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Yellow hypergiant
A yellow hypergiant is a massive star with an extended atmosphere, a spectral class from late A to early K, an initial mass of as much as 20–50 solar masses, but having lost as much as half that mass. They are amongst the most visually luminous stars, with absolute magnitude (MV) around −9, but also one of the rarest with just a handful known in the Milky Way. They are sometimes referred to as cool hypergiants in comparison to O- and B-type stars, and sometimes as warm hypergiants in comparison to red supergiants.
==Classification==
Although some of the yellow hypergiants are naked eye stars that have long been known as interesting objects, the term ''Yellow Hypergiant'' is relatively recent. The term "hypergiant"" was used as early as 1929, but not for the stars currently known as hypergiants. Luminosity class 0 was defined as an extension of the spectral luminosity classes defined by Morgan and Keenan to luminosity higher than bright supergiant (Ia). This was defined to account for the M star VV Cephei. In 1979, the name ''Hypergiant'' was suggested for some highly luminous mass-losing hot stars. It wasn't applied to cooler stars until 1991 when Rho Cassiopeiae was described explicitly as a yellow hypergiant. Yellow hypergiants as a class of stars were discussed at the 1992 ''Solar physics and astrophysics at interferometric resolution'' workshop.
The term hypergiant has been somewhat formalised, but is still quite vague. Formally the luminosity class 0 identifies a hypergiant, but alternatives Ia-0 and Ia+ are also used. Luminosity is determined from various spectral features that are sensitive to the surface gravity, such as the width of the Hβ line in hot stars, and the strength of the Balmer discontinuity in cool stars. Lower surface gravity indicates a larger star and hence higher luminosity. In cooler stars, the width of lines such as OI at 777.4 nm can be calibrated directly against the luminosity of the star.
One approach to definitively identifying yellow hypergiants is the ''Keenan-Smolinski'' criterion: absorption lines should be strongly broadened, beyond that expected for bright supergiant stars and indicating strong mass loss; and at least one broadened Hα component should be present. The Hα line in yellow hypergiants have complex profiles, often but not always strong emission combined with absorption components.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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