few decades or a few hundred years, before returning to their earlier status. Both Earth and Venus are known to have quasi-satellites.
Such objects, if associated with Earth or Venus or even hypothetically Mercury, are a special class of Aten asteroids. However, such objects could be associated with the outer planets as well.
Spectral classification
In 1975, an asteroid taxonomic system based on color, albedo, and spectral shape was developed by Chapman, Morrison, and Zellner. These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories: C-types for dark carbonaceous objects (75% of known asteroids), S-types for stony (silicaceous) objects (17% of known asteroids) and U for those that did not fit into either C or S. This classification has since been expanded to include many other asteroid types. The number of types continues to grow as more asteroids are studied.
The two most widely used taxonomies now used are the Tholen classification and SMASS classification. The former was proposed in 1984 by David J. Tholen, and was based on data collected from an eight-color asteroid survey performed in the 1980s. This resulted in 14 asteroid categories. In 2002, the Small Main-Belt Asteroid Spectroscopic Survey resulted in a modified version of the Tholen taxonomy with 24 different types. Both systems have three broad categories of C, S, and X asteroids, where X consists of mostly metallic asteroids, such as the M-type. There are also several smaller classes.
The proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.
Problems
Originally, spectral designations were based on inferences of an asteroid's composition. However, the correspondence between spectral class and composition is not always very good, and a variety of classifications are in use. This has led to significant confusion. Although asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of the same (or similar) materials.
Active asteroids | Asteroid | Wikipedia | 509 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Active asteroids are objects that have asteroid-like orbits but show comet-like visual characteristics. That is, they show comae, tails, or other visual evidence of mass-loss (like a comet), but their orbit remains within Jupiter's orbit (like an asteroid). These bodies were originally designated main-belt comets (MBCs) in 2006 by astronomers David Jewitt and Henry Hsieh, but this name implies they are necessarily icy in composition like a comet and that they only exist within the main-belt, whereas the growing population of active asteroids shows that this is not always the case.
The first active asteroid discovered is 7968 Elst–Pizarro. It was discovered (as an asteroid) in 1979 but then was found to have a tail by Eric Elst and Guido Pizarro in 1996 and given the cometary designation 133P/Elst-Pizarro. Another notable object is 311P/PanSTARRS: observations made by the Hubble Space Telescope revealed that it had six comet-like tails. The tails are suspected to be streams of material ejected by the asteroid as a result of a rubble pile asteroid spinning fast enough to remove material from it.
By smashing into the asteroid Dimorphos, NASA's Double Asteroid Redirection Test spacecraft made it an active asteroid. Scientists had proposed that some active asteroids are the result of impact events, but no one had ever observed the activation of an asteroid. The DART mission activated Dimorphos under precisely known and carefully observed impact conditions, enabling the detailed study of the formation of an active asteroid for the first time. Observations show that Dimorphos lost approximately 1 million kilograms after the collision. Impact produced a dust plume that temporarily brightened the Didymos system and developed a -long dust tail that persisted for several months. | Asteroid | Wikipedia | 370 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Observation and exploration
Until the age of space travel, objects in the asteroid belt could only be observed with large telescopes, their shapes and terrain remaining a mystery. The best modern ground-based telescopes and the Earth-orbiting Hubble Space Telescope can only resolve a small amount of detail on the surfaces of the largest asteroids. Limited information about the shapes and compositions of asteroids can be inferred from their light curves (variation in brightness during rotation) and their spectral properties. Sizes can be estimated by timing the lengths of star occultations (when an asteroid passes directly in front of a star). Radar imaging can yield good information about asteroid shapes and orbital and rotational parameters, especially for near-Earth asteroids. Spacecraft flybys can provide much more data than any ground or space-based observations; sample-return missions gives insights about regolith composition.
Ground-based observations
As asteroids are rather small and faint objects, the data that can be obtained from ground-based observations (GBO) are limited. By means of ground-based optical telescopes the visual magnitude can be obtained; when converted into the absolute magnitude it gives a rough estimate of the asteroid's size. Light-curve measurements can also be made by GBO; when collected over a long period of time it allows an estimate of the rotational period, the pole orientation (sometimes), and a rough estimate of the asteroid's shape. Spectral data (both visible-light and near-infrared spectroscopy) gives information about the object's composition, used to classify the observed asteroids. Such observations are limited as they provide information about only the thin layer on the surface (up to several micrometers). As planetologist Patrick Michel writes: | Asteroid | Wikipedia | 342 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Mid- to thermal-infrared observations, along with polarimetry measurements, are probably the only data that give some indication of actual physical properties. Measuring the heat flux of an asteroid at a single wavelength gives an estimate of the dimensions of the object; these measurements have lower uncertainty than measurements of the reflected sunlight in the visible-light spectral region. If the two measurements can be combined, both the effective diameter and the geometric albedo—the latter being a measure of the brightness at zero phase angle, that is, when illumination comes from directly behind the observer—can be derived. In addition, thermal measurements at two or more wavelengths, plus the brightness in the visible-light region, give information on the thermal properties. The thermal inertia, which is a measure of how fast a material heats up or cools off, of most observed asteroids is lower than the bare-rock reference value but greater than that of the lunar regolith; this observation indicates the presence of an insulating layer of granular material on their surface. Moreover, there seems to be a trend, perhaps related to the gravitational environment, that smaller objects (with lower gravity) have a small regolith layer consisting of coarse grains, while larger objects have a thicker regolith layer consisting of fine grains. However, the detailed properties of this regolith layer are poorly known from remote observations. Moreover, the relation between thermal inertia and surface roughness is not straightforward, so one needs to interpret the thermal inertia with caution.
Near-Earth asteroids that come into close vicinity of the planet can be studied in more details with radar; it provides information about the surface of the asteroid (for example can show the presence of craters and boulders). Such observations were conducted by the Arecibo Observatory in Puerto Rico (305 meter dish) and Goldstone Observatory in California (70 meter dish). Radar observations can also be used for accurate determination of the orbital and rotational dynamics of observed objects.
Space-based observations | Asteroid | Wikipedia | 403 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Both space and ground-based observatories conducted asteroid search programs; the space-based searches are expected to detect more objects because there is no atmosphere to interfere and because they can observe larger portions of the sky. NEOWISE observed more than 100,000 asteroids of the main belt, Spitzer Space Telescope observed more than 700 near-Earth asteroids. These observations determined rough sizes of the majority of observed objects, but provided limited detail about surface properties (such as regolith depth and composition, angle of repose, cohesion, and porosity).
Asteroids were also studied by the Hubble Space Telescope, such as tracking the colliding asteroids in the main belt, break-up of an asteroid, observing an active asteroid with six comet-like tails, and observing asteroids that were chosen as targets of dedicated missions.
Space probe missions
According to Patrick Michel
The internal structure of asteroids is inferred only from indirect evidence: bulk densities measured by spacecraft, the orbits of natural satellites in the case of asteroid binaries, and the drift of an asteroid's orbit due to the Yarkovsky thermal effect. A spacecraft near an asteroid is perturbed enough by the asteroid's gravity to allow an estimate of the asteroid's mass. The volume is then estimated using a model of the asteroid's shape. Mass and volume allow the derivation of the bulk density, whose uncertainty is usually dominated by the errors made on the volume estimate. The internal porosity of asteroids can be inferred by comparing their bulk density with that of their assumed meteorite analogues, dark asteroids seem to be more porous (>40%) than bright ones. The nature of this porosity is unclear.
Dedicated missions
The first asteroid to be photographed in close-up was 951 Gaspra in 1991, followed in 1993 by 243 Ida and its moon Dactyl, all of which were imaged by the Galileo probe en route to Jupiter. Other asteroids briefly visited by spacecraft en route to other destinations include 9969 Braille (by Deep Space 1 in 1999), 5535 Annefrank (by Stardust in 2002), 2867 Šteins and 21 Lutetia (by the Rosetta probe in 2008), and 4179 Toutatis (China's lunar orbiter Chang'e 2, which flew within in 2012). | Asteroid | Wikipedia | 476 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
The first dedicated asteroid probe was NASA's NEAR Shoemaker, which photographed 253 Mathilde in 1997, before entering into orbit around 433 Eros, finally landing on its surface in 2001. It was the first spacecraft to successfully orbit and land on an asteroid. From September to November 2005, the Japanese Hayabusa probe studied 25143 Itokawa in detail and returned samples of its surface to Earth on 13 June 2010, the first asteroid sample-return mission. In 2007, NASA launched the Dawn spacecraft, which orbited 4 Vesta for a year, and observed the dwarf planet Ceres for three years.
Hayabusa2, a probe launched by JAXA 2014, orbited its target asteroid 162173 Ryugu for more than a year and took samples that were delivered to Earth in 2020. The spacecraft is now on an extended mission and expected to arrive at a new target in 2031.
NASA launched the OSIRIS-REx in 2016, a sample return mission to asteroid 101955 Bennu. In 2021, the probe departed the asteroid with a sample from its surface. Sample was delivered to Earth in September 2023. The spacecraft continues its extended mission, designated OSIRIS-APEX, to explore near-Earth asteroid Apophis in 2029.
In 2021, NASA launched Double Asteroid Redirection Test (DART), a mission to test technology for defending Earth against potential hazardous objects. DART deliberately crashed into the minor-planet moon Dimorphos of the double asteroid Didymos in September 2022 to assess the potential of a spacecraft impact to deflect an asteroid from a collision course with Earth. In October, NASA declared DART a success, confirming it had shortened Dimorphos' orbital period around Didymos by about 32 minutes.
NASA's Lucy, launched in 2021, is a multiple-asteroid flyby probe focused on flying by 7 Jupiter trojans of varying types. While not yet set to reach its first main target, 3548 Eurybates, until 2027, it has made a flyby of main-belt asteroid 152830 Dinkinesh and is set to flyby another asteroid 52246 Donaldjohanson in 2025.
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NASA's Psyche, launched in October 2023, is intended to study the large metallic asteroid of the same name, and is on track to arrive there in 2029.
ESA's Hera, launched in October 2024, is intended study the results of the DART impact. It is expected to measure the size and morphology of the crater, and momentum transmitted by the impact, to determine the efficiency of the deflection produced by DART.
JAXA's DESTINY+ is a mission for a flyby of the Geminids meteor shower parent body 3200 Phaethon, as well as various minor bodies. Its launch is planned for 2024.
CNSA's Tianwen-2 is planned to launch in 2025. If all goes as planned, it will use solar electric propulsion to explore the co-orbital near-Earth asteroid 469219 Kamoʻoalewa and the active asteroid 311P/PanSTARRS. The spacecraft is tasked with collecting samples of the regolith of Kamo'oalewa.
Asteroid mining
The concept of asteroid mining was proposed in 1970s. Matt Anderson defines successful asteroid mining as "the development of a mining program that is both financially self-sustaining and profitable to its investors". It has been suggested that asteroids might be used as a source of materials that may be rare or exhausted on Earth, or materials for constructing space habitats. Materials that are heavy and expensive to launch from Earth may someday be mined from asteroids and used for space manufacturing and construction.
As resource depletion on Earth becomes more real, the idea of extracting valuable elements from asteroids and returning these to Earth for profit, or using space-based resources to build solar-power satellites and space habitats, becomes more attractive. Hypothetically, water processed from ice could refuel orbiting propellant depots.
From the astrobiological perspective, asteroid prospecting could provide scientific data for the search for extraterrestrial intelligence (SETI). Some astrophysicists have suggested that if advanced extraterrestrial civilizations employed asteroid mining long ago, the hallmarks of these activities might be detectable.
Threats to Earth
There is increasing interest in identifying asteroids whose orbits cross Earth's, and that could, given enough time, collide with Earth. The three most important groups of near-Earth asteroids are the Apollos, Amors, and Atens. | Asteroid | Wikipedia | 482 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
The near-Earth asteroid 433 Eros had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: 1221 Amor, 1862 Apollo, 2101 Adonis, and finally 69230 Hermes, which approached within 0.005 AU of Earth in 1937. Astronomers began to realize the possibilities of Earth impact.
Two events in later decades increased the alarm: the increasing acceptance of the Alvarez hypothesis that an impact event resulted in the Cretaceous–Paleogene extinction, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into Jupiter. The U.S. military also declassified the information that its military satellites, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to ten meters across.
All of these considerations helped spur the launch of highly efficient surveys, consisting of charge-coupled device (CCD) cameras and computers directly connected to telescopes. , it was estimated that 89% to 96% of near-Earth asteroids one kilometer or larger in diameter had been discovered. , the LINEAR system alone had discovered 147,132 asteroids. Among the surveys, 19,266 near-Earth asteroids have been discovered including almost 900 more than in diameter.
In June 2018, the National Science and Technology Council warned that the United States is unprepared for an asteroid impact event, and has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched.
Asteroid deflection strategies
Various collision avoidance techniques have different trade-offs with respect to metrics such as overall performance, cost, failure risks, operations, and technology readiness. There are various methods for changing the course of an asteroid/comet. These can be differentiated by various types of attributes such as the type of mitigation (deflection or fragmentation), energy source (kinetic, electromagnetic, gravitational, solar/thermal, or nuclear), and approach strategy (interception, rendezvous, or remote station). | Asteroid | Wikipedia | 438 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Strategies fall into two basic sets: fragmentation and delay. Fragmentation concentrates on rendering the impactor harmless by fragmenting it and scattering the fragments so that they miss the Earth or are small enough to burn up in the atmosphere. Delay exploits the fact that both the Earth and the impactor are in orbit. An impact occurs when both reach the same point in space at the same time, or more correctly when some point on Earth's surface intersects the impactor's orbit when the impactor arrives. Since the Earth is approximately 12,750 km in diameter and moves at approx. 30 km per second in its orbit, it travels a distance of one planetary diameter in about 425 seconds, or slightly over seven minutes. Delaying, or advancing the impactor's arrival by times of this magnitude can, depending on the exact geometry of the impact, cause it to miss the Earth.
"Project Icarus" was one of the first projects designed in 1967 as a contingency plan in case of collision with 1566 Icarus. The plan relied on the new Saturn V rocket, which did not make its first flight until after the report had been completed. Six Saturn V rockets would be used, each launched at variable intervals from months to hours away from impact. Each rocket was to be fitted with a single 100-megaton nuclear warhead as well as a modified Apollo Service Module and uncrewed Apollo Command Module for guidance to the target. The warheads would be detonated 30 meters from the surface, deflecting or partially destroying the asteroid. Depending on the subsequent impacts on the course or the destruction of the asteroid, later missions would be modified or cancelled as needed. The "last-ditch" launch of the sixth rocket would be 18 hours prior to impact.
Fiction
Asteroids and the asteroid belt are a staple of science fiction stories. Asteroids play several potential roles in science fiction: as places human beings might colonize, resources for extracting minerals, hazards encountered by spacecraft traveling between two other points, and as a threat to life on Earth or other inhabited planets, dwarf planets, and natural satellites by potential impact. | Asteroid | Wikipedia | 425 | 791 | https://en.wikipedia.org/wiki/Asteroid | Physical sciences | Astronomy | null |
Aries is one of the constellations of the zodiac. It is located in the Northern celestial hemisphere between Pisces to the west and Taurus to the east. The name Aries is Latin for ram. Its old astronomical symbol is (♈︎). It is one of the 48 constellations described by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations. It is a mid-sized constellation ranking 39th in overall size, with an area of 441 square degrees (1.1% of the celestial sphere).
Aries has represented a ram since late Babylonian times. Before that, the stars of Aries formed a farmhand. Different cultures have incorporated the stars of Aries into different constellations including twin inspectors in China and a porpoise in the Marshall Islands. Aries is a relatively dim constellation, possessing only four bright stars: Hamal (Alpha Arietis, second magnitude), Sheratan (Beta Arietis, third magnitude), Mesarthim (Gamma Arietis, fourth magnitude), and 41 Arietis (also fourth magnitude). The few deep-sky objects within the constellation are quite faint and include several pairs of interacting galaxies. Several meteor showers appear to radiate from Aries, including the Daytime Arietids and the Epsilon Arietids.
History and mythology | Aries (constellation) | Wikipedia | 272 | 798 | https://en.wikipedia.org/wiki/Aries%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
Aries is now recognized as an official constellation, albeit as a specific region of the sky, by the International Astronomical Union. It was originally defined in ancient texts as a specific pattern of stars, and has remained a constellation since ancient times; it now includes the ancient pattern and the surrounding stars. In the description of the Babylonian zodiac given in the clay tablets known as the MUL.APIN, the constellation, now known as Aries, was the final station along the ecliptic. The MUL.APIN was a comprehensive table of the rising and settings of stars, which likely served as an agricultural calendar. Modern-day Aries was known as , "The Agrarian Worker" or "The Hired Man". Although likely compiled in the 12th or 11th century BC, the MUL.APIN reflects a tradition that marks the Pleiades as the vernal equinox, which was the case with some precision at the beginning of the Middle Bronze Age. The earliest identifiable reference to Aries as a distinct constellation comes from the boundary stones that date from 1350 to 1000 BC. On several boundary stones, a zodiacal ram figure is distinct from the other characters. The shift in identification from the constellation as the Agrarian Worker to the Ram likely occurred in later Babylonian tradition because of its growing association with Dumuzi the Shepherd. By the time the MUL.APIN was created—in 1000 BC—modern Aries was identified with both Dumuzi's ram and a hired labourer. The exact timing of this shift is difficult to determine due to the lack of images of Aries or other ram figures.
In ancient Egyptian astronomy, Aries was associated with the god Amun-Ra, who was depicted as a man with a ram's head and represented fertility and creativity. Because it was the location of the vernal equinox, it was called the "Indicator of the Reborn Sun". During the times of the year when Aries was prominent, priests would process statues of Amon-Ra to temples, a practice that was modified by Persian astronomers centuries later. Aries acquired the title of "Lord of the Head" in Egypt, referring to its symbolic and mythological importance. | Aries (constellation) | Wikipedia | 449 | 798 | https://en.wikipedia.org/wiki/Aries%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
Aries was not fully accepted as a constellation until classical times. In Hellenistic astrology, the constellation of Aries is associated with the golden ram of Greek mythology that rescued Phrixus and Helle on orders from Hermes, taking Phrixus to the land of Colchis. Phrixus and Helle were the son and daughter of King Athamas and his first wife Nephele. The king's second wife, Ino, was jealous and wished to kill his children. To accomplish this, she induced famine in Boeotia, then falsified a message from the Oracle of Delphi that said Phrixus must be sacrificed to end the famine. Athamas was about to sacrifice his son atop Mount Laphystium when Aries, sent by Nephele, arrived. Helle fell off of Aries's back in flight and drowned in the Dardanelles, also called the Hellespont in her honour.
Historically, Aries has been depicted as a crouched, wingless ram with its head turned towards Taurus. Ptolemy asserted in his Almagest that Hipparchus depicted Alpha Arietis as the ram's muzzle, though Ptolemy did not include it in his constellation figure. Instead, it was listed as an "unformed star", and denoted as "the star over the head". John Flamsteed, in his Atlas Coelestis, followed Ptolemy's description by mapping it above the figure's head. Flamsteed followed the general convention of maps by depicting Aries lying down. Astrologically, Aries has been associated with the head and its humors. It was strongly associated with Mars, both the planet and the god. It was considered to govern Western Europe and Syria and to indicate a strong temper in a person.
The First Point of Aries, the location of the vernal equinox, is named for the constellation. This is because the Sun crossed the celestial equator from south to north in Aries more than two millennia ago. Hipparchus defined it in 130 BC. as a point south of Gamma Arietis. Because of the precession of the equinoxes, the First Point of Aries has since moved into Pisces and will move into Aquarius by around 2600 AD. The Sun now appears in Aries from late April through mid-May, though the constellation is still associated with the beginning of spring. | Aries (constellation) | Wikipedia | 500 | 798 | https://en.wikipedia.org/wiki/Aries%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
Medieval Muslim astronomers depicted Aries in various ways. Astronomers like al-Sufi saw the constellation as a ram, modelled on the precedent of Ptolemy. However, some Islamic celestial globes depicted Aries as a nondescript four-legged animal with what may be antlers instead of horns. Some early Bedouin observers saw a ram elsewhere in the sky; this constellation featured the Pleiades as the ram's tail. The generally accepted Arabic formation of Aries consisted of thirteen stars in a figure along with five "unformed" stars, four of which were over the animal's hindquarters and one of which was the disputed star over Aries's head. Al-Sufi's depiction differed from both other Arab astronomers' and Flamsteed's, in that his Aries was running and looking behind itself.
The obsolete constellations Apes, Vespa, Lilium, and Musca Borealis all centred on the same four stars, now known as 33, 35, 39, and 41 Arietis. In 1612, Petrus Plancius introduced Apes, a constellation representing a bee. In 1624, the same stars were used by Jakob Bartsch for Vespa, representing a wasp. In 1679, Augustin Royer used these stars for his constellation Lilium, representing the fleur-de-lis. None of these constellations became widely accepted. Johann Hevelius renamed the constellation "Musca" in 1690 in his Firmamentum Sobiescianum. To differentiate it from Musca, the southern fly, it was later renamed Musca Borealis but it did not gain acceptance and its stars were ultimately officially reabsorbed into Aries.
In 1922, the International Astronomical Union defined its recommended three-letter abbreviation, "Ari". The official boundaries of Aries were defined in 1930 by Eugène Delporte as a polygon of 12 segments. Its right ascension is between 1h 46.4m and 3h 29.4m and its declination is between 10.36° and 31.22° in the equatorial coordinate system. | Aries (constellation) | Wikipedia | 427 | 798 | https://en.wikipedia.org/wiki/Aries%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
In non-Western astronomy
In traditional Chinese astronomy, stars from Aries were used in several constellations. The brightest stars—Alpha, Beta, and Gamma Arietis—formed a constellation called 'Lou',variously translated as "bond" or "lasso" also "sickle", which was associated with the ritual sacrifice of cattle. This name was shared by the 16th lunar mansion, the location of the full moon closest to the autumnal equinox. This constellation has also been associated with harvest-time as it could represent a woman carrying a basket of food on her head. 35, 39, and 41 Arietis were part of a constellation called Wei (胃), which represented a fat abdomen and was the namesake of the 17th lunar mansion, which represented granaries. Delta and Zeta Arietis were a part of the constellation Tianyin (天陰), thought to represent the Emperor's hunting partner. Zuogeng (左更), a constellation depicting a marsh and pond inspector, was composed of Mu, Nu, Omicron, Pi, and Sigma Arietis. He was accompanied by Yeou-kang, a constellation depicting an official in charge of pasture distribution. | Aries (constellation) | Wikipedia | 245 | 798 | https://en.wikipedia.org/wiki/Aries%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
In a similar system to the Chinese, the first lunar mansion in Hindu astronomy was called "Aswini", after the traditional names for Beta and Gamma Arietis, the Aswins. Because the Hindu new year began with the vernal equinox, the Rig Veda contains over 50 new-year's related hymns to the twins, making them some of the most prominent characters in the work. Aries itself was known as "Aja" and "Mesha". In Hebrew astronomy Aries was named "Taleh"; it signified either Simeon or Gad, and generally symbolizes the "Lamb of the World". The neighboring Syrians named the constellation "Amru", and the bordering Turks named it "Kuzi". Half a world away, in the Marshall Islands, several stars from Aries were incorporated into a constellation depicting a porpoise, along with stars from Cassiopeia, Andromeda, and Triangulum. Alpha, Beta, and Gamma Arietis formed the head of the porpoise, while stars from Andromeda formed the body and the bright stars of Cassiopeia formed the tail. Other Polynesian peoples recognized Aries as a constellation. The Marquesas islanders called it Na-pai-ka; the Māori constellation Pipiri may correspond to modern Aries as well. In indigenous Peruvian astronomy, a constellation with most of the same stars as Aries existed. It was called the "Market Moon" and the "Kneeling Terrace", as a reminder of when to hold the annual harvest festival, Ayri Huay.
Features
Stars
Bright stars
Aries has three prominent stars forming an asterism, designated Alpha, Beta, and Gamma Arietis by Johann Bayer. Alpha (Hamal) and Beta (Sheratan) are commonly used for navigation. There is also one other star above the fourth magnitude, 41 Arietis (Bharani). α Arietis, called Hamal, is the brightest star in Aries. Its traditional name is derived from the Arabic word for "lamb" or "head of the ram" (ras al-hamal), which references Aries's mythological background. With a spectral class of K2 and a luminosity class of III, it is an orange giant with an apparent visual magnitude of 2.00, which lies 66 light-years from Earth. Hamal has a luminosity of and its absolute magnitude is −0.1. | Aries (constellation) | Wikipedia | 512 | 798 | https://en.wikipedia.org/wiki/Aries%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
β Arietis, also known as Sheratan, is a blue-white star with an apparent visual magnitude of 2.64. Its traditional name is derived from "sharatayn", the Arabic word for "the two signs", referring to both Beta and Gamma Arietis in their position as heralds of the vernal equinox. The two stars were known to the Bedouin as "qarna al-hamal", "horns of the ram". It is 59 light-years from Earth. It has a luminosity of and its absolute magnitude is 2.1. It is a spectroscopic binary star, one in which the companion star is only known through analysis of the spectra. The spectral class of the primary is A5. Hermann Carl Vogel determined that Sheratan was a spectroscopic binary in 1903; its orbit was determined by Hans Ludendorff in 1907. It has since been studied for its eccentric orbit. | Aries (constellation) | Wikipedia | 199 | 798 | https://en.wikipedia.org/wiki/Aries%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
γ Arietis, with a common name of Mesarthim, is a binary star with two white-hued components, located in a rich field of magnitude 8–12 stars. Its traditional name has conflicting derivations. It may be derived from a corruption of "al-sharatan", the Arabic word meaning "pair" or a word for "fat ram". However, it may also come from the Sanskrit for "first star of Aries" or the Hebrew for "ministerial servants", both of which are unusual languages of origin for star names. Along with Beta Arietis, it was known to the Bedouin as "qarna al-hamal". The primary is of magnitude 4.59 and the secondary is of magnitude 4.68. The system is 164 light-years from Earth. The two components are separated by 7.8 arcseconds, and the system as a whole has an apparent magnitude of 3.9. The primary has a luminosity of and the secondary has a luminosity of ; the primary is an A-type star with an absolute magnitude of 0.2 and the secondary is a B9-type star with an absolute magnitude of 0.4. The angle between the two components is 1°. Mesarthim was discovered to be a double star by Robert Hooke in 1664, one of the earliest such telescopic discoveries. The primary, γ1 Arietis, is an Alpha² Canum Venaticorum variable star that has a range of 0.02 magnitudes and a period of 2.607 days. It is unusual because of its strong silicon emission lines. | Aries (constellation) | Wikipedia | 340 | 798 | https://en.wikipedia.org/wiki/Aries%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
The constellation is home to several double stars, including Epsilon, Lambda, and Pi Arietis. ε Arietis is a binary star with two white components. The primary is of magnitude 5.2 and the secondary is of magnitude 5.5. The system is 290 light-years from Earth. Its overall magnitude is 4.63, and the primary has an absolute magnitude of 1.4. Its spectral class is A2. The two components are separated by 1.5 arcseconds. λ Arietis is a wide double star with a white-hued primary and a yellow-hued secondary. The primary is of magnitude 4.8 and the secondary is of magnitude 7.3. The primary is 129 light-years from Earth. It has an absolute magnitude of 1.7 and a spectral class of F0. The two components are separated by 36 arcseconds at an angle of 50°; the two stars are located 0.5° east of 7 Arietis. π Arietis is a close binary star with a blue-white primary and a white secondary. The primary is of magnitude 5.3 and the secondary is of magnitude 8.5. The primary is 776 light-years from Earth. The primary itself is a wide double star with a separation of 25.2 arcseconds; the tertiary has a magnitude of 10.8. The primary and secondary are separated by 3.2 arcseconds. | Aries (constellation) | Wikipedia | 298 | 798 | https://en.wikipedia.org/wiki/Aries%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
Most of the other stars in Aries visible to the naked eye have magnitudes between 3 and 5. δ Ari, called Boteïn, is a star of magnitude 4.35, 170 light-years away. It has an absolute magnitude of −0.1 and a spectral class of K2. ζ Arietis is a star of magnitude 4.89, 263 light-years away. Its spectral class is A0 and its absolute magnitude is 0.0. 14 Arietis is a star of magnitude 4.98, 288 light-years away. Its spectral class is F2 and its absolute magnitude is 0.6. 39 Arietis (Lilii Borea) is a similar star of magnitude 4.51, 172 light-years away. Its spectral class is K1 and its absolute magnitude is 0.0. 35 Arietis is a dim star of magnitude 4.55, 343 light-years away. Its spectral class is B3 and its absolute magnitude is −1.7. 41 Arietis, known both as c Arietis and Nair al Butain, is a brighter star of magnitude 3.63, 165 light-years away. Its spectral class is B8 and it has a luminosity of . Its absolute magnitude is −0.2. 53 Arietis is a runaway star of magnitude 6.09, 815 light-years away. Its spectral class is B2. It was likely ejected from the Orion Nebula approximately five million years ago, possibly due to supernovae. Finally, Teegarden's Star is the closest star to Earth in Aries. It is a red dwarf of magnitude 15.14 and spectral class M6.5V. With a proper motion of 5.1 arcseconds per year, it is the 24th closest star to Earth overall. | Aries (constellation) | Wikipedia | 375 | 798 | https://en.wikipedia.org/wiki/Aries%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
Variable stars
Aries has its share of variable stars, including R and U Arietis, Mira-type variable stars, and T Arietis, a semi-regular variable star. R Arietis is a Mira variable star that ranges in magnitude from a minimum of 13.7 to a maximum of 7.4 with a period of 186.8 days. It is 4,080 light-years away. U Arietis is another Mira variable star that ranges in magnitude from a minimum of 15.2 to a maximum of 7.2 with a period of 371.1 days. T Arietis is a semiregular variable star that ranges in magnitude from a minimum of 11.3 to a maximum of 7.5 with a period of 317 days. It is 1,630 light-years away. One particularly interesting variable in Aries is SX Arietis, a rotating variable star considered to be the prototype of its class, helium variable stars. SX Arietis stars have very prominent emission lines of Helium I and Silicon III. They are normally main-sequence B0p—B9p stars, and their variations are not usually visible to the naked eye. Therefore, they are observed photometrically, usually having periods that fit in the course of one night. Similar to α2s, SX Arietis stars have periodic changes in their light and magnetic field, which correspond to the periodic rotation; they differ from the α2 Canum Venaticorum variables in their higher temperature. There are between 39 and 49 SX Arietis variable stars currently known; ten are noted as being "uncertain" in the General Catalog of Variable Stars.
Deep sky objects | Aries (constellation) | Wikipedia | 348 | 798 | https://en.wikipedia.org/wiki/Aries%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
NGC 772 is a spiral galaxy with an integrated magnitude of 10.3, located southeast of β Arietis and 15 arcminutes west of 15 Arietis. It is a relatively bright galaxy and shows obvious nebulosity and ellipticity in an amateur telescope. It is 7.2 by 4.2 arcminutes, meaning that its surface brightness, magnitude 13.6, is significantly lower than its integrated magnitude. NGC 772 is a class SA(s)b galaxy, which means that it is an unbarred spiral galaxy without a ring that possesses a somewhat prominent bulge and spiral arms that are wound somewhat tightly. The main arm, on the northwest side of the galaxy, is home to many star forming regions; this is due to previous gravitational interactions with other galaxies. NGC 772 has a small companion galaxy, NGC 770, that is about 113,000 light-years away from the larger galaxy. The two galaxies together are also classified as Arp 78 in the Arp peculiar galaxy catalog. NGC 772 has a diameter of 240,000 light-years and the system is 114 million light-years from Earth. Another spiral galaxy in Aries is NGC 673, a face-on class SAB(s)c galaxy. It is a weakly barred spiral galaxy with loosely wound arms. It has no ring and a faint bulge and is 2.5 by 1.9 arcminutes. It has two primary arms with fragments located farther from the core. 171,000 light-years in diameter, NGC 673 is 235 million light-years from Earth. | Aries (constellation) | Wikipedia | 326 | 798 | https://en.wikipedia.org/wiki/Aries%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
NGC 678 and NGC 680 are a pair of galaxies in Aries that are only about 200,000 light-years apart. Part of the NGC 691 group of galaxies, both are at a distance of approximately 130 million light-years. NGC 678 is an edge-on spiral galaxy that is 4.5 by 0.8 arcminutes. NGC 680, an elliptical galaxy with an asymmetrical boundary, is the brighter of the two at magnitude 12.9; NGC 678 has a magnitude of 13.35. Both galaxies have bright cores, but NGC 678 is the larger galaxy at a diameter of 171,000 light-years; NGC 680 has a diameter of 72,000 light-years. NGC 678 is further distinguished by its prominent dust lane. NGC 691 itself is a spiral galaxy slightly inclined to our line of sight. It has multiple spiral arms and a bright core. Because it is so diffuse, it has a low surface brightness. It has a diameter of 126,000 light-years and is 124 million light-years away. NGC 877 is the brightest member of an 8-galaxy group that also includes NGC 870, NGC 871, and NGC 876, with a magnitude of 12.53. It is 2.4 by 1.8 arcminutes and is 178 million light-years away with a diameter of 124,000 light-years. Its companion is NGC 876, which is about 103,000 light-years from the core of NGC 877. They are interacting gravitationally, as they are connected by a faint stream of gas and dust. Arp 276 is a different pair of interacting galaxies in Aries, consisting of NGC 935 and IC 1801.
NGC 821 is an E6 elliptical galaxy. It is unusual because it has hints of an early spiral structure, which is normally only found in lenticular and spiral galaxies. NGC 821 is 2.6 by 2.0 arcminutes and has a visual magnitude of 11.3. Its diameter is 61,000 light-years and it is 80 million light-years away. Another unusual galaxy in Aries is Segue 2, a dwarf and satellite galaxy of the Milky Way, recently discovered to be a potential relic of the epoch of reionization. | Aries (constellation) | Wikipedia | 468 | 798 | https://en.wikipedia.org/wiki/Aries%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
Meteor showers
Aries is home to several meteor showers. The Daytime Arietid meteor shower is one of the strongest meteor showers that occurs during the day, lasting from 22 May to 2 July. It is an annual shower associated with the Marsden group of comets that peaks on 7 June with a maximum zenithal hourly rate of 54 meteors. Its parent body may be the asteroid Icarus. The meteors are sometimes visible before dawn, because the radiant is 32 degrees away from the Sun. They usually appear at a rate of 1–2 per hour as "earthgrazers", meteors that last several seconds and often begin at the horizon. Because most of the Daytime Arietids are not visible to the naked eye, they are observed in the radio spectrum. This is possible because of the ionized gas they leave in their wake. Other meteor showers radiate from Aries during the day; these include the Daytime Epsilon Arietids and the Northern and Southern Daytime May Arietids. The Jodrell Bank Observatory discovered the Daytime Arietids in 1947 when James Hey and G. S. Stewart adapted the World War II-era radar systems for meteor observations.
The Delta Arietids are another meteor shower radiating from Aries. Peaking on 9 December with a low peak rate, the shower lasts from 8 December to 14 January, with the highest rates visible from 8 to 14 December. The average Delta Arietid meteor is very slow, with an average velocity of per second. However, this shower sometimes produces bright fireballs. This meteor shower has northern and southern components, both of which are likely associated with 1990 HA, a near-Earth asteroid.
The Autumn Arietids also radiate from Aries. The shower lasts from 7 September to 27 October and peaks on 9 October. Its peak rate is low. The Epsilon Arietids appear from 12 to 23 October. Other meteor showers radiating from Aries include the October Delta Arietids, Daytime Epsilon Arietids, Daytime May Arietids, Sigma Arietids, Nu Arietids, and Beta Arietids. The Sigma Arietids, a class IV meteor shower, are visible from 12 to 19 October, with a maximum zenithal hourly rate of less than two meteors per hour on 19 October. | Aries (constellation) | Wikipedia | 464 | 798 | https://en.wikipedia.org/wiki/Aries%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
Planetary systems
Aries contains several stars with extrasolar planets. HIP 14810, a G5 type star, is orbited by three giant planets (those more than ten times the mass of Earth). HD 12661, like HIP 14810, is a G-type main sequence star, slightly larger than the Sun, with two orbiting planets. One planet is 2.3 times the mass of Jupiter, and the other is 1.57 times the mass of Jupiter. HD 20367 is a G0 type star, approximately the size of the Sun, with one orbiting planet. The planet, discovered in 2002, has a mass 1.07 times that of Jupiter and orbits every 500 days. In 2019, scientists conducting the CARMENES survey at the Calar Alto Observatory announced evidence of two Earth-mass exoplanets orbiting Teegarden's star, located in Aries, within its habitable zone. The star is a small red dwarf with only around a tenth of the mass and radius of the Sun. It has a large radial velocity. | Aries (constellation) | Wikipedia | 215 | 798 | https://en.wikipedia.org/wiki/Aries%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
Aquarius is an equatorial constellation of the zodiac, between Capricornus and Pisces. Its name is Latin for "water-carrier" or "cup-carrier", and its old astronomical symbol is (♒︎), a representation of water. Aquarius is one of the oldest of the recognized constellations along the zodiac (the Sun's apparent path). It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations. It is found in a region often called the Sea due to its profusion of constellations with watery associations such as Cetus the whale, Pisces the fish, and Eridanus the river.
At apparent magnitude 2.9, Beta Aquarii is the brightest star in the constellation.
History and mythology
Aquarius is identified as "The Great One" in the Babylonian star catalogues and represents the god Ea himself, who is commonly depicted holding an overflowing vase. The Babylonian star-figure appears on entitlement stones and cylinder seals from the second millennium. It contained the winter solstice in the Early Bronze Age. In Old Babylonian astronomy, Ea was the ruler of the southernmost quarter of the Sun's path, the "Way of Ea", corresponding to the period of 45 days on either side of winter solstice. Aquarius was also associated with the destructive floods that the Babylonians regularly experienced, and thus was negatively connoted. In Ancient Egypt astronomy, Aquarius was associated with the annual flood of the Nile; the banks were said to flood when Aquarius put his jar into the river, beginning spring.
In the Greek tradition, the constellation came to be represented simply as a single vase from which a stream poured down to Piscis Austrinus. The name in the Hindu zodiac is likewise kumbha "water-pitcher". | Aquarius (constellation) | Wikipedia | 382 | 799 | https://en.wikipedia.org/wiki/Aquarius%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
In Greek mythology, Aquarius is sometimes associated with Deucalion, the son of Prometheus who built a ship with his wife Pyrrha to survive an imminent flood. They sailed for nine days before washing ashore on Mount Parnassus. Aquarius is also sometimes identified with beautiful Ganymede, a youth in Greek mythology and the son of Trojan king Tros, who was taken to Mount Olympus by Zeus to act as cup-carrier to the gods. Neighboring Aquila represents the eagle, under Zeus' command, that snatched the young boy; some versions of the myth indicate that the eagle was in fact Zeus transformed. One tradition, stated that he was carried off by Eos. Yet another figure associated with the water bearer is Cecrops I, a king of Athens who sacrificed water instead of wine to the gods.
Depictions
In the first century, Ptolemy's Almagest established the common Western depiction of Aquarius. His water jar, an asterism itself, consists of Gamma, Pi, Eta, and Zeta Aquarii; it pours water in a stream of more than 20 stars terminating with Fomalhaut, now assigned solely to Piscis Austrinus. The water bearer's head is represented by 5th magnitude 25 Aquarii while his left shoulder is Beta Aquarii; his right shoulder and forearm are represented by Alpha and Gamma Aquarii respectively. | Aquarius (constellation) | Wikipedia | 287 | 799 | https://en.wikipedia.org/wiki/Aquarius%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
In Eastern astronomy
In Chinese astronomy, the stream of water flowing from the Water Jar was depicted as the "Army of Yu-Lin" (Yu-lim-kiun or Yulinjun, Hanzi: 羽林君). The name "Yu-lin" means "feathers and forests", referring to the numerous light-footed soldiers from the northern reaches of the empire represented by these faint stars. The constellation's stars were the most numerous of any Chinese constellation, numbering 45, the majority of which were located in modern Aquarius. The celestial army was protected by the wall Leibizhen (垒壁阵), which counted Iota, Lambda, Phi, and Sigma Aquarii among its 12 stars. 88, 89, and 98 Aquarii represent Fou-youe, the axes used as weapons and for hostage executions. Also in Aquarius is Loui-pi-tchin, the ramparts that stretch from 29 and 27 Piscium and 33 and 30 Aquarii through Phi, Lambda, Sigma, and Iota Aquarii to Delta, Gamma, Kappa, and Epsilon Capricorni. Similarly in the Hindu calendar Aquarius is depicted as Kumbha, and Kumbha, which means a pot or a jug, stands for the zodiac sign of Aquarius.
Near the border with Cetus, the axe Fuyue was represented by three stars; its position is disputed and may have instead been located in Sculptor. Tienliecheng also has a disputed position; the 13-star castle replete with ramparts may have possessed Nu and Xi Aquarii but may instead have been located south in Piscis Austrinus. The Water Jar asterism was seen to the ancient Chinese as the tomb, Fenmu. Nearby, the emperors' mausoleum Xiuliang stood, demarcated by Kappa Aquarii and three other collinear stars. Ku ("crying") and Qi ("weeping"), each composed of two stars, were located in the same region. | Aquarius (constellation) | Wikipedia | 416 | 799 | https://en.wikipedia.org/wiki/Aquarius%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
Three of the Chinese lunar mansions shared their name with constellations. Nu, also the name for the 10th lunar mansion, was a handmaiden represented by Epsilon, Mu, 3, and 4 Aquarii. The 11th lunar mansion shared its name with the constellation Xu ("emptiness"), formed by Beta Aquarii and Alpha Equulei; it represented a bleak place associated with death and funerals. Wei, the rooftop and 12th lunar mansion, was a V-shaped constellation formed by Alpha Aquarii, Theta Pegasi, and Epsilon Pegasi; it shared its name with two other Chinese constellations, in modern-day Scorpius and Aries.
Features
Stars
Despite both its prominent position on the zodiac and its large size, Aquarius has no particularly bright stars, its four brightest stars being less bright than (The Apparent Magnitude scale is reverse logarithmic, with increasingly bright objects having lower and lower (more negative) magnitudes.) Recent research has shown that there are several stars lying within its borders that possess planetary systems.
The two brightest stars, α Aquarii and β Aquarii, are luminous yellow supergiants, of spectral types G0Ib and G2Ib respectively, that were once hot blue-white B-class main sequence stars 5 to 9 times as massive as the Sun. The two are also moving through space perpendicular to the plane of the Milky Way. β Aquarii is the brightest star in Aquarius with apparent — only slightly brighter than α Aquarii. It also has the proper name of Sadalsuud. Having cooled and swollen to around 50 times the Sun's diameter, it is around 2200 times as luminous as the Sun. It is around 6.4 times as massive as the Sun and around 56 million years old. Sadalsuud is from Earth. α Aquarii, also known as Sadalmelik, has apparent It is distant from Earth, and is around 6.5 times as massive as the Sun, and 3000 times as luminous. It is 53 million years old. | Aquarius (constellation) | Wikipedia | 424 | 799 | https://en.wikipedia.org/wiki/Aquarius%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
γ Aquarii, also called Sadachbia, is a white main sequence star of spectral type star of spectral type A0V that is between 158 and 315 million years old and is around 2.5 times the Sun's mass (), and double its radius. Its magnitude is 3.85, and it is away, hence its luminosity is . The name Sadachbia comes from the Arabic for "lucky stars of the tents", sa'd al-akhbiya.
δ Aquarii, also known as Skat or Scheat is a blue-white spectral type A2 star with apparent magnitude 3.27 and luminosity .
ε Aquarii, also known as Albali, is a blue-white spectral type A1 star with apparent magnitude 3.77, absolute magnitude 1.2, and a luminosity of .
ζ Aquarii is a spectral type F2 double star; both stars are white. In combination, they appear to be magnitude 3.6 with luminosity . The primary has magnitude 4.53 and the secondary's magnitude is 4.31, but both have absolute The system's orbital period is 760 years; currently the two components are moving farther apart.
θ Aquarii, sometimes called Ancha, is spectral type G8 with apparent magnitude 4.16 and an absolute
κ Aquarii, also called Situla, has an apparent
λ Aquarii, also called Hudoor or Ekchusis, is spectral type M2 with magnitude 3.74 and luminosity .
ξ Aquarii, also called Bunda, is spectral type A7 with an apparent magnitude 4.69 and an absolute
π Aquarii, also called Seat, is spectral type B0 with apparent magnitude 4.66 and absolute | Aquarius (constellation) | Wikipedia | 364 | 799 | https://en.wikipedia.org/wiki/Aquarius%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
Planetary systems
Twelve exoplanet systems have been found in Aquarius as of 2013. Gliese 876, one of the nearest stars to Earth at a distance of 15 light-years, was the first red dwarf star to be found to possess a planetary system. It is orbited by four planets, including one terrestrial planet 6.6 times the mass of Earth. The planets vary in orbital period from 2 days to 124 days. 91 Aquarii is an orange giant star orbited by one planet, 91 Aquarii b. The planet's mass is 2.9 times the mass of Jupiter, and its orbital period is 182 days. Gliese 849 is a red dwarf star orbited by the first known long-period Jupiter-like planet, Gliese 849 b. The planet's mass is 0.99 times that of Jupiter and its orbital period is 1,852 days. | Aquarius (constellation) | Wikipedia | 188 | 799 | https://en.wikipedia.org/wiki/Aquarius%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
There are also less-prominent systems in Aquarius. WASP-6, a type G8 star of magnitude 12.4, is host to one exoplanet, WASP-6 b. The star is 307 parsecs from Earth and has a mass of 0.888 solar masses and a radius of 0.87 solar radii. WASP-6 b was discovered in 2008 by the transit method. It orbits its parent star every 3.36 days at a distance of 0.042 astronomical units (AU). It is 0.503 Jupiter masses but has a proportionally larger radius of 1.224 Jupiter radii. HD 206610, a K0 star located 194 parsecs from Earth, is host to one planet, HD 206610 b. The host star is larger than the Sun; more massive at 1.56 solar masses and larger at 6.1 solar radii. The planet was discovered by the radial velocity method in 2010 and has a mass of 2.2 Jupiter masses. It orbits every 610 days at a distance of 1.68 AU. Much closer to its sun is WASP-47 b, which orbits every 4.15 days only 0.052 AU from its sun, yellow dwarf (G9V) WASP-47. WASP-47 is close in size to the Sun, having a radius of 1.15 solar radii and a mass even closer at 1.08 solar masses. WASP-47 b was discovered in 2011 by the transit method, like WASP-6 b. It is slightly larger than Jupiter with a mass of 1.14 Jupiter masses and a radius of 1.15 Jupiter masses. | Aquarius (constellation) | Wikipedia | 343 | 799 | https://en.wikipedia.org/wiki/Aquarius%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
There are several more single-planet systems in Aquarius. HD 210277, a magnitude 6.63 yellow star located 21.29 parsecs from Earth, is host to one known planet: HD 210277 b. The 1.23 Jupiter mass planet orbits at nearly the same distance as Earth orbits the Sun1.1 AU, though its orbital period is significantly longer at around 442 days. HD 210277 b was discovered earlier than most of the other planets in Aquarius, detected by the radial velocity method in 1998. The star it orbits resembles the Sun beyond their similar spectral class; it has a radius of 1.1 solar radii and a mass of 1.09 solar masses. HD 212771 b, a larger planet at 2.3 Jupiter masses, orbits host star HD 212771 at a distance of 1.22 AU. The star itself, barely below the threshold of naked-eye visibility at magnitude 7.6, is a G8IV (yellow subgiant) star located 131 parsecs from Earth. Though it has a similar mass to the Sun1.15 solar massesit is significantly less dense with its radius of 5 solar radii. Its lone planet was discovered in 2010 by the radial velocity method, like several other exoplanets in the constellation.
As of 2013, there were only two known multiple-planet systems within the bounds of Aquarius: the Gliese 876 and HD 215152 systems. The former is quite prominent; the latter has only two planets and has a host star farther away at 21.5 parsecs. The HD 215152 system consists of the planets HD 215152 b and HD 215152 c orbiting their K0-type, magnitude 8.13 sun. Both discovered in 2011 by the radial velocity method, the two tiny planets orbit very close to their host star. HD 215152 c is the larger at 0.0097 Jupiter masses (still significantly larger than the Earth, which weighs in at 0.00315 Jupiter masses); its smaller sibling is barely smaller at 0.0087 Jupiter masses. The error in the mass measurements (0.0032 and respectively) is large enough to make this discrepancy statistically insignificant. HD 215152 c also orbits further from the star than HD 215152 b, 0.0852 AU compared to 0.0652. | Aquarius (constellation) | Wikipedia | 497 | 799 | https://en.wikipedia.org/wiki/Aquarius%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
On 23 February 2017, NASA announced that ultracool dwarf star TRAPPIST-1 in Aquarius has seven Earth-like rocky planets. Of these, as many as four may lie within the system's habitable zone, and may have liquid water on their surfaces. The discovery of the TRAPPIST-1 system is seen by astronomers as a significant step toward finding life beyond Earth.
Deep sky objects
Because of its position away from the galactic plane, the majority of deep-sky objects in Aquarius are galaxies, globular clusters, and planetary nebulae. Aquarius contains three deep sky objects that are in the Messier catalog: the globular clusters Messier 2, Messier 72, and the asterism Messier 73. While M73 was originally catalogued as a sparsely populated open cluster, modern analysis indicates the 6 main stars are not close enough together to fit this definition, reclassifying M73 as an asterism. Two well-known planetary nebulae are also located in Aquarius: the Saturn Nebula (NGC 7009), to the southeast of μ Aquarii; and the famous Helix Nebula (NGC 7293), southwest of δ Aquarii.
M2, also catalogued as NGC 7089, is a rich globular cluster located approximately 37,000 light-years from Earth. At magnitude 6.5, it is viewable in small-aperture instruments, but a 100 mm aperture telescope is needed to resolve any stars. M72, also catalogued as NGC 6981, is a small 9th magnitude globular cluster located approximately 56,000 light-years from Earth. M73, also catalogued as NGC 6994, is an open cluster with highly disputed status. | Aquarius (constellation) | Wikipedia | 361 | 799 | https://en.wikipedia.org/wiki/Aquarius%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
Aquarius is also home to several planetary nebulae. NGC 7009, also known as the Saturn Nebula, is an 8th magnitude planetary nebula located 3,000 light-years from Earth. It was given its moniker by the 19th century astronomer Lord Rosse for its resemblance to the planet Saturn in a telescope; it has faint protrusions on either side that resemble Saturn's rings. It appears blue-green in a telescope and has a central star of magnitude 11.3. Compared to the Helix Nebula, another planetary nebula in Aquarius, it is quite small. NGC 7293, also known as the Helix Nebula, is the closest planetary nebula to Earth at a distance of 650 light-years. It covers 0.25 square degrees, making it also the largest planetary nebula as seen from Earth. However, because it is so large, it is only viewable as a very faint object, though it has a fairly high integrated magnitude of 6.0.
One of the visible galaxies in Aquarius is NGC 7727, of particular interest for amateur astronomers who wish to discover or observe supernovae. A spiral galaxy (type S), it has an integrated magnitude of 10.7 and is 3 by 3 arcseconds. NGC 7252 is a tangle of stars resulting from the collision of two large galaxies and is known as the Atoms-for-Peace galaxy because of its resemblance to a cartoon atom.
Meteor showers
There are three major meteor showers with radiants in Aquarius: the Eta Aquariids, the Delta Aquariids, and the Iota Aquariids.
The Eta Aquariids are the strongest meteor shower radiating from Aquarius. It peaks between 5 and 6 May with a rate of approximately 35 meteors per hour. Originally discovered by Chinese astronomers in 401, Eta Aquariids can be seen coming from the Water Jar beginning on 21 April and as late as 12 May. The parent body of the shower is Halley's Comet, a periodic comet. Fireballs are common shortly after the peak, approximately between 9 May and 11 May. The normal meteors appear to have yellow trails. | Aquarius (constellation) | Wikipedia | 430 | 799 | https://en.wikipedia.org/wiki/Aquarius%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
The Delta Aquariids is a double radiant meteor shower that peaks first on 29 July and second on 6 August. The first radiant is located in the south of the constellation, while the second radiant is located in the northern circlet of Pisces asterism. The southern radiant's peak rate is about 20 meteors per hour, while the northern radiant's peak rate is about 10 meteors per hour.
The Iota Aquariids is a fairly weak meteor shower that peaks on 6 August, with a rate of approximately 8 meteors per hour.
Astrology
, the Sun appears in the constellation Aquarius from 16 February to 12 March. In tropical astrology, the Sun is considered to be in the sign Aquarius from 20 January to 19 February, and in sidereal astrology, from 15 February to 14 March.
Aquarius is also associated with the Age of Aquarius, a concept popular in 1960s counterculture and Medieval Alchemy. The date of the start of The Age of Aquarius is a topic of much debate. | Aquarius (constellation) | Wikipedia | 212 | 799 | https://en.wikipedia.org/wiki/Aquarius%20%28constellation%29 | Physical sciences | Zodiac | Astronomy |
Anacondas or water boas are a group of large boas of the genus Eunectes. They are a semiaquatic group of snakes found in tropical South America. Three to five extant and one extinct species are currently recognized, including one of the largest snakes in the world, E. murinus, the green anaconda.
Description
Although the name applies to a group of snakes, it is often used to refer only to one species, in particular, the common or green anaconda (Eunectes murinus), which is the largest snake in the world by weight, and the second longest after the reticulated python.
Origin
The recent fossil record of Eunectes is relatively sparse compared to other vertebrates and other genera of snakes. The fossil record of this group is effected by an artifact called the Pull of the Recent. Fossils of recent ancestors are not known, so the living species 'pull' the historical range of the genus to the present.
Etymology
The name Eunectes is derived from .
The South American names anacauchoa and anacaona were suggested in an account by Peter Martyr d'Anghiera. The idea of a South American origin was questioned by Henry Walter Bates who, in his travels in South America, failed to find any similar name in use. The word anaconda is derived from the name of a snake from Ceylon (Sri Lanka) that John Ray described in Latin in his (1693) as .
Ray used a catalogue of snakes from the Leyden museum supplied by Dr. Tancred Robinson. The description of its habit was based on Andreas Cleyer, who in 1684 described a gigantic snake that crushed large animals by coiling around their bodies and crushing their bones. Henry Yule in his 1886 work Hobson-Jobson, notes that the word became more popular due to a piece of fiction published in 1768 in the Scots Magazine by a certain R. Edwin. Edwin described a 'tiger' being crushed to death by an anaconda, when there were never any tigers in Sri Lanka.
Yule and Frank Wall noted that the snake was a python and suggested a Tamil origin meaning elephant killer. A Sinhalese origin was also suggested by Donald Ferguson who pointed out that the word ( lightning/large and stem/trunk) was used in Sri Lanka for the small whip snake (Ahaetulla pulverulenta) and somehow got misapplied to the python before myths were created. | Anaconda | Wikipedia | 505 | 809 | https://en.wikipedia.org/wiki/Anaconda | Biology and health sciences | Snakes | Animals |
The name commonly used for the anaconda in Brazil is sucuri, sucuriju or sucuriuba.
Distribution and habitat
Found in tropical South America from Ecuador, Brazil, Colombia and Venezuela south to Argentina.
Feeding
All five species are aquatic snakes that prey on other aquatic animals, including fish, river fowl, and caiman. Videos exist of anacondas preying on domestic animals such as goats and sometimes even jaguars that venture too close to the water.
Relationship with humans
While encounters between people and anacondas may be dangerous, they do not regularly hunt humans. Nevertheless, threat from anacondas is a familiar trope in comics, movies, and adventure stories (often published in pulp magazines or adventure magazines) set in the Amazon jungle. Local communities and some European explorers have given accounts of giant anacondas, legendary snakes of much greater proportion than any confirmed specimen.
Although charismatic, there is little known on the biology of wild anacondas. Most of our knowledge comes from the work of Dr. Jesús A. Rivas and his team working in the Venezuelan Llanos.
Species
Rivas et al. revised the taxonomy of Eunectes, describing a new species of green anaconda (Eunectes akayima) and merging E. deschauenseei and E. beniensis with E. notaeus, which resulted in the recognition of only three species of anaconda. The result of their phylogenetic analysis is represented below:
In a response paper, Dubois et al. questioned the results of the mtDNA analysis above and the validity of Eunectes akayima. The name of the new species was considered a nomen nudum.
Mating system
The mating seasons in Eunectes varies both between species and within species depending on locality, although the trend appears to be the dry season. The green anaconda (E. murinus) is the most well-studied species of Eunectes in terms of their mating system, followed by the yellow anaconda (E. notaeus); unfortunately E. deschauenseei and E. beniensis are much less common, making the specific details of their mating systems less well understood. | Anaconda | Wikipedia | 450 | 809 | https://en.wikipedia.org/wiki/Anaconda | Biology and health sciences | Snakes | Animals |
Sexual dimorphism
Sexual size dimorphism in Eunectes is the opposite of most other vertebrates. Females are larger than males in most snakes, and green anacondas (E. murinus) have one of the most extreme size differences, where females average roughly and males average only around . This size difference has several benefits for both sexes. Large size in females leads to higher fecundity and larger offspring; as a result male mate choice favours larger females. Large size is also favoured in males because larger males tend to be more successful at reproducing, both because of their size advantage in endurance rivalry and their advantage in sperm competition because larger males are able to produce more sperm. One reason that males are so much smaller in Eunectes is that large males can be confused for females, which interferes with their ability to mate when smaller males mistakenly coil them in breeding balls; as a result, there is an optimum size for males where they are large enough to successfully compete, but not large enough to risk other males trying to mate with them.
Breeding balls
During the mating season female anacondas release pheromones to attract males for breeding, which can result in polyandrous breeding balls; these breeding balls have been observed in E. murinus, E. notaeus, and E. deschauenseei, and likely also occur in E. beniensis. In the green anaconda (E. murinus), up to 13 males have been observed in a breeding ball, which have been recorded to last two weeks on average. In anaconda breeding balls, several males coil around one female and attempt to position themselves as close to her cloaca as possible where they use their pelvic spurs to "tickle" and encourage her to allow penetration. Since there are often many males present and only one male can mate with the female at a time, the success of a male often depends on his persistence and endurance, because physical combat is not a part of the Eunectes mating ritual, apart from firmly pushing against other males in an attempt to secure the best position on the female.
Sexual cannibalism | Anaconda | Wikipedia | 447 | 809 | https://en.wikipedia.org/wiki/Anaconda | Biology and health sciences | Snakes | Animals |
Cannibalism is quite easy in anacondas since females are so much larger than males, but sexual cannibalism has only been confirmed in E. murinus. Females gain the direct benefit of a post-copulatory high-protein meal when they consume their mates, along with the indirect benefit of additional resources to use for the formation of offspring; cannibalism in general (outside of the breeding season) has been confirmed in all but E. deschauenseei, although it is likely that it occurs in all Eunectes species.
Asexual reproduction
Although sexual reproduction is by far the most common in Eunectes, E. murinus has been observed to undergo facultative parthenogenesis. In both cases, the females had lived in isolation from other anacondas for over eight years, and DNA analysis showed that the few fully formed offspring were genetically identical to the mothers; although this is not commonly observed, it is likely possible in all species of Eunectes and several other species of Boidae.
Indigenous mythology
According to the founding myth of the Huni Kuin, a man named Yube fell in love with an anaconda woman and was turned into an anaconda as well. He began to live with her in the deep world of waters. In this world, Yube discovered a hallucinogenic drink with healing powers and access to knowledge. One day, without telling his anaconda wife, Yube decided to return to the land of men and resume his old human form. The myth also explains the origin of cipó or ayahuasca — a hallucinogenic drink taken ritualistically by the Huni Kuin. | Anaconda | Wikipedia | 349 | 809 | https://en.wikipedia.org/wiki/Anaconda | Biology and health sciences | Snakes | Animals |
In mathematics, the axiom of choice, abbreviated AC or AoC, is an axiom of set theory equivalent to the statement that a Cartesian product of a collection of non-empty sets is non-empty. Informally put, the axiom of choice says that given any collection of sets, each containing at least one element, it is possible to construct a new set by choosing one element from each set, even if the collection is infinite. Formally, it states that for every indexed family of nonempty sets, there exists an indexed set such that for every . The axiom of choice was formulated in 1904 by Ernst Zermelo in order to formalize his proof of the well-ordering theorem.
The axiom of choice is equivalent to the statement that every partition has a transversal.
In many cases, a set created by choosing elements can be made without invoking the axiom of choice, particularly if the number of sets from which to choose the elements is finite, or if a canonical rule on how to choose the elements is available — some distinguishing property that happens to hold for exactly one element in each set. An illustrative example is sets picked from the natural numbers. From such sets, one may always select the smallest number, e.g. given the sets {{4, 5, 6}, {10, 12}, {1, 400, 617, 8000}}, the set containing each smallest element is {4, 10, 1}. In this case, "select the smallest number" is a choice function. Even if infinitely many sets are collected from the natural numbers, it will always be possible to choose the smallest element from each set to produce a set. That is, the choice function provides the set of chosen elements. But no definite choice function is known for the collection of all non-empty subsets of the real numbers. In that case, the axiom of choice must be invoked.
Bertrand Russell coined an analogy: for any (even infinite) collection of pairs of shoes, one can pick out the left shoe from each pair to obtain an appropriate collection (i.e. set) of shoes; this makes it possible to define a choice function directly. For an infinite collection of pairs of socks (assumed to have no distinguishing features such as being a left sock rather than a right sock), there is no obvious way to make a function that forms a set out of selecting one sock from each pair without invoking the axiom of choice. | Axiom of choice | Wikipedia | 508 | 840 | https://en.wikipedia.org/wiki/Axiom%20of%20choice | Mathematics | Discrete mathematics | null |
Although originally controversial, the axiom of choice is now used without reservation by most mathematicians, and is included in the standard form of axiomatic set theory, Zermelo–Fraenkel set theory with the axiom of choice (ZFC). One motivation for this is that a number of generally accepted mathematical results, such as Tychonoff's theorem, require the axiom of choice for their proofs. Contemporary set theorists also study axioms that are not compatible with the axiom of choice, such as the axiom of determinacy. The axiom of choice is avoided in some varieties of constructive mathematics, although there are varieties of constructive mathematics in which the axiom of choice is embraced.
Statement
A choice function (also called selector or selection) is a function f, defined on a collection X of nonempty sets, such that for every set A in X, f(A) is an element of A. With this concept, the axiom can be stated:
Formally, this may be expressed as follows:
Thus, the negation of the axiom may be expressed as the existence of a collection of nonempty sets which has no choice function. Formally, this may be derived making use of the logical equivalence of to .
Each choice function on a collection X of nonempty sets is an element of the Cartesian product of the sets in X. This is not the most general situation of a Cartesian product of a family of sets, where a given set can occur more than once as a factor; however, one can focus on elements of such a product that select the same element every time a given set appears as factor, and such elements correspond to an element of the Cartesian product of all distinct sets in the family. The axiom of choice asserts the existence of such elements; it is therefore equivalent to:
Given any family of nonempty sets, their Cartesian product is a nonempty set.
Nomenclature
In this article and other discussions of the Axiom of Choice the following abbreviations are common:
AC – the Axiom of Choice. More rarely, AoC is used.
ZF – Zermelo–Fraenkel set theory omitting the Axiom of Choice.
ZFC – Zermelo–Fraenkel set theory, extended to include the Axiom of Choice. | Axiom of choice | Wikipedia | 474 | 840 | https://en.wikipedia.org/wiki/Axiom%20of%20choice | Mathematics | Discrete mathematics | null |
Variants
There are many other equivalent statements of the axiom of choice. These are equivalent in the sense that, in the presence of other basic axioms of set theory, they imply the axiom of choice and are implied by it.
One variation avoids the use of choice functions by, in effect, replacing each choice function with its range:
Given any set X, if the empty set is not an element of X and the elements of X are pairwise disjoint, then there exists a set C such that its intersection with any of the elements of X contains exactly one element.
This can be formalized in first-order logic as:
∀x (
∃o (o ∈ x ∧ ¬∃n (n ∈ o)) ∨
∃a ∃b ∃c (a ∈ x ∧ b ∈ x ∧ c ∈ a ∧ c ∈ b ∧ ¬(a = b)) ∨
∃c ∀e (e ∈ x → ∃a (a ∈ e ∧ a ∈ c ∧ ∀b ((b ∈ e ∧ b ∈ c) → a = b))))
Note that P ∨ Q ∨ R is logically equivalent to (¬P ∧ ¬Q) → R.
In English, this first-order sentence reads:
Given any set X,
X contains the empty set as an element or
the elements of X are not pairwise disjoint or
there exists a set C such that its intersection with any of the elements of X contains exactly one element.
This guarantees for any partition of a set X the existence of a subset C of X containing exactly one element from each part of the partition. | Axiom of choice | Wikipedia | 318 | 840 | https://en.wikipedia.org/wiki/Axiom%20of%20choice | Mathematics | Discrete mathematics | null |
Another equivalent axiom only considers collections X that are essentially powersets of other sets:
For any set A, the power set of A (with the empty set removed) has a choice function.
Authors who use this formulation often speak of the choice function on A, but this is a slightly different notion of choice function. Its domain is the power set of A (with the empty set removed), and so makes sense for any set A, whereas with the definition used elsewhere in this article, the domain of a choice function on a collection of sets is that collection, and so only makes sense for sets of sets. With this alternate notion of choice function, the axiom of choice can be compactly stated as
Every set has a choice function.
which is equivalent to
For any set A there is a function f such that for any non-empty subset B of A, f(B) lies in B.
The negation of the axiom can thus be expressed as:
There is a set A such that for all functions f (on the set of non-empty subsets of A), there is a B such that f(B) does not lie in B.
Restriction to finite sets
The usual statement of the axiom of choice does not specify whether the collection of nonempty sets is finite or infinite, and thus implies that every finite collection of nonempty sets has a choice function. However, that particular case is a theorem of the Zermelo–Fraenkel set theory without the axiom of choice (ZF); it is easily proved by the principle of finite induction. In the even simpler case of a collection of one set, a choice function just corresponds to an element, so this instance of the axiom of choice says that every nonempty set has an element; this holds trivially. The axiom of choice can be seen as asserting the generalization of this property, already evident for finite collections, to arbitrary collections.
Usage
Until the late 19th century, the axiom of choice was often used implicitly, although it had not yet been formally stated. For example, after having established that the set X contains only non-empty sets, a mathematician might have said "let F(s) be one of the members of s for all s in X" to define a function F. In general, it is impossible to prove that F exists without the axiom of choice, but this seems to have gone unnoticed until Zermelo. | Axiom of choice | Wikipedia | 501 | 840 | https://en.wikipedia.org/wiki/Axiom%20of%20choice | Mathematics | Discrete mathematics | null |
Examples
The nature of the individual nonempty sets in the collection may make it possible to avoid the axiom of choice even for certain infinite collections. For example, suppose that each member of the collection X is a nonempty subset of the natural numbers. Every such subset has a smallest element, so to specify our choice function we can simply say that it maps each set to the least element of that set. This gives us a definite choice of an element from each set, and makes it unnecessary to add the axiom of choice to our axioms of set theory.
The difficulty appears when there is no natural choice of elements from each set. If we cannot make explicit choices, how do we know that our selection forms a legitimate set (as defined by the other ZF axioms of set theory)? For example, suppose that X is the set of all non-empty subsets of the real numbers. First we might try to proceed as if X were finite. If we try to choose an element from each set, then, because X is infinite, our choice procedure will never come to an end, and consequently we shall never be able to produce a choice function for all of X. Next we might try specifying the least element from each set. But some subsets of the real numbers do not have least elements. For example, the open interval (0,1) does not have a least element: if x is in (0,1), then so is x/2, and x/2 is always strictly smaller than x. So this attempt also fails. | Axiom of choice | Wikipedia | 320 | 840 | https://en.wikipedia.org/wiki/Axiom%20of%20choice | Mathematics | Discrete mathematics | null |
Additionally, consider for instance the unit circle S, and the action on S by a group G consisting of all rational rotations, that is, rotations by angles which are rational multiples of π. Here G is countable while S is uncountable. Hence S breaks up into uncountably many orbits under G. Using the axiom of choice, we could pick a single point from each orbit, obtaining an uncountable subset X of S with the property that all of its translates by G are disjoint from X. The set of those translates partitions the circle into a countable collection of pairwise disjoint sets, which are all pairwise congruent. Since X is not measurable for any rotation-invariant countably additive finite measure on S, finding an algorithm to form a set from selecting a point in each orbit requires that one add the axiom of choice to our axioms of set theory. See non-measurable set for more details.
In classical arithmetic, the natural numbers are well-ordered: for every nonempty subset of the natural numbers, there is a unique least element under the natural ordering. In this way, one may specify a set from any given subset. One might say, "Even though the usual ordering of the real numbers does not work, it may be possible to find a different ordering of the real numbers which is a well-ordering. Then our choice function can choose the least element of every set under our unusual ordering." The problem then becomes that of constructing a well-ordering, which turns out to require the axiom of choice for its existence; every set can be well-ordered if and only if the axiom of choice holds.
Criticism and acceptance
A proof requiring the axiom of choice may establish the existence of an object without explicitly defining the object in the language of set theory. For example, while the axiom of choice implies that there is a well-ordering of the real numbers, there are models of set theory with the axiom of choice in which no individual well-ordering of the reals is definable. Similarly, although a subset of the real numbers that is not Lebesgue measurable can be proved to exist using the axiom of choice, it is consistent that no such set is definable. | Axiom of choice | Wikipedia | 474 | 840 | https://en.wikipedia.org/wiki/Axiom%20of%20choice | Mathematics | Discrete mathematics | null |
The axiom of choice asserts the existence of these intangibles (objects that are proved to exist, but which cannot be explicitly constructed), which may conflict with some philosophical principles. Because there is no canonical well-ordering of all sets, a construction that relies on a well-ordering may not produce a canonical result, even if a canonical result is desired (as is often the case in category theory). This has been used as an argument against the use of the axiom of choice.
Another argument against the axiom of choice is that it implies the existence of objects that may seem counterintuitive. One example is the Banach–Tarski paradox, which says that it is possible to decompose the 3-dimensional solid unit ball into finitely many pieces and, using only rotations and translations, reassemble the pieces into two solid balls each with the same volume as the original. The pieces in this decomposition, constructed using the axiom of choice, are non-measurable sets.
Moreover, paradoxical consequences of the axiom of choice for the no-signaling principle in physics have recently been pointed out.
Despite these seemingly paradoxical results, most mathematicians accept the axiom of choice as a valid principle for proving new results in mathematics. But the debate is interesting enough that it is considered notable when a theorem in ZFC (ZF plus AC) is logically equivalent (with just the ZF axioms) to the axiom of choice, and mathematicians look for results that require the axiom of choice to be false, though this type of deduction is less common than the type that requires the axiom of choice to be true.
Theorems of ZF hold true in any model of that theory, regardless of the truth or falsity of the axiom of choice in that particular model. The implications of choice below, including weaker versions of the axiom itself, are listed because they are not theorems of ZF. The Banach–Tarski paradox, for example, is neither provable nor disprovable from ZF alone: it is impossible to construct the required decomposition of the unit ball in ZF, but also impossible to prove there is no such decomposition. Such statements can be rephrased as conditional statements—for example, "If AC holds, then the decomposition in the Banach–Tarski paradox exists." Such conditional statements are provable in ZF when the original statements are provable from ZF and the axiom of choice. | Axiom of choice | Wikipedia | 510 | 840 | https://en.wikipedia.org/wiki/Axiom%20of%20choice | Mathematics | Discrete mathematics | null |
In constructive mathematics
As discussed above, in the classical theory of ZFC, the axiom of choice enables nonconstructive proofs in which the existence of a type of object is proved without an explicit instance being constructed. In fact, in set theory and topos theory, Diaconescu's theorem shows that the axiom of choice implies the law of excluded middle. The principle is thus not available in constructive set theory, where non-classical logic is employed.
The situation is different when the principle is formulated in Martin-Löf type theory. There and higher-order Heyting arithmetic, the appropriate statement of the axiom of choice is (depending on approach) included as an axiom or provable as a theorem. A cause for this difference is that the axiom of choice in type theory does not have the extensionality properties that the axiom of choice in constructive set theory does. The type theoretical context is discussed further below.
Different choice principles have been thoroughly studied in the constructive contexts and the principles' status varies between different school and varieties of the constructive mathematics.
Some results in constructive set theory use the axiom of countable choice or the axiom of dependent choice, which do not imply the law of the excluded middle. Errett Bishop, who is notable for developing a framework for constructive analysis, argued that an axiom of choice was constructively acceptable, saying
Although the axiom of countable choice in particular is commonly used in constructive mathematics, its use has also been questioned.
Independence
It has been known since as early as 1922 that the axiom of choice may fail in a variant of ZF with urelements, through the technique of permutation models introduced by Abraham Fraenkel and developed further by Andrzej Mostowski. The basic technique can be illustrated as follows: Let xn and yn be distinct urelements for , and build a model where each set is symmetric under the interchange xn ↔ yn for all but a finite number of n. Then the set can be in the model but sets such as cannot, and thus X cannot have a choice function. | Axiom of choice | Wikipedia | 429 | 840 | https://en.wikipedia.org/wiki/Axiom%20of%20choice | Mathematics | Discrete mathematics | null |
In 1938, Kurt Gödel showed that the negation of the axiom of choice is not a theorem of ZF by constructing an inner model (the constructible universe) that satisfies ZFC, thus showing that ZFC is consistent if ZF itself is consistent. In 1963, Paul Cohen employed the technique of forcing, developed for this purpose, to show that, assuming ZF is consistent, the axiom of choice itself is not a theorem of ZF. He did this by constructing a much more complex model that satisfies ZF¬C (ZF with the negation of AC added as axiom) and thus showing that ZF¬C is consistent. Cohen's model is a symmetric model, which is similar to permutation models, but uses "generic" subsets of the natural numbers (justified by forcing) in place of urelements.
Together these results establish that the axiom of choice is logically independent of ZF. The assumption that ZF is consistent is harmless because adding another axiom to an already inconsistent system cannot make the situation worse. Because of independence, the decision whether to use the axiom of choice (or its negation) in a proof cannot be made by appeal to other axioms of set theory. It must be made on other grounds.
One argument in favor of using the axiom of choice is that it is convenient because it allows one to prove some simplifying propositions that otherwise could not be proved. Many theorems provable using choice are of an elegant general character: the cardinalities of any two sets are comparable, every nontrivial ring with unity has a maximal ideal, every vector space has a basis, every connected graph has a spanning tree, and every product of compact spaces is compact, among many others. Frequently, the axiom of choice allows generalizing a theorem to "larger" objects. For example, it is provable without the axiom of choice that every vector space of finite dimension has a basis, but the generalization to all vector spaces requires the axiom of choice. Likewise, a finite product of compact spaces can be proven to be compact without the axiom of choice, but the generalization to infinite products (Tychonoff's theorem) requires the axiom of choice. | Axiom of choice | Wikipedia | 466 | 840 | https://en.wikipedia.org/wiki/Axiom%20of%20choice | Mathematics | Discrete mathematics | null |
The proof of the independence result also shows that a wide class of mathematical statements, including all statements that can be phrased in the language of Peano arithmetic, are provable in ZF if and only if they are provable in ZFC. Statements in this class include the statement that P = NP, the Riemann hypothesis, and many other unsolved mathematical problems. When attempting to solve problems in this class, it makes no difference whether ZF or ZFC is employed if the only question is the existence of a proof. It is possible, however, that there is a shorter proof of a theorem from ZFC than from ZF.
The axiom of choice is not the only significant statement that is independent of ZF. For example, the generalized continuum hypothesis (GCH) is not only independent of ZF, but also independent of ZFC. However, ZF plus GCH implies AC, making GCH a strictly stronger claim than AC, even though they are both independent of ZF.
Stronger axioms
The axiom of constructibility and the generalized continuum hypothesis each imply the axiom of choice and so are strictly stronger than it. In class theories such as Von Neumann–Bernays–Gödel set theory and Morse–Kelley set theory, there is an axiom called the axiom of global choice that is stronger than the axiom of choice for sets because it also applies to proper classes. The axiom of global choice follows from the axiom of limitation of size. Tarski's axiom, which is used in Tarski–Grothendieck set theory and states (in the vernacular) that every set belongs to Grothendieck universe, is stronger than the axiom of choice.
Equivalents
There are important statements that, assuming the axioms of ZF but neither AC nor ¬AC, are equivalent to the axiom of choice. The most important among them are Zorn's lemma and the well-ordering theorem. In fact, Zermelo initially introduced the axiom of choice in order to formalize his proof of the well-ordering theorem. | Axiom of choice | Wikipedia | 429 | 840 | https://en.wikipedia.org/wiki/Axiom%20of%20choice | Mathematics | Discrete mathematics | null |
Set theory
Tarski's theorem about choice: For every infinite set A, there is a bijective map between the sets A and A×A.
Trichotomy: If two sets are given, then either they have the same cardinality, or one has a smaller cardinality than the other.
Given two non-empty sets, one has a surjection to the other.
Every surjective function has a right inverse.
The Cartesian product of any family of nonempty sets is nonempty. In other words, every family of nonempty sets has a choice function (i.e. a function which maps each of the nonempty sets to one of its elements).
König's theorem: Colloquially, the sum of a sequence of cardinals is strictly less than the product of a sequence of larger cardinals. (The reason for the term "colloquially" is that the sum or product of a "sequence" of cardinals cannot itself be defined without some aspect of the axiom of choice.)
Well-ordering theorem: Every set can be well-ordered. Consequently, every cardinal has an initial ordinal.
Zorn's lemma: Every non-empty partially ordered set in which every chain (i.e., totally ordered subset) has an upper bound contains at least one maximal element.
Hausdorff maximal principle: Every partially ordered set has a maximal chain. Equivalently, in any partially ordered set, every chain can be extended to a maximal chain.
Tukey's lemma: Every non-empty collection of finite character has a maximal element with respect to inclusion.
Antichain principle: Every partially ordered set has a maximal antichain. Equivalently, in any partially ordered set, every antichain can be extended to a maximal antichain.
The powerset of any ordinal can be well-ordered.
Abstract algebra
Every vector space has a basis (i.e., a linearly independent spanning subset). In other words, vector spaces are equivalent to free modules.
Krull's theorem: Every unital ring (other than the trivial ring) contains a maximal ideal. Equivalently, in any nontrivial unital ring, every ideal can be extended to a maximal ideal.
For every non-empty set S there is a binary operation defined on S that gives it a group structure. (A cancellative binary operation is enough, see group structure and the axiom of choice.) | Axiom of choice | Wikipedia | 505 | 840 | https://en.wikipedia.org/wiki/Axiom%20of%20choice | Mathematics | Discrete mathematics | null |
Every free abelian group is projective.
Baer's criterion: Every divisible abelian group is injective.
Every set is a projective object in the category Set of sets.
Functional analysis
The closed unit ball of the dual of a normed vector space over the reals has an extreme point.
Point-set topology
The Cartesian product of any family of connected topological spaces is connected.
Tychonoff's theorem: The Cartesian product of any family of compact topological spaces is compact.
In the product topology, the closure of a product of subsets is equal to the product of the closures.
Mathematical logic
If S is a set of sentences of first-order logic and B is a consistent subset of S, then B is included in a set that is maximal among consistent subsets of S. The special case where S is the set of all first-order sentences in a given signature is weaker, equivalent to the Boolean prime ideal theorem; see the section "Weaker forms" below.
Graph theory
Every connected graph has a spanning tree. Equivalently, every nonempty graph has a spanning forest. | Axiom of choice | Wikipedia | 227 | 840 | https://en.wikipedia.org/wiki/Axiom%20of%20choice | Mathematics | Discrete mathematics | null |
Category theory
Several results in category theory invoke the axiom of choice for their proof. These results might be weaker than, equivalent to, or stronger than the axiom of choice, depending on the strength of the technical foundations. For example, if one defines categories in terms of sets, that is, as sets of objects and morphisms (usually called a small category), then there is no category of all sets, and so it is difficult for a category-theoretic formulation to apply to all sets. On the other hand, other foundational descriptions of category theory are considerably stronger, and an identical category-theoretic statement of choice may be stronger than the standard formulation, à la class theory, mentioned above.
Examples of category-theoretic statements which require choice include:
Every small category has a skeleton.
If two small categories are weakly equivalent, then they are equivalent.
Every continuous functor on a small-complete category which satisfies the appropriate solution set condition has a left-adjoint (the Freyd adjoint functor theorem).
Weaker forms
There are several weaker statements that are not equivalent to the axiom of choice but are closely related. One example is the axiom of dependent choice (DC). A still weaker example is the axiom of countable choice (ACω or CC), which states that a choice function exists for any countable set of nonempty sets. These axioms are sufficient for many proofs in elementary mathematical analysis, and are consistent with some principles, such as the Lebesgue measurability of all sets of reals, that are disprovable from the full axiom of choice.
Given an ordinal parameter α ≥ ω+2 — for every set S with rank less than α, S is well-orderable. Given an ordinal parameter α ≥ 1 — for every set S with Hartogs number less than ωα, S is well-orderable. As the ordinal parameter is increased, these approximate the full axiom of choice more and more closely.
Other choice axioms weaker than axiom of choice include the Boolean prime ideal theorem and the axiom of uniformization. The former is equivalent in ZF to Tarski's 1930 ultrafilter lemma: every filter is a subset of some ultrafilter. | Axiom of choice | Wikipedia | 481 | 840 | https://en.wikipedia.org/wiki/Axiom%20of%20choice | Mathematics | Discrete mathematics | null |
Results requiring AC (or weaker forms) but weaker than it
One of the most interesting aspects of the axiom of choice is the large number of places in mathematics where it shows up. Here are some statements that require the axiom of choice in the sense that they are not provable from ZF but are provable from ZFC (ZF plus AC). Equivalently, these statements are true in all models of ZFC but false in some models of ZF. | Axiom of choice | Wikipedia | 96 | 840 | https://en.wikipedia.org/wiki/Axiom%20of%20choice | Mathematics | Discrete mathematics | null |
Set theory
The ultrafilter lemma (with ZF) can be used to prove the Axiom of choice for finite sets: Given and a collection of non-empty sets, their product is not empty.
The union of any countable family of countable sets is countable (this requires countable choice but not the full axiom of choice).
If the set A is infinite, then there exists an injection from the natural numbers N to A (see Dedekind infinite).
Eight definitions of a finite set are equivalent.
Every infinite game in which is a Borel subset of Baire space is determined.
Every infinite cardinal κ satisfies 2×κ = κ.
Measure theory
The Vitali theorem on the existence of non-measurable sets, which states that there exists a subset of the real numbers that is not Lebesgue measurable.
There exist Lebesgue-measurable subsets of the real numbers that are not Borel sets. That is, the Borel σ-algebra on the real numbers (which is generated by all real intervals) is strictly included the Lebesgue-measure σ-algebra on the real numbers.
The Hausdorff paradox.
The Banach–Tarski paradox.
Algebra
Every field has an algebraic closure.
Every field extension has a transcendence basis.
Every infinite-dimensional vector space contains an infinite linearly independent subset (this requires dependent choice, but not the full axiom of choice).
Stone's representation theorem for Boolean algebras needs the Boolean prime ideal theorem.
The Nielsen–Schreier theorem, that every subgroup of a free group is free.
The additive groups of R and C are isomorphic.
Functional analysis
The Hahn–Banach theorem in functional analysis, allowing the extension of linear functionals.
The theorem that every Hilbert space has an orthonormal basis.
The Banach–Alaoglu theorem about compactness of sets of functionals.
The Baire category theorem about complete metric spaces, and its consequences, such as the open mapping theorem and the closed graph theorem.
On every infinite-dimensional topological vector space there is a discontinuous linear map.
General topology
A uniform space is compact if and only if it is complete and totally bounded.
Every Tychonoff space has a Stone–Čech compactification.
Mathematical logic | Axiom of choice | Wikipedia | 473 | 840 | https://en.wikipedia.org/wiki/Axiom%20of%20choice | Mathematics | Discrete mathematics | null |
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