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25.751840591430664 136 WikiText2 |
23350 The oldest accurately dated star chart was the result of ancient Egyptian astronomy in 1534 BC . The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC , during the Kassite Period ( ca . 1531 โ 1155 BC ) . |
40.14085006713867 51 WikiText2 |
23351 The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC , with the help of Timocharis . The star catalog of Hipparchus ( 2nd century BC ) included 1020 stars , and was used to assemble Ptolemy 's star catalogue . Hipparchus is known for the discovery of the first recorded no... |
34.68022918701172 77 WikiText2 |
23352 In spite of the apparent immutability of the heavens , Chinese astronomers were aware that new stars could appear . In 185 AD , they were the first to observe and write about a supernova , now known as the SN 185 . The brightest stellar event in recorded history was the SN 1006 supernova , which was observed in ... |
30.027252197265625 98 WikiText2 |
23353 Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars . They built the first large observatory research institutes , mainly for the purpose of producing Zij star catalogues . Among t... |
56.87480926513672 154 WikiText2 |
23354 According to Josep Puig , the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material , citing his observation of the conjunction of Jupiter and Mars on 500... |
35.35270309448242 221 WikiText2 |
23355 The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667 . Edmond Halley published the first measurements of the proper motion of a pair of nearby " fixed " stars , demonstrating that they had changed positions since the time of the ancient Greek astronomer... |
50.182559967041016 55 WikiText2 |
23356 William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky . During the 1780s he established a series of gauges in 600 directions and counted the stars observed along each line of sight . From this he deduced that the number of stars steadily increased toward one side ... |
25.06373405456543 129 WikiText2 |
23357 The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi . By comparing the spectra of stars such as Sirius to the Sun , they found differences in the strength and number of their absorption lines โ the dark lines in a stellar spectra caused by the atmosphere 's absorption of ... |
49.26408386230469 88 WikiText2 |
23358 The first direct measurement of the distance to a star ( 61 Cygni at 11 @.@ 4 light @-@ years ) was made in 1838 by Friedrich Bessel using the parallax technique . Parallax measurements demonstrated the vast separation of the stars in the heavens . Observation of double stars gained increasing importance during ... |
42.51604080200195 265 WikiText2 |
23359 Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century . In 1913 , the Hertzsprung @-@ Russell diagram was developed , propelling the astrophysical study of stars . Successful models were developed to explain the interiors of stars and stellar evo... |
44.76145553588867 102 WikiText2 |
23360 With the exception of supernovae , individual stars have primarily been observed in the Local Group , and especially in the visible part of the Milky Way ( as demonstrated by the detailed star catalogues available for our galaxy ) . But some stars have been observed in the M100 galaxy of the Virgo Cluster , abou... |
21.60479164123535 156 WikiText2 |
23361 = = Designations = = |
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23362 The concept of a constellation was known to exist during the Babylonian period . Ancient sky watchers imagined that prominent arrangements of stars formed patterns , and they associated these with particular aspects of nature or their myths . Twelve of these formations lay along the band of the ecliptic and thes... |
55.04510498046875 77 WikiText2 |
23363 As well as certain constellations and the Sun itself , individual stars have their own myths . To the Ancient Greeks , some " stars " , known as planets ( Greek ฯฮปฮฑฮฝฮฎฯฮทฯ ( planฤtฤs ) , meaning " wanderer " ) , represented various important deities , from which the names of the planets Mercury , Venus , Mars , Ju... |
31.283227920532227 100 WikiText2 |
23364 Circa 1600 , the names of the constellations were used to name the stars in the corresponding regions of the sky . The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation . Later a numbering system based on the star 's right as... |
44.5109748840332 95 WikiText2 |
23365 The only internationally recognized authority for naming celestial bodies is the International Astronomical Union ( IAU ) . A number of private companies sell names of stars , which the British Library calls an unregulated commercial enterprise . The IAU has disassociated itself from this commercial practice , a... |
39.89923858642578 137 WikiText2 |
23366 = = Units of measurement = = |
388.1640930175781 7 WikiText2 |
23367 Although stellar parameters can be expressed in SI units or CGS units , it is often most convenient to express mass , luminosity , and radii in solar units , based on the characteristics of the Sun : |
63.6916618347168 38 WikiText2 |
23368 Large lengths , such as the radius of a giant star or the semi @-@ major axis of a binary star system , are often expressed in terms of the astronomical unit โ approximately equal to the mean distance between the Earth and the Sun ( 150 million km or 93 million miles ) . |
50.418739318847656 57 WikiText2 |
23369 = = Formation and evolution = = |
799.8424682617188 7 WikiText2 |
23370 Stars condense from regions of space of higher density , yet those regions are less dense than within a vacuum chamber . These regions - known as molecular clouds - consist mostly of hydrogen , with about 23 to 28 percent helium and a few percent heavier elements . One example of such a star @-@ forming region i... |
63.066898345947266 119 WikiText2 |
23371 All stars spend the majority of their existence as main sequence stars , fueled primarily by the nuclear fusion of hydrogen into helium within their cores . However , stars of different masses have markedly different properties at various stages of their development . The ultimate fate of more massive stars diff... |
39.52186584472656 82 WikiText2 |
23372 Very low mass stars , with masses below 0 @.@ 5 M โ , are fully convective and distribute helium evenly throughout the whole star while on the main sequence . Therefore , they never undergo shell burning , never become red giants , which cease fusing and become helium white dwarfs and slowly cool after exhaustin... |
65.396484375 96 WikiText2 |
23373 Low mass stars ( including the Sun ) , with a mass between 0 @.@ 5 M โ and 1 @.@ 8 โ 2 @.@ 5 M โ depending on composition , do become red giants as their core hydrogen is depleted and they begin to burn helium in core in a helium flash ; they develop a degenerate carbon @-@ oxygen core later on the asymptotic gi... |
56.985267639160156 102 WikiText2 |
23374 Intermediate @-@ mass stars , between 1 @.@ 8 โ 2 @.@ 5 M โ and 5 โ 10 M โ , pass through evolutionary stages similar to low mass stars , but after a relatively short period on the RGB they ignite helium without a flash and spend an extended period in the red clump before forming a degenerate carbon @-@ oxygen c... |
109.4989242553711 73 WikiText2 |
23375 Massive stars generally have a minimum mass of 7 โ 10 M โ ( possibly as low as 5 โ 6 M โ ) . After exhausting the hydrogen at the core these stars become supergiants and go on to fuse elements heavier than helium . They end their lives when their cores collapse and they explode as supernovae . |
33.378013610839844 60 WikiText2 |
23376 = = = Star formation = = = |
145.47882080078125 8 WikiText2 |
23377 The formation of a star begins with gravitational instability within a molecular cloud , caused by regions of higher density - often triggered by compression of clouds by radiation from massive stars , expanding bubbles in the interstellar medium , the collision of different molecular clouds , or the collision o... |
51.81310272216797 87 WikiText2 |
23378 As the cloud collapses , individual conglomerations of dense dust and gas form " Bok globules " . As a globule collapses and the density increases , the gravitational energy converts into heat and the temperature rises . When the protostellar cloud has approximately reached the stable condition of hydrostatic eq... |
59.61901092529297 95 WikiText2 |
23379 Early stars of less than 2 M โ are called T Tauri stars , while those with greater mass are Herbig Ae / Be stars . These newly formed stars emit jets of gas along their axis of rotation , which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig โ Haro... |
54.7838020324707 91 WikiText2 |
23380 Early in their development , T Tauri stars follow the Hayashi track โ they contract and decrease in luminosity while remaining at roughly the same temperature . Less massive T Tauri stars follow this track to the main sequence , while more massive stars turn onto the Henyey track . |
126.00977325439453 50 WikiText2 |
23381 Most stars are observed to be members of binary star systems , and the properties of those binaries are the result of the conditions in which they formed . A gas cloud must lose its angular momentum in order to collapse and form a star . The fragmentation of the cloud into multiple stars distributes some of that... |
49.986568450927734 118 WikiText2 |
23382 = = = Main sequence = = = |
183.69354248046875 8 WikiText2 |
23383 Stars spend about 90 % of their existence fusing hydrogen into helium in high @-@ temperature and high @-@ pressure reactions near the core . Such stars are said to be on the main sequence , and are called dwarf stars . Starting at zero @-@ age main sequence , the proportion of helium in a star 's core will stea... |
40.90819549560547 131 WikiText2 |
23384 Every star generates a stellar wind of particles that causes a continual outflow of gas into space . For most stars , the mass lost is negligible . The Sun loses 10 โ 14 M โ every year , or about 0 @.@ 01 % of its total mass over its entire lifespan . However , very massive stars can lose 10 โ 7 to 10 โ 5 M โ ea... |
42.58911895751953 102 WikiText2 |
23385 The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it . The Sun 's is expected to live 10 billion ( 1010 ) years . Massive stars consume their fuel very rapidly and are short @-@ lived . Low mass stars consume their fuel very slowly . Stars l... |
30.488435745239258 236 WikiText2 |
23386 Besides mass , the elements heavier than helium can play a significant role in the evolution of stars . Astronomers label all elements heavier than helium " metals " , and call the chemical concentration of these elements in a star , its metallicity . A star 's metallicity can influence the time the star takes t... |
28.47385025024414 130 WikiText2 |
23387 = = = Post โ main sequence = = = |
333.9183044433594 10 WikiText2 |
23388 As stars of at least 0 @.@ 4 M โ exhaust their supply of hydrogen at their core , they start to fuse hydrogen in a shell outside the helium core . Their outer layers expand and cool greatly as they form a red giant . In about 5 billion years , when the Sun enters the helium burning phase , it will expand to a ma... |
71.59598541259766 96 WikiText2 |
23389 As the hydrogen shell burning produces more helium , the core increases in mass and temperature . In a red giant of up to 2 @.@ 25 M โ , the mass of the helium core becomes degenerate prior to helium fusion . Finally , when the temperature increases sufficiently , helium fusion begins explosively in what is call... |
45.403717041015625 135 WikiText2 |
23390 After the star has fused the helium of its core , the carbon product fuses producing a hot core with an outer shell of fusing helium . The star then follows an evolutionary path called the asymptotic giant branch ( AGB ) that parallels the other described red giant phase , but with a higher luminosity . The more... |
66.85414123535156 76 WikiText2 |
23391 = = = = Massive stars = = = = |
145.18484497070312 10 WikiText2 |
23392 During their helium @-@ burning phase , stars of more than nine solar masses expand to form red supergiants . When this fuel is exhausted at the core , they continue to fuse elements heavier than helium . |
108.34059143066406 40 WikiText2 |
23393 The core contracts and the temperature and pressure rises enough to fuse carbon ( see Carbon burning process ) . This process continues , with the successive stages being fueled by neon ( see neon burning process ) , oxygen ( see oxygen burning process ) , and silicon ( see silicon burning process ) . Near the e... |
51.715171813964844 106 WikiText2 |
23394 The final stage occurs when a massive star begins producing iron . Since iron nuclei are more tightly bound than any heavier nuclei , any fusion beyond iron does not produce a net release of energy . To a very limited degree such a process proceeds , but it consumes energy . Likewise , since they are more tightl... |
58.01069641113281 134 WikiText2 |
23395 = = = = Collapse = = = = |
52.176204681396484 9 WikiText2 |
23396 As a star 's core shrinks , the intensity of radiation from that surface increases , creating such radiation pressure on the outer shell of gas that it will push those layers away , forming a planetary nebula . If what remains after the outer atmosphere has been shed is less than 1 @.@ 4 M โ , it shrinks to a re... |
47.191837310791016 138 WikiText2 |
23397 In larger stars , fusion continues until the iron core has grown so large ( more than 1 @.@ 4 M โ ) that it can no longer support its own mass . This core will suddenly collapse as its electrons are driven into its protons , forming neutrons , neutrinos , and gamma rays in a burst of electron capture and inverse... |
42.18328857421875 134 WikiText2 |
23398 A supernova explosion blows away the star 's outer layers , leaving a remnant such as the Crab Nebula . The core is compressed into a neutron star , which sometimes manifests itself as a pulsar or X @-@ ray burster . In the case of the largest stars , the remnant is a black hole greater than 4 M โ ) s . In a neu... |
36.719276428222656 120 WikiText2 |
23399 The blown @-@ off outer layers of dying stars include heavy elements , which may be recycled during the formation of new stars . These heavy elements allow the formation of rocky planets . The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium . |
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