Editing Supergiant (section)

===Luminosity===
The supergiants lie more or less on a horizontal band occupying the entire upper portion of the HR diagram, but there are some variations at different spectral types.  These variations are due partly to different methods for assigning luminosity classes at different spectral types, and partly to actual physical differences in the stars.

The bolometric luminosity of a star reflects its total output of [[electromagnetic radiation]] at all [[wavelength]]s.  For very hot and very cool stars, the bolometric luminosity is dramatically higher than the visual luminosity, sometimes several magnitudes or a factor of five or more.  This [[bolometric correction]] is approximately one magnitude for mid B, late K, and early M stars, increasing to three magnitudes (a factor of 15) for O and mid M stars.

All supergiants are larger and more luminous than main-sequence stars of the same temperature.  This means that hot supergiants lie on a relatively narrow band above bright main-sequence stars.  A B0 main-sequence star has an absolute magnitude of about &minus;5, meaning that all B0 supergiants are significantly brighter than absolute magnitude &minus;5. Bolometric luminosities for even the faintest blue supergiants are tens of thousands of times the Sun ({{solar luminosity}}). The brightest can be {{solar luminosity|over a million}} and are often unstable, such as [[Alpha Cygni variable|α Cygni variable]]s and [[luminous blue variable]]s.

The very hottest supergiants with early O spectral types occur in an extremely narrow range of luminosities above the highly luminous early O main-sequence and giant stars.  They are not classified separately into normal (Ib) and luminous (Ia) supergiants, although they commonly have other spectral type modifiers such as "f" for nitrogen and helium emission (e.g. O2 If for [[HD 93129A]]).<ref name=sota>{{cite journal|bibcode= 2011ApJS..193...24S|title= The Galactic O-Star Spectroscopic Survey. I. Classification System and Bright Northern Stars in the Blue-violet at R ~ 2500|journal= The Astrophysical Journal Supplement|volume= 193|issue= 2|pages= 24|last1= Sota|first1= A.|last2= Maíz Apellániz|first2= J.|last3= Walborn|first3= N. R.|last4= Alfaro|first4= E. J.|last5= Barbá|first5= R. H.|last6= Morrell|first6= N. I.|last7= Gamen|first7= R. C.|last8= Arias|first8= J. I.|year= 2011|doi= 10.1088/0067-0049/193/2/24|arxiv = 1101.4002 |s2cid= 119248206}}</ref>

[[Yellow supergiant]]s can be considerably fainter than absolute magnitude &minus;5, with some examples around &minus;2 (e.g. [[14 Persei]]).  With bolometric corrections around zero, they may only be a few hundred times the luminosity of the Sun. These are not massive stars, though; instead, they are stars of intermediate mass that have particularly low surface gravities, often due to instability such as [[Classical Cepheid variable|Cepheid]] pulsations.  These intermediate-mass stars' being classified as supergiants during a relatively long-lasting phase of their evolution accounts for the large number of low-luminosity yellow supergiants.  The most luminous yellow stars, the [[yellow hypergiant]]s, are amongst the visually brightest stars, with absolute magnitudes around &minus;9, although still less than {{solar luminosity|a million}}.

There is a strong upper limit to the luminosity of [[red supergiant]]s at around {{solar luminosity|half a million}}. Stars that would be brighter than this shed their outer layers so rapidly that they remain hot supergiants after they leave the main sequence. The majority of red supergiants were {{solar mass|10-15}} main-sequence stars and now have luminosities below {{solar luminosity|100,000}}, and there are very few bright supergiant (Ia) M class stars.<ref name=sowell/> The least luminous stars classified as red supergiants are some of the brightest AGB and post-AGB stars, highly expanded and unstable low-mass stars such as the [[RV Tauri variable]]s. The majority of AGB stars are assigned giant or bright giant luminosity classes, but particularly unstable stars such as [[W Virginis variable]]s may be given a supergiant classification (e.g. [[W Virginis]] itself). The faintest red supergiants are around absolute magnitude &minus;3.