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 our galaxy. They are sometimes referred to as cool hypergiants in comparison to O and B stars, and sometimes as warm hypergiants in comparison to red supergiants.
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.
Yellow hypergiants occupy a region of the Hertzsprung-Russell diagram above the instability strip, a region where relatively few stars are found and where those stars are generally unstable. The spectral and temperature ranges are approximately A0-K0 and 4,000-8,000K respectively. The area is bounded on the high temperature side by the "Yellow Evolutionary Void" where stars of this luminosity become extremely unstable and experience severe mass loss. The Yellow Evolutionary Void clearly separates yellow hypergiants from luminous blue variables. Although yellow hypergiants at their hottest and luminous blue variables at their coolest can have approximately the same temperature near 8,000K, At the lower temperature bound, yellow hypergiants and red supergiants are not clearly separated; RW Cephei (4,500K, 555,000 L☉) is an example of a star that shares characteristics of both yellow hypergiants and red supergiants.
Yellow hypergiants have a fairly narrow range of luminosities above 300,000 L☉ (eg. V382 Carinae at 316,000 L☉) and below than the Humphrey-Davidson limit at around 600,000 L☉. With their output peaking in the middle of the visual range, these are the most visually bright stars known with absolute magnitudes around -9 or -10.
They are large and somewhat unstable, with very low surface gravities. Where yellow supergiants have surface gravities (log g) below about 2, the yellow hyergiants have log g near zero, sometimes even negative. In addition they pulsate irregularly, producing small variations in temperature and brightness. This produces very high mass loss rates, and nebulosity is common around the stars. Occasional larger outbursts can temporarily obscure the stars.
They are massive stars with above 10 M☉, but there appears to be an upper limit with the most massive known yellow hypergiants less than 70 M☉.
Chemically, most yellow hypergiants show strong surface enhancement of nitrogen and also of sodium and some other heavy elements. Carbon and oxygen are depleted, while helium is enhanced as expected for a post-main sequence star.
Yellow hypergiants have clearly evolved off the main sequence and so have depleted the hydrogen in their cores. The majority of yellow hypergiants are postulated to be post-red supergiants evolving blueward, while more stable and less luminous yellow supergiants are likely to be evolving to red supergiants for the first time. However there is strong chemical and surface gravity evidence that the brightest of the yellow supergiants, HD 33579, is currently expanding from a blue supergiant to a red supergiant.
These stars are doubly rare because they are very massive, initially hot class O main sequence stars more than 15 times as massive as the Sun, but also because they spend only a few thousand years in the unstable yellow void phase of their lives. In fact, it is difficult to explain even the small number of observed yellow hypergiants, relative to red supergiants of comparable luminosity, from simple models of stellar evolution. The most luminous red supergiants may execute multiple "blue loops", shedding much of their atmosphere, but without actually ever reaching the blue supergiant stage, each one taking only a few decades at most. Conversely, some apparent yellow hypergiants may be hotter stars, such as the "missing" LBVs, masked within a cool pseudo-photosphere.
Recent discoveries of blue supergiant supernova progenitors have also raised the question of whether stars could explode directly from the yellow hypergiant stage. A handful of possible yellow supergiant supernova progenitors have been discovered, but they all appear to be of relatively low mass and luminosity, not hypergiants. SN 2013cu is a type IIb supernova whose progenitor has been directly and clearly observed. It was an evolved star around 8,000K showing extreme mass loss of helium and nitrogen enriched material. Although the luminosity is not known, only a yellow hypergiant or luminous blue variable in outburst would have these properties.
Modern models suggest that stars with a certain range of masses and rotation rates may explode as supernovae without ever becoming blue supergiants again, but many will eventually pass right through the yellow void and become low mass low luminosity Luminous Blue Variables and possibly Wolf-Rayet stars after that. Specifically, more massive stars and those with higher mass loss rates due to rotation or high metallicity will evolve beyond the yellow hypergiant stage to hotter temperatures before reaching core collpase.
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. Because of their extreme luminosity and internal structure, yellow hypergiants suffer high rates of mass loss and are generally surrounded by envelopes of expelled material. A photogenic example of the nebulae that can result is IRAS 17163-3907, known as the Fried Egg, which has expelled several solar masses of material in just a few hundred years.
The yellow hypergiant is an expected phase of evolution as the most luminous red supergiants evolve bluewards, but they may also represent a different sort of star. LBVs during eruption have such dense winds that they form a pseudo-photosphere which appears as a larger cooler star despite the underlying blue supergiant being largely unchanged. These are observed to have a very narrow range of temperatures around 8,000K. At the bistability jump which occurs around 21,000K blue supergiant winds become several times denser and could be result in an even cooler pseudo-photosphere. No LBVs are observed just below the luminosity where the bistability jump crosses the S Doradus instability strip (not to be confused with the Cepheid instability strip), but it is theorised that they do exist and appear as yellow hypergiants because of their pseudo-photospheres.
Known yellow hypergiants
- Rho Cassiopeiae
- V509 Cassiopeiae
- IRC+10420 (V1302 Aql)
- IRAS 18357-0604
- V766 Centauri (= HR 5171A)
- HD 179821
- IRAS 17163-3907
- V382 Carinae
In other galaxies:
- HD 7583 (R45 in SMC)
- HD 33579 (in LMC)
- HD 269723 (R117 in LMC)
- HD 269953 (R150 in LMC)
- HD 268757 (R59 in LMC)
- Variable A (in M33)
- B324 (in M33)
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