Yellow hypergiant

Generally speaking, a yellow hypergiant is a massive star with an extended atmosphere, which can be classified as spectral class from late A to K, with a mass of as much as 20-50 solar masses. Yellow hypergiants, such as Rho Cassiopeiae in the constellation Cassiopeia, have been observed to experience periodic eruptions, resulting in periodic or continuous dimming of the star, respectively. Yellow hypergiants appear to be extremely rare in the universe. Due to their extremely rapid rate of consumption of nuclear fuel, yellow hypergiants generally only remain on the main sequence for a few million years before destroying themselves in a massive supernova or hypernova. Yellow hypergiants are post-red supergiants, rapidly evolving toward the blue supergiant phase. They are in a so-called "Yellow Evolutionary Void," a part of the Hertzsprung-Russell diagram where post-red supergiants exhibit atmospheric instability while evolving blueward; however, there exists strong chemical and surface gravity evidence that the brightest of the yellow supergiants, HD 33579, has not yet been to the red region. They can quickly run out of core nuclear fuel and implode to become Type II supernovae.

Internal structure

According to the current physical models of stars, a yellow hypergiant should possess a convective core surrounded by a radiative zone, as opposed to a sun-sized star, which consists of a radiative core surrounded by a convective zone (Seeds, 2005). Due to the extremely high pressures which exist at the core of a yellow hypergiant, portions of the core or perhaps the entire core may be composed of degenerate matter.

Chromosphere

Due to the sheer size of these stars, in addition to powerful magnetic fields and their extreme energy output, yellow hypergiants are less effective at retaining surface material than other kinds of stars. They therefore have very large, extended atmospheres. A disk of dust and gas can also sometimes be identified, allowing the possibility of planetary systems around these stars.

Known yellow hypergiants

• Rho Cassiopeiae
• V382 Carinae
• V509 Cassiopeiae (= HR 8752)
• IRC+10420
• HD 33579
• IRAS 17163-3907, aka The Fried Egg Nebula

See also

• Hypergiant
• supergiant

References

• Seeds, Michael A., Astronomy: The Solar System and Beyond, Brooks/Cole 2005.
• arXiv.org>astro-ph>0407202 explanation on YHG:s as a missing LBV link.
• http://alobel.freeshell.org/rcas.html
• http://www.solstation.com/x-objects/rho-cas.htm

External links

• The Millennium Outburst of the Star Rho Cassiopeiae