A yellow supergiant (YSG) is a star, generally of spectral type F or G, having a supergiant luminosity class (e.g. Ia or Ib). They are stars that have evolved away from the main sequence, expanding and becoming more luminous.
Yellow supergiants are smaller than red supergiants; naked eye examples include Polaris. Many of them are variable stars, mostly pulsating Cepheids such as δ Cephei itself.
Yellow supergiants generally have spectral types of F and G, although sometimes late A or early K stars are included. [1] [2] [3] These spectral types are characterised by hydrogen lines that are very strong in class A, weakening through F and G until they are very weak or absent in class K. Calcium H and K lines are present in late A spectra, but stronger in class F, and strongest in class G, before weakening again in cooler stars. Lines of ionised metals are strong in class A, weaker in class F and G, and absent from cooler stars. In class G, neutral metal lines are also found, along with CH molecular bands. [4]
Supergiants are identified in the Yerkes spectral classification by luminosities classes Ia and Ib, with intermediates such as Iab and Ia/ab sometimes being used. These luminosity classes are assigned using spectral lines that are sensitive to luminosity. Historically, the Ca H and K line strengths have been used for yellow stars, as well as the strengths of various metal lines. [5] The neutral oxygen lines, such as the 777.3 nm triplet, have also been used since they are extremely sensitive to luminosity across a wide range of spectral types. [6] Modern atmospheric models can accurately match all the spectral line strengths and profiles to give a spectral classification, or even skip straight to the physical parameters of the star, but in practice luminosity classes are still usually assigned by comparison against standard stars. [4]
Some yellow supergiant spectral standard stars: [7]
Yellow supergiants have a relatively narrow range of temperatures corresponding to their spectral types, from about 4,000 K to 7,000 K. [9] Their luminosities range from about 1,000 L☉ upwards, with the most luminous stars exceeding 100,000 L☉. The high luminosities indicate that they are much larger than the sun, from about 30 R☉ to several hundred R☉. [10]
The masses of yellow supergiants vary greatly, from less than the sun for stars such as W Virginis to 20 M☉ or more (e.g. V810 Centauri). Corresponding surface gravities (log(g) cgs) are around 1–2 for high-mass supergiants, but can be as low as 0 for low-mass supergiants. [9] [11]
Yellow supergiants are rare stars, much less common than red supergiants and main sequence stars. In M31 (Andromeda Galaxy), 16 yellow supergiants are seen associated with evolution from class O stars, of which there are around 25,000 visible. [12]
Many yellow supergiants are in a region of the HR diagram known as the instability strip because their temperatures and luminosities cause them to be dynamically unstable. Most yellow supergiants observed in the instability strip are Cepheid variables, named for δ Cephei, which pulsate with well-defined periods that are related to their luminosities. This means they can be used as standard candles for determining the distance of stars knowing only their period of variability. Cepheids with longer periods are cooler and more luminous. [13]
Two distinct types of Cepheid variable have been identified, which have different period-luminosity relationships: Classical Cepheid variables are young massive population I stars; type II Cepheids are older population II stars with low masses, including W Virginis variables, BL Herculis variables and RV Tauri variables. The Classical Cepheids are more luminous than the type II Cepheids with the same period. [14]
R Coronae Borealis variables are often yellow supergiants, but their variability is produced by a different mechanism from the Cepheids. At irregular intervals, they become obscured by dust condensation around the star and their brightness drops dramatically. [15]
Supergiants are stars that have evolved away from the main sequence after exhausting the hydrogen in their cores. Yellow supergiants are a heterogenous group of stars crossing the standard categories of stars in the HR diagram at various different stages of their evolution.
Stars more massive than 8–12 M☉ spend a few million years on the main sequence as class O and early B stars until the dense hydrogen in their cores becomes depleted. Then they expand and cool to become supergiants. They spend a few thousand years as a yellow supergiant while cooling, then spend one to four million years as a red supergiant, typically. Supergiants make up less than 1% of stars; though different proportions in the visible early eras of the universe. The relatively brief phases and concentration of matter explains the rarity of these stars. [16]
Some red supergiants undergo a blue loop, temporarily re-heating and becoming yellow or even blue supergiants before cooling again. Stellar models show that blue loops rely on particular chemical makeups and other assumptions, but they are most likely for stars of low red supergiant mass. While cooling for the first time or when performing a sufficiently extended blue loop, yellow supergiants will cross the instability strip and pulsate as Classical Cepheid variables with periods around ten days and longer. [17] [18]
Intermediate mass stars leave the main sequence by cooling along the subgiant branch until they reach the red-giant branch. Stars more massive than about 2 M☉ have a sufficiently large helium core that it begins fusion before becoming degenerate. These stars will perform a blue loop.
For masses between about 5 M☉ and 12 M☉, the blue loop can extend to F and G spectral types at luminosities reaching 1,000 L☉. These stars may develop supergiant luminosity classes, especially if they are pulsating. When these stars cross the instability strip they will pulsate as short period Cepheids. Blue loops in these stars can last for around 10 million years, so this type of yellow supergiant is more common than the more luminous types. [19] [20]
Stars with masses similar to the sun develop degenerate helium cores after they leave the main sequence and ascend to the tip of the red-giant branch where they ignite helium in a flash. They then fuse core helium on the horizontal branch with luminosities too low to be considered supergiants.
Stars leaving the blue half of the horizontal branch to be classified in the asymptotic giant branch (AGB) pass through the yellow classifications and will pulsate as BL Herculis variables. Such yellow stars may be given a supergiant luminosity class despite their low masses but assisted by luminous pulsation. In the AGB thermal pulses from the helium-fusing shell of stars may cause a blue loop across the instability strip. Such stars will pulsate as W Virginis variables and again may be classified as relatively low luminosity yellow supergiants. [14] When the hydrogen-fusing shell of a low or intermediate mass star of the AGB nears its surface, the cool outer layers are rapidly lost, which causes the star to heat up, eventually becoming a white dwarf. These stars have masses lower than the sun, but luminosities that can be 10,000 L☉ or higher, so they will become yellow supergiants for a short time. Post-AGB stars are believed to pulsate as RV Tauri variables when they cross the instability strip. [21]
The evolutionary status of yellow supergiant R Coronae Borealis variables is unclear. They may be post-AGB stars reignited by a late helium shell flash, or they could be formed from white dwarf mergers. [22]
It is expected that first-time yellow supergiants mature to the red supergiant stage without any supernova. The cores of some post-red supergiant yellow supergiants might collapse and trigger a supernova. A handful of supernovae have been associated with apparent yellow supergiant progenitors that are not luminous enough to be post-red supergiants. If these are confirmed then an explanation must be found for how a star of moderate mass still with a helium core would cause a core-collapse supernova. The obvious candidate in such cases is always some form of binary interaction. [23]
Particularly luminous and unstable yellow supergiants are often grouped into a separate class of stars called the yellow hypergiants. These are mostly thought to be post-red supergiant stars, very massive stars that have lost a considerable portion of their outer layers and are now evolving towards becoming blue supergiants and Wolf-Rayet stars. [24]
A variable star is a star whose brightness as seen from Earth fluctuates.
Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,400 K to over 20,000 K.
Red supergiants (RSGs) are stars with a supergiant luminosity class of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.
In astronomy, a blue giant is a hot star with a luminosity class of III (giant) or II. In the standard Hertzsprung–Russell diagram, these stars lie above and to the right of the main sequence.
A blue supergiant (BSG) is a hot, luminous star, often referred to as an OB supergiant. They have luminosity class I and spectral class B9 or earlier.
A giant star is a star with substantially larger radius and luminosity than a main-sequence star of the same surface temperature. They lie above the main sequence on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III. The terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung about 1905.
The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass stars. Red-giant-branch stars have an inert helium core surrounded by a shell of hydrogen fusing via the CNO cycle. They are K- and M-class stars much larger and more luminous than main-sequence stars of the same temperature.
RV Tauri variables are luminous variable stars that have distinctive light variations with alternating deep and shallow minima.
Alpha Cygni variables are variable stars which exhibit non-radial pulsations, meaning that some portions of the stellar surface are contracting at the same time other parts expand. They are supergiant stars of spectral types B or A. Variations in brightness on the order of 0.1 magnitudes are associated with the pulsations, which often seem irregular, due to beating of multiple pulsation periods. The pulsations typically have periods of several days to several weeks.
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.
The unqualified term instability strip usually refers to a region of the Hertzsprung–Russell diagram largely occupied by several related classes of pulsating variable stars: Delta Scuti variables, SX Phoenicis variables, and rapidly oscillating Ap stars (roAps) near the main sequence; RR Lyrae variables where it intersects the horizontal branch; and the Cepheid variables where it crosses the supergiants.
A hypergiant (luminosity class 0 or Ia+) is a very rare type of star that has an extremely high luminosity, mass, size and mass loss because of its extreme stellar winds. The term hypergiant is defined as luminosity class 0 (zero) in the MKK system. However, this is rarely seen in the literature or in published spectral classifications, except for specific well-defined groups such as the yellow hypergiants, RSG (red supergiants), or blue B(e) supergiants with emission spectra. More commonly, hypergiants are classed as Ia-0 or Ia+, but red supergiants are rarely assigned these spectral classifications. Astronomers are interested in these stars because they relate to understanding stellar evolution, especially with star formation, stability, and their expected demise as supernovae.
Classical Cepheids are a type of Cepheid variable star. They are population I variable stars that exhibit regular radial pulsations with periods of a few days to a few weeks and visual amplitudes from a few tenths of a magnitude to about 2 magnitudes.
TZ Cassiopeaie(TZ Cas, HIP 117763, SAO 20912) is a variable star in the constellation Cassiopeia with an apparent magnitude of around +9 to +10. It is approximately 8,000 light-years away from Earth. The star is a red supergiant star with a spectral type of M3 and a temperature below 4000 Kelvin.
PZ Cassiopeiae is a red supergiant star located in the Cassiopeia constellation, and a semi-regular variable star.
An O-type star is a hot, blue-white star of spectral type O in the Yerkes classification system employed by astronomers. They have temperatures in excess of 30,000 kelvin (K). Stars of this type have strong absorption lines of ionised helium, strong lines of other ionised elements, and hydrogen and neutral helium lines weaker than spectral type B.
UY Scuti (BD-12°5055) is a red supergiant star in the constellation Scutum. It is considered one of the largest known stars by radius and is also a pulsating variable star, with a maximum brightness of magnitude 8.29 and a minimum of magnitude 10.56. It has an estimated radius of 1,708 solar radii (1.188×109 kilometres; 7.94 astronomical units), thus a volume nearly 5 billion times that of the Sun. It is approximately 2.9 kiloparsecs (9,500 light-years) from Earth. If placed at the center of the Solar System, its photosphere would at least engulf the orbit of Jupiter.
14 Persei is a single star in the northern constellation Perseus, located roughly 1,900 light years away from the Sun. It is visible to the naked eye as a faint, yellow-hued star with an apparent visual magnitude is 5.43. The object is slowly moving closer to the Earth with a heliocentric radial velocity of −1.2 km/s.
R Sagittae is an RV Tauri variable star in the constellation Sagitta that varies from magnitude 8.0 to 10.5 in 70.77 days. It is a post-AGB low mass yellow supergiant that varies between spectral types G0Ib and G8Ib as it pulsates. Its variable star designation of "R" indicates that it was the first star discovered to be variable in the constellation. It was discovered in 1859 by Joseph Baxendell, though classified as a semi regular variable until RV Tauri variables were identified as a distinct class in 1905.
In the field of stellar evolution, a blue loop is a stage in the life of an evolved star where it changes from a cool star to a hotter one before cooling again. The name derives from the shape of the evolutionary track on a Hertzsprung–Russell diagram which forms a loop towards the blue side of the diagram.
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