Editing Star (section)

==Radiation==
[[File:Eta Carinae.jpg|thumb|upright=1.2|[[Eta Carinae]] is an unstable [[blue hypergiant]] star, roughly 100 times more massive than the Sun, over 700 times wider, and 4 million times more luminous. In a 19th century event termed the Great Eruption, Eta Carinae brightened and violently ejected mass to form the surrounding [[Homunculus Nebula]] (pictured).]]
The energy produced by stars, a product of nuclear fusion, radiates to space as both [[electromagnetic radiation]] and [[particle radiation]]. The particle radiation emitted by a star is manifested as the stellar wind,<ref>{{cite news
 | last=Koppes | first=Steve
 | title=University of Chicago physicist receives Kyoto Prize for lifetime achievements in science
 | publisher=The University of Chicago News Office
 | date=2003-06-20 | url=http://www-news.uchicago.edu/releases/03/030620.parker.shtml
 | access-date=2012-06-15}}</ref> which streams from the outer layers as electrically charged [[proton]]s and [[alpha particle|alpha]] and [[beta particle]]s. A steady stream of almost massless neutrinos emanate directly from the star's core.<ref name='carroll_ostlie_ch11'>{{cite book |last1=Carroll |first1=Bradley W. |last2=Ostlie |first2=Dale A. |title=An Introduction to Modern Astrophysics |location=Cambridge, United Kingdom |publisher=Cambridge University Press |isbn=978-1108422161 |edition=2nd |chapter=Chapter 11|date= 2017 }}</ref>

The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single [[atomic nucleus]] of a new heavier element, [[gamma ray]] [[photon]]s are released from the nuclear fusion product. This energy is converted to other forms of [[electromagnetic energy]] of lower frequency, such as visible light, by the time it reaches the star's outer layers.<ref name='carroll_ostlie_ch10'>{{cite book |last1=Carroll |first1=Bradley W. |last2=Ostlie |first2=Dale A. |title=An Introduction to Modern Astrophysics |location=Cambridge, United Kingdom |publisher=Cambridge University Press |isbn=978-1108422161 |edition=2nd |chapter=Chapter 10|year= 2017 }}</ref>

The color of a star, as determined by the most intense [[frequency]] of the visible light, depends on the temperature of the star's outer layers, including its [[photosphere]].<ref>{{cite web
 |url=http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html
 |title=The Colour of Stars
 |publisher=Australian Telescope Outreach and Education
 |access-date=2006-08-13
 |url-status=dead
 |archive-url=https://web.archive.org/web/20120318151427/http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html
 |archive-date=2012-03-18
}}</ref> Besides visible light, stars emit forms of electromagnetic radiation that are invisible to the [[human eye]]. In fact, stellar electromagnetic radiation spans the entire [[electromagnetic spectrum]], from the longest [[wavelength]]s of [[radio frequency|radio waves]] through [[infrared]], visible light, [[ultraviolet]], to the shortest of [[X-ray]]s, and gamma rays. From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.<ref name='carroll_ostlie_ch11' />

Using the [[stellar spectrum]], astronomers can determine the surface temperature, [[surface gravity]], metallicity and [[rotation]]al velocity of a star. If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be calculated for stars in [[Binary star|binary systems]] by measuring their orbital velocities and distances. [[Gravitational microlensing]] has been used to measure the mass of a single star.<ref>{{cite news
 | title=Astronomers Measure Mass of a Single Star – First Since the Sun
 | publisher=Hubble News Desk | date=2004-07-15 | url=http://hubblesite.org/newscenter/archive/releases/2004/24/text/
 | access-date=2006-05-24}}</ref>) With these parameters, astronomers can estimate the age of the star.<ref>{{cite journal
 | last1=Garnett | first1=D. R. | last2=Kobulnicky | first2=H. A.
 | title=Distance Dependence in the Solar Neighborhood Age-Metallicity Relation
 | journal=The Astrophysical Journal | date=2000
 | volume=532
 | issue= 2 | pages=1192–1196
 | doi= 10.1086/308617
| bibcode=2000ApJ...532.1192G|arxiv= astro-ph/9912031| s2cid=18473242 }}</ref>

===Luminosity===
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 (physics)|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.<ref>{{cite news
 | author=Staff
 | date=2006-01-10
 | title=Rapidly Spinning Star Vega has Cool Dark Equator
 | publisher=National Optical Astronomy Observatory
 | url=http://www.noao.edu/outreach/press/pr06/pr0603.html
 | access-date=2007-11-18
 | archive-date=2019-05-24
 | archive-url=https://web.archive.org/web/20190524103812/https://www.noao.edu/outreach/press/pr06/pr0603.html
 | url-status=dead
 }}</ref>

Patches of the star's surface with a lower temperature and luminosity than average are known as [[sunspot|starspots]]. Small, ''dwarf'' stars such as the Sun generally have essentially featureless disks with only small starspots. ''Giant'' stars have much larger, more obvious starspots,<ref name= "Michelson Starspots">{{cite journal
 |last1=Michelson |first1=A. A. |last2=Pease |first2=F. G.
 |title=Starspots: A Key to the Stellar Dynamo |journal=Living Reviews in Solar Physics
 |volume=2 |issue=1 |pages=8 |date=2005
 |url=http://solarphysics.livingreviews.org/Articles/lrsp-2005-8/|bibcode=2005LRSP....2....8B |doi=10.12942/lrsp-2005-8 |doi-access=free }}</ref> and they exhibit strong stellar [[limb darkening]]. That is, the brightness decreases towards the edge of the stellar disk.<ref>{{cite journal
 |last1=Manduca |first1=A. |last2=Bell |first2=R. A. |last3=Gustafsson |first3=B.
 |title=Limb darkening coefficients for late-type giant model atmospheres
 |journal=Astronomy and Astrophysics |date=1977 |volume=61 |issue=6 |pages=809–813
 |bibcode=1977A&A....61..809M}}</ref> Red dwarf [[flare star]]s such as [[UV Ceti]] may possess prominent starspot features.<ref>{{cite journal   |last1=Chugainov |first1=P. F.
 |title=On the Cause of Periodic Light Variations of Some Red Dwarf Stars
 |journal=Information Bulletin on Variable Stars |date=1971 |volume=520 |pages=1–3 |bibcode=1971IBVS..520....1C}}</ref>

===Magnitude===
{{Main|Apparent magnitude|Absolute 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, the [[Extinction (astronomy)|extinction]] effect of [[interstellar dust]] and gas, 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 the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years).<ref name="Lawrence2019">{{cite book |author=Lawrence |first=J. L. |url=https://books.google.com/books?id=YTyUDwAAQBAJ&pg=PA252 |title=Celestial Calculations: A Gentle Introduction to Computational Astronomy |date=2019 |publisher=Massachusetts Institute of Technology Press |isbn=978-0-262-53663-9 |pages=252}}</ref>

{| class="wikitable" style="float: right; margin-left: 1em;"
|+ Number of stars brighter than magnitude
!Apparent<br />magnitude
!Number&nbsp;<br />of&nbsp;stars<ref>{{cite web |url= http://www.nso.edu/PR/answerbook/magnitude.html |archive-url= https://web.archive.org/web/20080206074842/http://www.nso.edu/PR/answerbook/magnitude.html |archive-date= 2008-02-06 |title= Magnitude |publisher= National Solar Observatory – Sacramento Peak |access-date= 2006-08-23}}</ref>
|- style="text-align: center;"
||0
||4
|- style="text-align: center;"
||1
||15
|- style="text-align: center;"
||2
||48
|- style="text-align: center;"
||3
||171
|- style="text-align: center;"
||4
||513
|- style="text-align: center;"
||5
||1,602
|- style="text-align: center;"
||6
||4,800
|- style="text-align: center;"
||7
||14,000
|}

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<ref name="luminosity">{{cite web|url=http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_luminosity.html |title=Luminosity of Stars |publisher=Australian Telescope Outreach and Education |access-date=2006-08-13 |url-status=dead |archive-url=https://web.archive.org/web/20140809120004/http://www.atnf.csiro.au/outreach//education/senior/astrophysics/photometry_specparallax.html |archive-date=2014-08-09 }}</ref> (the [[nth root|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 star|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.<ref name="Nicolson1999">{{cite book |author=Nicolson |first=Iain |url=https://books.google.com/books?id=5iacbufd4kEC&pg=PA134 |title=Unfolding Our Universe |date=1999 |publisher=Cambridge University Press |isbn=978-0-521-59270-3 |pages=134}}</ref>

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 (Δ''L'') between two stars is calculated by subtracting the magnitude number of the brighter star (''m''<sub>b</sub>) from the magnitude number of the fainter star (''m''<sub>f</sub>), then using the difference as an exponent for the base number 2.512; that is to say:

:<math> \Delta{m} = m_\mathrm{f} - m_\mathrm{b} </math>
:<math>2.512^{\Delta{m}} = \Delta{L}</math>

Relative to both luminosity and distance from Earth, a star's absolute magnitude (''M'') and apparent magnitude (''m'') are not equivalent;<ref name="luminosity" /> 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, the latter star appears the brighter of the two. 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.<ref>{{cite book|title=Astounding Science Fact & Fiction|url=https://books.google.com/books?id=zLkOAQAAIAAJ|year=1960|publisher=Street & Smith|page=7}}</ref>

The [[List of most luminous stars|most luminous known stars]] have absolute magnitudes of roughly −12, corresponding to 6&nbsp;million times the luminosity of the Sun.<ref>{{cite journal |last1=Bestenlehner |first1=Joachim M. |last2=Crowther |first2=Paul A. |last3=Caballero-Nieves |first3=Saida M. |last4=Schneider |first4=Fabian R. N. |last5=Simón-Díaz |first5=Sergio |last6=Brands |first6=Sarah A. |last7=de Koter |first7=Alex |last8=Gräfener |first8=Götz |last9=Herrero |first9=Artemio |last10=Langer |first10=Norbert |last11=Lennon |first11=Daniel J. |last12=Maíz Apellániz |first12=Jesus |last13=Puls |first13=Joachim |last14=Vink |first14=Jorick S. |date=2020-10-17 |title=The R136 star cluster dissected with Hubble Space Telescope/STIS – II. Physical properties of the most massive stars in R136 |journal=Monthly Notices of the Royal Astronomical Society |volume=499 |issue=2 |pages=1918–1936 |arxiv=2009.05136 |bibcode=2020MNRAS.499.1918B |doi=10.1093/mnras/staa2801 |doi-access=free}}</ref> Theoretically, the least luminous stars are at the lower limit of mass at which stars are capable of supporting nuclear fusion of hydrogen in the core; stars just above this limit have been located in the [[NGC 6397]] cluster. The faintest red dwarfs in the cluster are absolute magnitude 15, while a 17th absolute magnitude white dwarf has been discovered.<ref>{{cite web
 | date=August 17, 2006 | url=http://hubblesite.org/newscenter/archive/releases/2006/37/image/a/
 | title=Faintest Stars in Globular Cluster NGC 6397
 | publisher=HubbleSite | access-date=2006-06-08}}</ref><ref>{{cite journal |last1=Richer |first1=H. B. |title=Probing the Faintest Stars in a Globular Star Cluster |journal=Science |date=18 August 2006 |volume=313 |issue=5789 |pages=936–940 |doi=10.1126/science.1130691|pmid=16917054 |arxiv=astro-ph/0702209 |bibcode=2006Sci...313..936R |s2cid=27339792 }}</ref>