Editing Luminosity (section)

== Stellar luminosity ==
A star's luminosity can be determined from two stellar characteristics: size and [[effective temperature]].<ref name="AUSTRALIA2004">{{cite web |title=Luminosity of Stars |publisher=[[Australia Telescope National Facility]]|url=http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_luminosity.html |date=12 July 2004 |archive-url=https://web.archive.org/web/20140809144429/http://www.atnf.csiro.au/outreach//education/senior/astrophysics/photometry_luminosity.html |archive-date=9 August 2014}}</ref> The former is typically represented in terms of solar [[radius|radii]], ''R''<sub>⊙</sub>, while the latter is represented in [[kelvin]]s, but in most cases neither can be measured directly.  To determine a star's radius, two other metrics are needed: the star's [[angular diameter]] and its distance from Earth.  Both can be measured with great accuracy in certain cases, with cool supergiants often having large angular diameters, and some cool evolved stars having [[astrophysical maser|maser]]s in their atmospheres that can be used to measure the parallax using [[VLBI]].  However, for most stars the angular diameter or parallax, or both, are far below our ability to measure with any certainty.  Since the effective temperature is merely a number that represents the temperature of a black body that would reproduce the luminosity, it obviously cannot be measured directly, but it can be estimated from the spectrum.

An alternative way to measure stellar luminosity is to measure the star's apparent brightness and distance.  A third component needed to derive the luminosity is the degree of [[Extinction (astronomy)|interstellar extinction]] that is present, a condition that usually arises because of gas and dust present in the [[interstellar medium]] (ISM), the [[Earth's atmosphere]], and [[circumstellar dust|circumstellar matter]].  Consequently, one of astronomy's central challenges in determining a star's luminosity is to derive accurate measurements for each of these components, without which an accurate luminosity figure remains elusive.<ref name="KARTTUNEN1">{{cite book  | last1 = Karttunen  | first1 = Hannu | title = Fundamental Astronomy  | publisher = [[Springer-Verlag]]  | date = 2003  | page = 289  | url=https://books.google.com/books?id=OEhHqwW-kgQC | isbn = 978-3-540-00179-9}}</ref> Extinction can only be measured directly if the actual and observed luminosities are both known, but it can be estimated from the observed colour of a star, using models of the expected level of reddening from the interstellar medium.

In the current system of [[stellar classification]], stars are grouped according to temperature, with the massive, very young and energetic [[O-type main sequence star|Class O]] stars boasting temperatures in excess of 30,000&nbsp;[[kelvin|K]] while the less massive, typically older [[Stellar classification#Class M|Class M]] stars exhibit temperatures less than 3,500&nbsp;K. Because luminosity is proportional to temperature to the fourth power, the large variation in stellar temperatures produces an even vaster variation in stellar luminosity.<ref name="LEDREW1">{{cite journal | author=Ledrew, Glenn | title=The Real Starry Sky | journal=Journal of the Royal Astronomical Society of Canada |date=February 2001 | volume=95 | pages=32–33 | url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?2001JRASC..95...32L&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf | access-date=2 July 2012 | bibcode=2001JRASC..95...32L}}</ref> Because the luminosity depends on a high power of the stellar mass, high mass luminous stars have much shorter lifetimes.  The most luminous stars are always young stars, no more than a few million years for the most extreme.  In the [[Hertzsprung–Russell diagram]], the x-axis represents temperature or spectral type while the y-axis represents luminosity or magnitude.  The vast majority of stars are found along the [[main sequence]] with blue Class O stars found at the top left of the chart while red Class M stars fall to the bottom right.  Certain stars like [[Deneb]] and [[Betelgeuse]] are found above and to the right of the main sequence, more luminous or cooler than their equivalents on the main sequence.  Increased luminosity at the same temperature, or alternatively cooler temperature at the same luminosity, indicates that these stars are larger than those on the main sequence and they are called giants or supergiants.

Blue and white supergiants are high luminosity stars somewhat cooler than the most luminous main sequence stars.  A star like [[Deneb]], for example, has a luminosity around 200,000 ''L''<sub>⊙</sub>, a spectral type of A2, and an effective temperature around 8,500&nbsp;K, meaning it has a radius around {{convert|203|solar radius|m|abbr=on|lk=on}}. For comparison, the red supergiant [[Betelgeuse]] has a luminosity around 100,000 ''L''<sub>⊙</sub>, a spectral type of M2, and a temperature around 3,500&nbsp;K, meaning its radius is about {{convert|1000|solar radius|m|abbr=on|lk=on}}. Red supergiants are the largest type of star, but the most luminous are much smaller and hotter, with temperatures up to 50,000&nbsp;K and more and luminosities of several million ''L''<sub>⊙</sub>, meaning their radii are just a few tens of ''R''<sub>⊙</sub>.  For example, [[R136a1]] has a temperature over 46,000&nbsp;K and a luminosity of more than 6,100,000 ''L''<sub>⊙</sub><ref name="census">{{cite journal|last1=Doran|first1=E. I.|last2=Crowther|first2=P. A.| last3=de Koter|first3=A.|last4=Evans|first4=C. J.|last5=McEvoy|first5=C.|last6=Walborn|first6=N. R.|last7=Bastian|first7=N.|last8=Bestenlehner|first8=J. M.| last9=Gräfener|first9=G.| last10=Herrero|first10=A.|last11=Kohler|first11=K.|last12=Maiz Apellaniz|first12=J.|last13=Najarro|first13=F.| last14=Puls|first14=J.| last15=Sana|first15=H.| last16=Schneider|first16=F. R. N.|last17=Taylor|first17=W. D.|last18=van Loon|first18=J. Th.|last19=Vink|first19=J. S.| title=The VLT-FLAMES Tarantula Survey - XI. A census of the hot luminous stars and their feedback in 30 Doradus|journal=Astronomy & Astrophysics| volume=558| pages=A134| arxiv=1308.3412v1| date=2013| doi=10.1051/0004-6361/201321824|bibcode=2013A&A...558A.134D|s2cid=118510909}}</ref> (mostly in the UV), it is only {{convert|39|solar radius|m|abbr=on|lk=on}}.