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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 ... |
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 dar... |
= = = 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 i... |
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 se... |
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 subtractin... |
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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 ... |
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... |
= = 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 increasin... |
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 s... |
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 star... |
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... |
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 Ce... |
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 ... |
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 temperatu... |
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 . Ev... |
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 sta... |
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 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... |
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 ... |
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 rapid... |
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 rela... |
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 massiv... |
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 ... |
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... |
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 " Rollerc... |
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 undergroun... |
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 ... |
= = 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 ... |
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 Doofensh... |
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 R... |
= = 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 ... |
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 Am... |
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 Pov... |
= = = 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 ... |
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 ... |
= = = 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 re... |
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 butt... |
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 ... |
= = 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 ... |
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... |
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