A yellow hypergiant (YHG) is a massive star with an extended atmosphere, a spectral class from A to K, and, starting with an initial mass of about 20–60 solar masses, has 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 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 with O- and B-type stars, and sometimes as warm hypergiants in comparison with red supergiants.
Classification
The term "hypergiant" was used as early as 1929, but not for the stars currently known as hypergiants.[1] Hypergiants are defined by their '0' luminosity class, and are higher in luminosity than the brightest supergiants of class Ia,[2] although they were not referred to as hypergiants until the late 1970s.[3] Another criterion for hypergiants was also suggested in 1979 for some other highly luminous mass-losing hot stars,[4] but was not applied to cooler stars. In 1991, Rho Cassiopeiae was the first to be described as a yellow hypergiant,[5] likely becoming grouped as a new class of luminous stars during discussions at the Solar physics and astrophysics at interferometric resolution workshop in 1992.[6]
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+.[7] 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.[8] In cooler stars, the strength of observed oxygen lines, such as O I at 777.4 nm., can be used to calibrate directly against stellar luminosity.[9]
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 of 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.[10]
The terminology of yellow hypergiants is further complicated by referring to them as either cool hypergiants or warm hypergiants, depending on the context. Cool hypergiants refers to all sufficiently luminous and unstable stars cooler than blue hypergiants and LBVs, including both yellow and red hypergiants.[11] The term warm hypergiants has been used for highly luminous class A and F stars in M31 and M33 that are not LBVs,[12] as well as more generally for yellow hypergiants.[13]
Characteristics
Visual light curve for ρ Cassiopeiae from 1933 to 2015
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-K2 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” 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 8,000 K. At the lower temperature bound, yellow hypergiants and red supergiants are not clearly separated; RW Cephei (roughly 4,000 K, 295,000 L☉) is an example of a star that shares characteristics of both yellow hypergiants and red supergiants.[14][15]
Yellow hypergiants have a fairly narrow range of luminosities above 200,000 L☉ (e.g. V382 Carinae at 212,000 L☉) and below 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 −9.5 .[5]
They are large and somewhat unstable, with very low surface gravities. Where yellow supergiants have surface gravities (log g) below about 2, the yellow hypergiants have log g around zero. 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.[16] Occasional larger outbursts can temporarily obscure the stars.[17]
Yellow hypergiants form from massive stars after they have evolved away from the main sequence. Most observed yellow hypergiants have been through a red supergiant phase and are evolving back towards higher temperatures, but a few are seen in the brief first transition from main sequence to red supergiant. Supergiants with an initial mass less than 20 M☉ will explode as a supernova while still red supergiants, while stars more massive than about 60 M☉ will never cool beyond blue supergiant temperatures. The exact mass ranges depend on metallicity and rotation.[18] Yellow supergiants cooling for the first time may be massive stars of up to 60 M☉ or more,[15] but post-red supergiant stars will have lost around half their initial mass.[19]
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.
Evolution
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,[14] while more stable and less luminous yellow supergiants are likely to be evolving to red supergiants for the first time. 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.[15]
These stars are doubly rare because they are very massive, initially hot class O-type 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.[14]
Recent discoveries of blue supergiant supernova progenitors have also raised the question of whether stars could explode directly from the yellow hypergiant stage.[20] 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.[21][22] 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.[23]
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.[24] 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 collapse.[25]
Structure
IRAS 17163-3907 is a yellow hypergiant that clearly shows the expelled material that probably surrounds all yellow hypergiants.
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.[26] Because of their extreme luminosity and internal structure,[27] yellow hypergiants suffer high rates of mass loss[28] and are generally surrounded by envelopes of expelled material. An 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.[29]
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.[30]
Known yellow hypergiants
Yellow hypergiant HR 5171 A, seen as the bright yellow star at the center of the image.
File:Artist’s impression of the yellow hypergiant star HR 5171.ogvPlay media
Artist's impression of the binary system containing yellow hypergiant HR 5171 A
Rho Cassiopeiae
V509 Cassiopeiae
R Puppis[31]
IRC+10420 (V1302 Aql)
IRAS 18357-0604[32]
V766 Centauri (= HR 5171A) (possibly a red supergiant[33])
HD 179821
IRAS 17163-3907
V382 Carinae
RSGC1-F15[34]
In Westerlund 1:[35]
W4
W8a
W12a
W16a
W32
W265
In other galaxies:
HD 7583 (R45 in SMC)[10]
HD 33579 (in LMC)
HD 269723 (R117 in LMC)[10]
HD 269953 (R150 in LMC)[10]
HD 268757 (R59 in LMC)[10]
Variable A (in M33)[36]
B324 (in M33)[36]
LGGS J013250.70+304510.6[37]
Sextans A 7[38]
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vte
Accretion Molecular cloud Bok globule Young stellar object
Protostar Pre-main-sequence Herbig Ae/Be T Tauri FU Orionis Herbig–Haro object Hayashi track Henyey track
Main sequence Red-giant branch Horizontal branch
Red clump Asymptotic giant branch
super-AGB Blue loop Protoplanetary nebula Planetary nebula PG1159 Dredge-up OH/IR Instability strip Luminous blue variable Blue straggler Stellar population Supernova Superluminous supernova / Hypernova
Early Late Main sequence
O B A F G K M Brown dwarf WR OB Subdwarf
O B Subgiant Giant
Blue Red Yellow Bright giant Supergiant
Blue Red Yellow Hypergiant
Yellow Carbon
S CN CH White dwarf Chemically peculiar
Am Ap/Bp HgMn Helium-weak Barium Extreme helium Lambda Boötis Lead Technetium Be
Shell B[e]
White dwarf
Helium planet Black dwarf Neutron
Radio-quiet Pulsar
Binary X-ray Magnetar Stellar black hole X-ray binary
Burster
Hypothetical
Blue dwarf Green Black dwarf Exotic
Boson Electroweak Strange Preon Planck Dark Dark-energy Quark Q Black Gravastar Frozen Quasi-star Thorne–Żytkow object Iron Blitzar
Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova
Symbiotic Remnant Luminous red nova
Structure
Core Convection zone
Microturbulence Oscillations Radiation zone Atmosphere
Photosphere Starspot Chromosphere Stellar corona Stellar wind
Bubble Bipolar outflow Accretion disk Asteroseismology
Helioseismology Eddington luminosity Kelvin–Helmholtz mechanism
Properties
Designation Dynamics Effective temperature Luminosity Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation Starlight Variable Photometric system Color index Hertzsprung–Russell diagram Color–color diagram
Star systems
Binary
Contact Common envelope Eclipsing Symbiotic Multiple Cluster
Open Globular Super Planetary system
Earth-centric
observations
Sun
Solar System Sunlight Pole star Circumpolar Constellation Asterism Magnitude
Apparent Extinction Photographic Radial velocity Proper motion Parallax Photometric-standard
Lists
Proper names
Arabic Chinese Extremes Most massive Highest temperature Lowest temperature Largest volume Smallest volume Brightest
Historical Most luminous Nearest
Nearest bright With exoplanets Brown dwarfs White dwarfs Milky Way novae Supernovae
Candidates Remnants Planetary nebulae Timeline of stellar astronomy
Related articles
Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event
vte
Variable stars
Pulsating
Cepheids and
cepheid-like
Type I (Classical cepheids, Delta Scuti) Type II (BL Herculis, W Virginis, RV Tauri) RR Lyrae Rapidly oscillating Ap SX Phoenicis
Blue-white with
early spectra
Alpha Cygni Beta Cephei Slowly pulsating B-type PV Telescopii Blue large-amplitude pulsator
Long-period
Mira Semiregular Slow irregular
Other
Gamma Doradus Solar-like oscillations White dwarf
Eruptive
Protostar and PMS
Herbig Ae/Be Orion
FU Orionis T Tauri
Giants and
supergiants
Luminous blue variable R Coronae Borealis (DY Persei) Yellow hypergiant
Eruptive binary
Double periodic FS Canis Majoris RS Canum Venaticorum
Other
Flare Gamma Cassiopeiae Lambda Eridani Wolf–Rayet
Cataclysmic
AM Canum Venaticorum Dwarf nova Luminous red nova Nova Polar Intermediate polar Supernova
Hypernova SW Sextantis Symbiotic
Symbiotic nova Z Andromedae
Rotating
Non-spherical
Rotating ellipsoidal
Stellar spots
BY Draconis FK Comae Berenices
Magnetic fields
Alpha² Canum Venaticorum Pulsar SX Arietis
Eclipsing
Algol Beta Lyrae Planetary transit W Ursae Majoris
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