text stringlengths 1 3.86k |
|---|
The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time . It has units of power . The luminosity of a star is determined by its radius and surface temperature . Many stars do not radiate uniformly across their entire surface . The rapidly rotating star Vega , for example , has a higher energy flux ( power per unit area ) at its poles than along its equator . |
Patches of the star 's surface with a lower temperature and luminosity than average are known as starspots . Small , dwarf stars such as our Sun generally have essentially featureless disks with only small starspots . Giant stars have much larger , more obvious starspots , and they also exhibit strong stellar limb darkening . That is , the brightness decreases towards the edge of the stellar disk . Red dwarf flare stars such as UV Ceti may also possess prominent starspot features . |
= = = Magnitude = = = |
The apparent brightness of a star is expressed in terms of its apparent magnitude . It is a function of the star 's luminosity , its distance from Earth , and the altering of the star 's light as it passes through Earth 's atmosphere . Intrinsic or absolute magnitude is directly related to a star 's luminosity , and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs ( 32 @.@ 6 light @-@ years ) . |
Both the apparent and absolute magnitude scales are logarithmic units : one whole number difference in magnitude is equal to a brightness variation of about 2 @.@ 5 times ( the 5th root of 100 or approximately 2 @.@ 512 ) . This means that a first magnitude star ( + 1 @.@ 00 ) is about 2 @.@ 5 times brighter than a second magnitude ( + 2 @.@ 00 ) star , and about 100 times brighter than a sixth magnitude star ( + 6 @.@ 00 ) . The faintest stars visible to the naked eye under good seeing conditions are about magnitude + 6 . |
On both apparent and absolute magnitude scales , the smaller the magnitude number , the brighter the star ; the larger the magnitude number , the fainter the star . The brightest stars , on either scale , have negative magnitude numbers . The variation in brightness ( ΔL ) between two stars is calculated by subtracting the magnitude number of the brighter star ( mb ) from the magnitude number of the fainter star ( mf ) , then using the difference as an exponent for the base number 2 @.@ 512 ; that is to say : |
<formula> |
<formula> |
Relative to both luminosity and distance from Earth , a star 's absolute magnitude ( M ) and apparent magnitude ( m ) are not equivalent ; for example , the bright star Sirius has an apparent magnitude of − 1 @.@ 44 , but it has an absolute magnitude of + 1 @.@ 41 . |
The Sun has an apparent magnitude of − 26 @.@ 7 , but its absolute magnitude is only + 4 @.@ 83 . Sirius , the brightest star in the night sky as seen from Earth , is approximately 23 times more luminous than the Sun , while Canopus , the second brightest star in the night sky with an absolute magnitude of − 5 @.@ 53 , is approximately 14 @,@ 000 times more luminous than the Sun . Despite Canopus being vastly more luminous than Sirius , however , Sirius appears brighter than Canopus . This is because Sirius is merely 8 @.@ 6 light @-@ years from the Earth , while Canopus is much farther away at a distance of 310 light @-@ years . |
As of 2006 , the star with the highest known absolute magnitude is LBV 1806 @-@ 20 , with a magnitude of − 14 @.@ 2 . This star is at least 5 @,@ 000 @,@ 000 times more luminous than the Sun . The least luminous stars that are currently known are located in the NGC 6397 cluster . The faintest red dwarfs in the cluster were magnitude 26 , while a 28th magnitude white dwarf was also discovered . These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth . |
= = Classification = = |
The current stellar classification system originated in the early 20th century , when stars were classified from A to Q based on the strength of the hydrogen line . It thought that the hydrogen line strength was a simple linear function of temperature . Rather , it was more complicated ; it strengthened with increasing temperature , it peaked near 9000 K , and then declined at greater temperatures . When the classifications were reordered by temperature , it more closely resembled the modern scheme . |
Stars are given a single @-@ letter classification according to their spectra , ranging from type O , which are very hot , to M , which are so cool that molecules may form in their atmospheres . The main classifications in order of decreasing surface temperature are : O , B , A , F , G , K , and M. A variety of rare spectral types are given special classifications . The most common of these are types L and T , which classify the coldest low @-@ mass stars and brown dwarfs . Each letter has 10 sub @-@ divisions , numbered from 0 to 9 , in order of decreasing temperature . However , this system breaks down at extreme high temperatures as classes O0 and O1 may not exist . |
In addition , stars may be classified by the luminosity effects found in their spectral lines , which correspond to their spatial size and is determined by their surface gravity . These range from 0 ( hypergiants ) through III ( giants ) to V ( main sequence dwarfs ) ; some authors add VII ( white dwarfs ) . Most stars belong to the main sequence , which consists of ordinary hydrogen @-@ burning stars . These fall along a narrow , diagonal band when graphed according to their absolute magnitude and spectral type . The Sun is a main sequence G2V yellow dwarf of intermediate temperature and ordinary size . |
Additional nomenclature , in the form of lower @-@ case letters added to the end of the spectral type to indicate peculiar features of the spectrum . For example , an " e " can indicate the presence of emission lines ; " m " represents unusually strong levels of metals , and " var " can mean variations in the spectral type . |
White dwarf stars have their own class that begins with the letter D. This is further sub @-@ divided into the classes DA , DB , DC , DO , DZ , and DQ , depending on the types of prominent lines found in the spectrum . This is followed by a numerical value that indicates the temperature . |
= = Variable stars = = |
Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties . Of the intrinsically variable stars , the primary types can be subdivided into three principal groups . |
During their stellar evolution , some stars pass through phases where they can become pulsating variables . Pulsating variable stars vary in radius and luminosity over time , expanding and contracting with periods ranging from minutes to years , depending on the size of the star . This category includes Cepheid and Cepheid @-@ like stars , and long @-@ period variables such as Mira . |
Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events . This group includes protostars , Wolf @-@ Rayet stars , and flare stars , as well as giant and supergiant stars . |
Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties . This group includes novae and supernovae . A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions , including the nova and a Type 1a supernova . The explosion is created when the white dwarf accretes hydrogen from the companion star , building up mass until the hydrogen undergoes fusion . Some novae are also recurrent , having periodic outbursts of moderate amplitude . |
Stars can also vary in luminosity because of extrinsic factors , such as eclipsing binaries , as well as rotating stars that produce extreme starspots . A notable example of an eclipsing binary is Algol , which regularly varies in magnitude from 2 @.@ 3 to 3 @.@ 5 over a period of 2 @.@ 87 days . |
= = Structure = = |
The interior of a stable star is in a state of hydrostatic equilibrium : the forces on any small volume almost exactly counterbalance each other . The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star . The pressure gradient is established by the temperature gradient of the plasma ; the outer part of the star is cooler than the core . The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen @-@ burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star . |
As atomic nuclei are fused in the core , they emit energy in the form of gamma rays . These photons interact with the surrounding plasma , adding to the thermal energy at the core . Stars on the main sequence convert hydrogen into helium , creating a slowly but steadily increasing proportion of helium in the core . Eventually the helium content becomes predominant , and energy production ceases at the core . Instead , for stars of more than 0 @.@ 4 M ☉ , fusion occurs in a slowly expanding shell around the degenerate helium core . |
In addition to hydrostatic equilibrium , the interior of a stable star will also maintain an energy balance of thermal equilibrium . There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior . The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below . |
The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer , since convective heat transfer is inefficient in that zone . In this region the plasma will not be perturbed , and any mass motions will die out . If this is not the case , however , then the plasma becomes unstable and convection will occur , forming a convection zone . This can occur , for example , in regions where very high energy fluxes occur , such as near the core or in areas with high opacity ( making radiatative heat transfer inefficient ) as in the outer envelope . |
The occurrence of convection in the outer envelope of a main sequence star depends on the star 's mass . Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers . Smaller stars such as the Sun are just the opposite , with the convective zone located in the outer layers . Red dwarf stars with less than 0 @.@ 4 M ☉ are convective throughout , which prevents the accumulation of a helium core . For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified . |
The photosphere is that portion of a star that is visible to an observer . This is the layer at which the plasma of the star becomes transparent to photons of light . From here , the energy generated at the core becomes free to propagate into space . It is within the photosphere that sun spots , regions of lower than average temperature , appear . |
Above the level of the photosphere is the stellar atmosphere . In a main sequence star such as the Sun , the lowest level of the atmosphere , just above the photosphere , is the thin chromosphere region , where spicules appear and stellar flares begin . Above this is the transition region , where the temperature rapidly increases within a distance of only 100 km ( 62 mi ) . Beyond this is the corona , a volume of super @-@ heated plasma that can extend outward to several million kilometres . The existence of a corona appears to be dependent on a convective zone in the outer layers of the star . Despite its high temperature , and the corona emits very little light , due to its low gas density . The corona region of the Sun is normally only visible during a solar eclipse . |
From the corona , a stellar wind of plasma particles expands outward from the star , until it interacts with the interstellar medium . For the Sun , the influence of its solar wind extends throughout a bubble @-@ shaped region called the heliosphere . |
= = Nuclear fusion reaction pathways = = |
A variety of nuclear fusion reactions take place in the cores of stars , that depend upon their mass and composition . When nuclei fuse , the mass of the fused product is less than the mass of the original parts . This lost mass is converted to electromagnetic energy , according to the mass @-@ energy equivalence relationship E = mc2 . |
The hydrogen fusion process is temperature @-@ sensitive , so a moderate increase in the core temperature will result in a significant increase in the fusion rate . As a result , the core temperature of main sequence stars only varies from 4 million kelvin for a small M @-@ class star to 40 million kelvin for a massive O @-@ class star . |
In the Sun , with a 10 @-@ million @-@ kelvin core , hydrogen fuses to form helium in the proton @-@ proton chain reaction : |
41H → 22H + 2e + + 2νe ( 2 x 0 @.@ 4 MeV ) |
2e + + 2e- → 2γ ( 2 x 1 @.@ 0 MeV ) |
21H + 22H → 23He + 2γ ( 2 x 5 @.@ 5 MeV ) |
23He → 4He + 21H ( 12 @.@ 9 MeV ) |
These reactions result in the overall reaction : |
41H → 4He + 2e + + 2γ + 2νe ( 26 @.@ 7 MeV ) |
where e + is a positron , γ is a gamma ray photon , νe is a neutrino , and H and He are isotopes of hydrogen and helium , respectively . The energy released by this reaction is in millions of electron volts , which is actually only a tiny amount of energy . However enormous numbers of these reactions occur constantly , producing all the energy necessary to sustain the star 's radiation output . In comparison , the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5 @.@ 7 eV . |
In more massive stars , helium is produced in a cycle of reactions catalyzed by carbon called the carbon @-@ nitrogen @-@ oxygen cycle . |
In evolved stars with cores at 100 million kelvin and masses between 0 @.@ 5 and 10 M ☉ , helium can be transformed into carbon in the triple @-@ alpha process that uses the intermediate element beryllium : |
4He + 4He + 92 keV → 8 * Be |
4He + 8 * Be + 67 keV → 12 * C |
12 * C → 12C + γ + 7 @.@ 4 MeV |
For an overall reaction of : |
34He → 12C + γ + 7 @.@ 2 MeV |
In massive stars , heavier elements can also be burned in a contracting core through the neon burning process and oxygen burning process . The final stage in the stellar nucleosynthesis process is the silicon burning process that results in the production of the stable isotope iron @-@ 56 , an endothermic process that consumes energy , and so further energy can only be produced through gravitational collapse . |
The example below shows the amount of time required for a star of 20 M ☉ to consume all of its nuclear fuel . As an O @-@ class main sequence star , it would be 8 times the solar radius and 62 @,@ 000 times the Sun 's luminosity . |
= Perry the Platypus = |
Perry the Platypus , also known as Agent P or simply Perry , is an anthropomorphic platypus from the animated series Phineas and Ferb . Perry was created by the series ' co @-@ founders , Dan Povenmire and Jeff " Swampy " Marsh . He first appeared along with the majority of the main cast in the pilot episode " Rollercoaster . " Perry is featured as the star of the B @-@ plot for every episode of the series , alongside his nemesis Dr. Heinz Doofenshmirtz . A mostly silent character , his lone vocal characteristic ( a rattling of Perry 's beak ) was provided by Dee Bradley Baker . |
Perry is the pet platypus of the Flynn @-@ Fletcher family , and is perceived as mindless and domesticated . In secret , however , he lives a double life as a member of an all @-@ animal espionage organization referred to as O.W.C.A. ( The Organization Without a Cool Acronym ) . Many secret entrances to his underground lair exist all around the house ; such as the side of the house , most notably the tree that his owners sit under in the backyard , and several other everyday objects that seem to elude the family 's attention . He engages in daily battles with Dr. Heinz Doofenshmirtz , an evil scientist who desires to take over the Tri @-@ state area with obscure contraptions that work perfectly according to his intended function but fail in his application of them every time . |
Perry was made a platypus because of the animal 's striking appearance and the lack of public knowledge of the animal , which allowed the writers to make things up about the species . Critical reception for the character from both professionals and fans have been considerably positive . Merchandising of the character include plush toys , t @-@ shirts , wooden toys , glasses , and coloring books , along with appearances in literature and a 2009 video game for the Nintendo DS . |
= = Role in Phineas and Ferb = = |
Perry is the docile pet platypus of the blended Flynn @-@ Fletcher family , who adopted him because his unfocused gaze made it seem as if he were looking at both Phineas and Ferb at the same time , as shown in the 2011 movie , Phineas and Ferb the Movie : Across the 2nd Dimension . Unbeknownst to them , Perry lives a double life as a crime @-@ fighting spy working for the " Organization Without a Cool Acronym " / The OWCA , going by the codename " Agent P. " He reports to his superior , Major Monogram , via telecast in his large , high @-@ tech , underground hideout . Every day , he engages in battles with the evil scientist Dr. Heinz Doofenshmirtz , who tries using inventions to take over the tri @-@ state area . Perry is always able to foil Doofenshmirtz 's plans and in doing so accidentally leads to the destruction of whatever form of contraption his owners , Phineas Flynn and Ferb Fletcher , are building in order to make summer better . Phineas and Ferb are aware that something happens to get rid of their scheme for the day , but do not know that Perry is the cause behind it and are largely dismissive of it . Their sister , Candace , also does not know that Perry is behind the destruction and is driven to near insanity trying to figure it out . Throughout the series , Perry is aware of Phineas and Ferb 's inventions , but is largely uninterested in them , save whenever he notices that their latest invention may help him thwart Doofenshmirtz . |
Perry and Doofenshmirtz at first seem to loathe each other in the beginning of the series , and have been arch @-@ nemeses since the day they met . However , they are often cordial and friendly towards one another and it is said by Doofenshmirtz that Perry is his best friend , and Perry will often act to save Doofenshmirtz 's life when his plot inevitably blows up in his face . Habitually , their daily brawls involve Doofenshmirtz devising a scheme , which Perry goes to stop after being briefed by Major Monogram . He is trapped by Doofenshmirtz while trying to do so and is told of the doctor 's scheme , usually pertaining to some backstory or pet peeve . He then escapes and the two fight , Perry coming out victorious . The two rely on this daily structure , Doofenshmirtz even specifically mentioning it in " Journey to the Center of Candace " and in episodes such as " It 's About Time ! " in which Doofenshmirtz temporarily replaces Perry with secret agent Peter The Panda and they become depressed about not having each other to fight . Perry realizes he misses Doof too . When Perry does not arrive on the scene of Doofenshmirtz 's evil plan , the doctor hesitates to execute his plans and fears for where Perry has gone , though he notes that he " hopes something terrible has happened to him . " . Sometimes , they decided not to fight and have fun , as shown in " Happy New Year ! " and " Candace Disconnected " . On other occasions , depending on whatever situation Doofenshmirtz is facing , Perry would often help Doofenshmirtz with his non @-@ evil plans , such as helping him overcome evil scientist 's block , helping him put on a birthday party for his 16 @-@ year @-@ old daughter Vanessa , impressing a square dancing girl with programmable square dancing boots that Doofenshmritz created , working together to stop a raging platypus hunter from hunting them down , or helping him convince his rich ex @-@ wife to help pay off his mortgage debt . Also , Perry tends to use Doofenshmirtz 's inventions to erase evidence of whatever contraptions Phineas and Ferb had made , leaving Candace unable to bust them . Examples include when Perry asked to borrow Doofenshmirtz 's robot Norm to pick up footage from the city surveillance cameras in order to preserve his job as an agent , as well as using Doofenshmirtz 's Pick @-@ Him @-@ Up @-@ inator to rescue a lost Candace and bring her home . |
In Phineas and Ferb the Movie : Across the 2nd Dimension , an alternate reality ( yet more evil and ruthless ) version of Perry ( known as Platyborg ) appears as the secondary antagonist , with the regular Perry serving as a supporting protagonist . Platyborg would later return in the episode sequel , Tales from the Resistance : Back to the 2nd Dimension , this time , as a supporting protagonist . |
= = Character = = |
= = = Creation and conception = = = |
While working on the animated television series Rocko 's Modern Life , Phineas and Ferb co @-@ founders Dan Povenmire and Jeff " Swampy " Marsh utilized several recurring elements in the episodes that they wrote . Among these were actions sequences and chase scenes . Povenmire and Marsh wanted to reuse these elements in their series and chose Perry to execute it . The pair gave him a consistent and continuous nemesis in the form of Doofenshmirtz as a means of allowing viewers to get to know him . |
While choosing a species for Perry , Povenmire and Marsh wanted to keep in mind selecting one that was uncommon , an animal that kids could not " pick out at a pet store and beg [ their parents ] for . " They chose a platypus because of the animal 's obscure and striking appearance . The animal was scarcely used in American animated programs , so the pair opined that Perry " would not have to compete with preconceived notions that viewers might bring to a more conventional critter . " The blank storyboard they were given allowed them to " make stuff up " since " no one knows very much about [ them ] . " |
Perry has a theme song tentatively entitled " Perry , " performed by Randy Crenshaw and Laura Dickinson , and written by Povenmire and Marsh , who write the majority of songs in the series . The song , along with the number " Gitchee Gitchee Goo " from the episode " Flop Starz , " was the first musical composition Povenmire and Marsh pitched to The Walt Disney Company . They were nervous doing so , because , as Povenmire explained , " Disney has a big history of music -- what if they hate it ? " Their reaction , however , was considerably positive and the pair was asked to write a song for each episode , which they vehemently agreed to . The opening lyrics for the song describe Perry as a standard textbook definition of a platypus : " He 's a semi @-@ aquatic egg @-@ laying mammal of action . " |
= = = Design = = = |
Like the other characters of the series , Perry was structured in a simple style to allow young viewers to easily draw him . In keeping with the show 's general design scheme , Perry is constructed of geometric shapes in a style reminiscent of deceased Looney Tunes animator Tex Avery . Povenmire uses different design styles for drawing Perry depending on how he is portrayed . When Perry is portrayed as a domesticated and mindless pet , Povenmire begins with a square shaped like a loaf of bread . He then draws his front legs and feet before drawing his bill , which is set at a certain angle . Povenmire then draws his eyeballs , which are never focused and look in opposing directions in a daze . He concludes the figure by adding his hind legs , tail , hair , and finally color . |
When portrayed as a secret agent , Povenmire starts with a similar bread loaf square design , but draws it standing up vertically and places a fedora on the top of his head , which is combined with the square torso . In contrast to his mindless and wild @-@ eyed expression as a pet , the secret agent version of Perry is detailed with eyes " full of steely blue resolve . " Povenmire proceeds by drawing his arms , which bear hands that are open and prepared for fighting or any danger . His legs are bent , as well prepared for an act of danger or action needed . Povenmire finishes the design by adding his beaver tail and color . |
= = = Personality = = = |
Perry 's undercover identity as a pet leads to characters throughout the series to deem him as a " mindless domestic pet " that " doesn 't do much " . This definition of the character led to Phineas and Ferb spearheading production of a new toyline based on Perry called " Perry the Inaction Figure , " whose tagline revolved around it not doing anything but allowing the customer to make it whatever they desire it to be . In all actuality , Perry is a skilled fighter who is able to perform several implausible judo fighting moves and escapes . He has access to several different types of technology and inventions provided to him by The O.W.C.A. , including a hover craft dubbed the " Platypusmobile , " a jet ski , and a whistle set that allows him to summon different types of animals . |
Although his appearance as a domestic pet is mostly a cover , Perry has nonetheless expressed care and devotion for the Flynn @-@ Fletcher family . In the episode " The Ballad of Bad Beard " , Perry managed to drag Candace out of the Dark Cave before it caved in because of Dr. Doofenshmirthz 's " self destruction button " that which Candace had accidentally pressed while under the orange moss hallucination , thinking it was a vending machine . He did this even though he knew that Candace saw that Perry was a secret agent talking to Agent E ( Eagle ) and to Major Monogram in a cracked egg . Candace thought she was just hallucinating because of the orange moss she accidentally touched earlier in the episode . Also , when an invention of Doofenshmirtz 's in the episode " Journey to the Center of Candace " might cause severe harm to Phineas and Ferb , Perry thoroughly beats up Doofenshmirtz and quickly handcuffs him . He then ties him to a pipe and , instead of just leaving him like he usually does in the series , calls for special forces from the O.W.C.A. to come and arrest the doctor , though they never show up . In " Oh , There You Are , Perry , " Doofenshmirtz is downgraded to a low threat level and Perry is reassigned to a new nemesis . This causes him to have to leave his home with the Flynn @-@ Fletchers , which makes him sad and he does not enjoy his new villain . Phineas and Ferb are fearful about where Perry could have gone to and put up several flyers around town for people to find him . They decide to throw a concert on the roof of a building , singing a song about how much they love Perry and want him to come home . Concurrently , Perry is reassigned to Doofenshmirtz again and returns to the brothers once more . |
Though Perry is anthropomorphic , he does not speak , only communicating through an " aggravated purr " type of noise made by flapping his bill . This noise is provided by actor Dee Bradley Baker ; to this day , Povenmire and Marsh do not understand how Baker produces the noise . Thomas Sangster , who plays Ferb , is notoriously good at making the noise and is considered second only to Baker himself . When in his " Agent P " persona , Perry is almost completely silent . |
= = Reception = = |
As a character , Perry has been well received by both fans and critics . As Cynthia Littleton of Variety magazine wrote , " Perry the platypus is a breakout star from the Disney Channel toon ' Phineas and Ferb . ' " The New York Times writer Susan Stewart called him " intrepid . " Jean Yoo , a press member for Disney Channel , said that Perry is " suave " and " makes James Bond look like a rank amateur . " Aaron H. Bynum of Animator Insider opined that he is " indubitably confident . " Carly H. of Scholastic , Inc. declared Perry " pretty darn awesome . " Josh Jackson , editor of Paste , described Perry and Doofenshmirtz 's relationship as " pitch @-@ perfect . " |
Certain reviewers have also commented negatively about Perry and his subplot , which Sherry Robinson of the St. Petersburg Times considers " pretty trippy . " Kevin McDonough of Sun Coast Today described it as " complicated " and " loosely connected " to the rest of the series , writing that he is " not sure what this accomplishes except to add the noise of explosions to the already constant din of singing and screaming . " Ed Liu of Toon Zone feels it is " truly puzzling what Perry the secret agent is doing in this show in the first place . " Liu considers his subplot to be a type of " throw everything against the wall and see what sticks " element and writes that " many of the earlier episodes of the show don 't manage to do a very good job of balancing the subplot and the main one . " |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.