Yellow hypergiant

A yellow hypergiant is a massive star with an extended atmosphere, a spectral class from A to K, and an initial mass of about 20–60 solar masses but having lost as much as half that mass. They are amongst the most visually luminous stars, with absolute magnitude (M_V) around −9, but also one of the rarest with just 15 known in the Milky Way and six of those in just a single cluster. 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.

The term "hypergiant"" was used as early as 1929, but not for the stars currently known as hypergiants. Hypergiants are defined by their '0' luminosity class, and are higher in luminosity than the brightest supergiants of class Ia, although they were not referred to as hypergiants until the late 1970s. Another criteria for hypergiants was also suggested in 1979 for some other highly luminous mass-losing hot stars, but was not applied to cooler stars. In 1991, Rho Cassiopeiae was the first to be described as a yellow hypergiant, likely becoming grouped as a new class of luminous stars during discussions at the Solar physics and astrophysics at interferometric resolution workshop in 1992.

Definitions of the term hypergiant remains vague, and although luminosity class 0 is for hypergiants, they are more commonly designated by the alternative luminosity classes Ia-0 and Ia⁺. Their great stellar luminosities are determined from various spectral features, which are sensitive to surface gravity, such as Hβ line widths in hot stars or a strong Balmer discontinuity in cooler stars. Lower surface gravity often indicates larger stars, and hence, higher luminosities. In cooler stars, the width of observed oxygen lines, such as O I at 777.4 nm., can be used to calibrate directly against stellar luminosity.

One astrophysical method used to definitively identify yellow hypergiants is the so-called Keenan-Smolinski criterion. Here all absorption lines should be strongly broadened, beyond those expected with bright supergiant stars, and also show strong evidence of significant mass loss. Furthermore, at least one broadened Hα component should also be present. They may also display very complex Hα profiles, typically having strong emission lines combined with absorption lines.

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