Red supergiants (RSGs) are stars with a supergiant luminosity class (Yerkes class I) and a stellar classification K or M. [1] They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares A are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.
Stars are classified as supergiants on the basis of their spectral luminosity class. This system uses certain diagnostic spectral lines to estimate the surface gravity of a star, hence determining its size relative to its mass. Larger stars are more luminous at a given temperature and can now be grouped into bands of differing luminosity. [2]
The luminosity differences between stars are most apparent at low temperatures, where giant stars are much brighter than main-sequence stars. Supergiants have the lowest surface gravities and hence are the largest and brightest at a particular temperature.
The Yerkes or Morgan-Keenan (MK) classification system [3] is almost universal. It groups stars into five main luminosity groups designated by roman numerals:
Specific to supergiants, the luminosity class is further divided into normal supergiants of class Ib and brightest supergiants of class Ia. The intermediate class Iab is also used. Exceptionally bright, low surface gravity, stars with strong indications of mass loss may be designated by luminosity class 0 (zero) although this is rarely seen. [4] More often the designation Ia-0 will be used, [5] and more commonly still Ia+. [6] These hypergiant spectral classifications are very rarely applied to red supergiants, although the term red hypergiant is sometimes used for the most extended and unstable red supergiants like VY Canis Majoris and NML Cygni. [7] [8]
The "red" part of "red supergiant" refers to the cool temperature. Red supergiants are the coolest supergiants, M-type, and at least some K-type stars although there is no precise cutoff. K-type supergiants are uncommon compared to M-type because they are a short-lived transition stage and somewhat unstable. The K-type stars, especially early or hotter K types, are sometimes described as orange supergiants (e.g. Zeta Cephei), or even as yellow (e.g. yellow hypergiant HR 5171 Aa).[ citation needed ]
| Spectral type | Temperature (K) |
|---|---|
| K1–1.5 | 4,100 |
| K2–3 | 4,015 |
| K5–M0 | 3,840 |
| M0 | 3,790 |
| M1 | 3,745 |
| M1.5 | 3,710 |
| M2 | 3,660 |
| M2.5 | 3,615 |
| M3 | 3,605 |
| M3.5 | 3,550 |
| M4–4.5 | 3,535 |
| M5 | 3,450 |
Red supergiants are cool and large. They have spectral types of K and M, hence surface temperatures below 4,100 K. [9] They are typically several hundred to over a thousand times the radius of the Sun, [9] although size is not the primary factor in a star being designated as a supergiant. A bright cool giant star can easily be larger than a hotter supergiant. For example, Alpha Herculis is classified as a giant star with a radius of between 264 to 303 R☉ while Epsilon Pegasi is a K2 supergiant of only 185 R☉.
Although red supergiants are much cooler than the Sun, they are so much larger that they are highly luminous, typically tens or hundreds of thousands L☉. [9] There is a theoretical upper limit to the radius of a red supergiant at around 1,500 R☉. [9] In the Hayashi limit, stars above this radius would be too unstable and simply do not form.
Red supergiants have masses between about 10 M☉ and 30 or 40 M☉. [10] Main-sequence stars more massive than about 40 M☉ do not expand and cool to become red supergiants. Red supergiants at the upper end of the possible mass and luminosity range are the largest known. Their low surface gravities and high luminosities cause extreme mass loss, millions of times higher than the Sun, producing observable nebulae surrounding the star. [11] By the end of their lives red supergiants may have lost a substantial fraction of their initial mass. The more massive supergiants lose mass much more rapidly and all red supergiants appear to reach a similar mass of the order of 10 M☉ by the time their cores collapse. The exact value depends on the initial chemical makeup of the star and its rotation rate. [12]
Most red supergiants show some degree of visual variability, but only rarely with a well-defined period or amplitude. Therefore, they are usually classified as irregular or semiregular variables. They even have their own sub-classes, SRC and LC for slow semi-regular and slow irregular supergiant variables respectively. Variations are typically slow and of small amplitude, but amplitudes up to four magnitudes are known. [13]
Statistical analysis of many known variable red supergiants shows a number of likely causes for variation: just a few stars show large amplitudes and strong noise indicating variability at many frequencies, thought to indicate powerful stellar winds that occur towards the end of the life of a red supergiant; more common are simultaneous radial mode variations over a few hundred days and probably non-radial mode variations over a few thousand days; only a few stars appear to be truly irregular, with small amplitudes, likely due to photospheric granulation. Red supergiant photospheres contain a relatively small number of very large convection cells compared to stars like the Sun. This causes variations in surface brightness that can lead to visible brightness variations as the star rotates. [14]
The spectra of red supergiants are similar to other cool stars, dominated by a forest of absorption lines of metals and molecular bands. Some of these features are used to determine the luminosity class, for example certain near-infrared cyanogen band strengths and the Ca II triplet. [15]
Maser emission is common from the circumstellar material around red supergiants. Most commonly this arises from H2O and SiO, but hydroxyl (OH) emission also occurs from narrow regions. [16] In addition to high resolution mapping of the circumstellar material around red supergiants, [17] VLBI or VLBA observations of masers can be used to derive accurate parallaxes and distances to their sources. [18] Currently this has been applied mainly to individual objects, but it may become useful for analysis of galactic structure and discovery of otherwise obscured red supergiant stars. [19]
Surface abundances of red supergiants are dominated by hydrogen even though hydrogen at the core has been completely consumed. In the latest stages of mass loss, before a star explodes, surface helium may become enriched to levels comparable with hydrogen. In theoretical extreme mass loss models, sufficient hydrogen may be lost that helium becomes the most abundant element at the surface. When pre-red supergiant stars leave the main sequence, oxygen is more abundant than carbon at the surface, and nitrogen is less abundant than either, reflecting abundances from the formation of the star. Carbon and oxygen are quickly depleted and nitrogen enhanced as a result of the dredge-up of CNO-processed material from the fusion layers. [20]
Red supergiants are observed to rotate slowly or very slowly. Models indicate that even rapidly rotating main-sequence stars should be braked by their mass loss so that red supergiants hardly rotate at all. Those red supergiants such as Betelgeuse that do have modest rates of rotation may have acquired it after reaching the red supergiant stage, perhaps through binary interaction. The cores of red supergiants are still rotating and the differential rotation rate can be very large. [21]
Supergiant luminosity classes are easy to determine and apply to large numbers of stars, but they group several very different types of stars into a single category. An evolutionary definition restricts the term supergiant to those massive stars which start core helium fusion without developing a degenerate helium core and without undergoing a helium flash. They will universally go on to burn heavier elements and undergo core-collapse resulting in a supernova. [22]
Less massive stars may develop a supergiant spectral luminosity class at relatively low luminosity, around 1,000 L☉ when they are on the asymptotic giant branch (AGB) undergoing helium shell burning. Researchers now prefer to categorize these as AGB stars distinct from supergiants because they are less massive, have different chemical compositions at the surface, undergo different types of pulsation and variability, and will evolve differently, usually producing a planetary nebula and white dwarf. [23] Most AGB stars will not become supernovae although there is interest in a class of super-AGB stars , those almost massive enough to undergo full carbon fusion, which may produce peculiar supernovae although without ever developing an iron core. [24] One notable group of low mass high luminosity stars are the RV Tauri variables, AGB or post-AGB stars lying on the instability strip and showing distinctive semi-regular variations.
Red supergiants develop from main-sequence stars with masses between about 8 M☉ and 30 or 40 M☉. [10] Higher-mass stars never cool sufficiently to become red supergiants. Lower-mass stars develop a degenerate helium core during a red giant phase, undergo a helium flash before fusing helium on the horizontal branch, evolve along the AGB while burning helium in a shell around a degenerate carbon-oxygen core, then rapidly lose their outer layers to become a white dwarf with a planetary nebula. [12] AGB stars may develop spectra with a supergiant luminosity class as they expand to extreme dimensions relative to their small mass, and they may reach luminosities tens of thousands times the sun's. Intermediate "super-AGB" stars, around 9 M☉, can undergo carbon fusion and may produce an electron capture supernova through the collapse of an oxygen-neon core. [24]
Main-sequence stars, burning hydrogen in their cores, with masses between 10 and 30 or 40 M☉ will have temperatures between about 25,000K and 32,000K and spectral types of early B, possibly very late O. They are already very luminous stars of 10,000–100,000 L☉ due to rapid CNO cycle fusion of hydrogen and they have fully convective cores. In contrast to the Sun, the outer layers of these hot main-sequence stars are not convective. [12]
These pre-red supergiant main-sequence stars exhaust the hydrogen in their cores after 5–20 million years. They then start to burn a shell of hydrogen around the now-predominantly helium core, and this causes them to expand and cool into supergiants. Their luminosity increases by a factor of about three. The surface abundance of helium is now up to 40% but there is little enrichment of heavier elements. [12]
The supergiants continue to cool and most will rapidly pass through the Cepheid instability strip, although the most massive will spend a brief period as yellow hypergiants. They will reach late K or M class and become a red supergiant. Helium fusion in the core begins smoothly either while the star is expanding or once it is already a red supergiant, but this produces little immediate change at the surface. Red supergiants develop deep convection zones reaching from the surface over halfway to the core and these cause strong enrichment of nitrogen at the surface, with some enrichment of heavier elements. [26]
Some red supergiants undergo blue loops where they temporarily increase in temperature before returning to the red supergiant state. This depends on the mass, rate of rotation, and chemical makeup of the star. While many red supergiants will not experience a blue loop, some can have several. Temperatures can reach 10,000K at the peak of the blue loop. The exact reasons for blue loops vary in different stars, but they are always related to the helium core increasing as a proportion of the mass of the star and forcing higher mass-loss rates from the outer layers. [21]
All red supergiants will exhaust the helium in their cores within one or two million years and then start to burn carbon. This continues with fusion of heavier elements until an iron core builds up, which then inevitably collapses to produce a supernova. The time from the onset of carbon fusion until the core collapse is no more than a few thousand years. In most cases, core-collapse occurs while the star is still a red supergiant, the large remaining hydrogen-rich atmosphere is ejected, and this produces a type II supernova spectrum. The opacity of this ejected hydrogen decreases as it cools and this causes an extended delay to the drop in brightness after the initial supernova peak, the characteristic of a Type II-P supernova. [12] [26]
The most luminous red supergiants, at near solar metallicity, are expected to lose most of their outer layers before their cores collapse, hence they evolve back to yellow hypergiants and luminous blue variables. Such stars can explode as type II-L supernovae, still with hydrogen in their spectra but not with sufficient hydrogen to cause an extended brightness plateau in their light curves. Stars with even less hydrogen remaining may produce the uncommon type IIb supernova, where there is so little hydrogen remaining that the hydrogen lines in the initial type II spectrum fade to the appearance of a Type Ib supernova. [27]
The observed progenitors of type II-P supernovae all have temperatures between 3,500K and 4,400K and luminosities between 10,000 L☉ and 300,000 L☉. This matches the expected parameters of lower mass red supergiants. A small number of progenitors of type II-L and type IIb supernovae have been observed, all having luminosities around 100,000 L☉ and somewhat higher temperatures up to 6,000K. These are a good match for slightly higher mass red supergiants with high mass-loss rates. There are no known supernova progenitors corresponding to the most luminous red supergiants, and it is expected that these evolve to Wolf Rayet stars before exploding. [21]
Red supergiants are necessarily no more than about 25 million years old and such massive stars are expected to form only in relatively large clusters of stars, so they are expected to be found mostly near prominent clusters. However they are fairly short-lived compared to other phases in the life of a star and only form from relatively uncommon massive stars, so there will generally only be small numbers of red supergiants in each cluster at any one time. The massive Hodge 301 cluster in the Tarantula Nebula contains three. [28] Until the 21st century the largest number of red supergiants known in a single cluster was five in NGC 7419. [29] Most red supergiants are found singly, for example Betelgeuse in the Orion OB1 association and Antares in the Scorpius–Centaurus association.
Since 2006, a series of massive clusters have been identified near the base of the Crux-Scutum Arm of the galaxy, each containing multiple red supergiants. RSGC1 contains at least 12 red supergiants, RSGC2 (also known as Stephenson 2) contains at least 26, RSGC3 contains at least 8, and RSGC4 (also known as Alicante 8) also contains at least 8. A total of 80 confirmed red supergiants have been identified within a small area of the sky in the direction of these clusters. These four clusters appear to be part of a massive burst of star formation 10–20 million years ago at the near end of the bar at the centre of the galaxy. [30] Similar massive clusters have been found near the far end of the galactic bar, but not such large numbers of red supergiants. [31]
Red supergiants are rare stars, but they are visible at great distance and are often variable so there are a number of well-known naked-eye examples:
Mira was historically thought to be a red supergiant star, but is now widely accepted to be an asymptotic giant branch star. [32]
Some red supergiants are larger and more luminous, and are hence also referred to as red hypergiants:
A survey expected to capture virtually all Magellanic Cloud red supergiants [33] detected around a dozen M class stars Mv−7 and brighter, around a quarter of a million times more luminous than the Sun, and from about 1,000 times the radius of the Sun upwards.
Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the current age of the universe. The table shows the lifetimes of stars as a function of their masses. All stars are formed from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star.
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.
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 are usually considered to be those with luminosity class I and spectral class B9 or earlier, although sometimes A-class supergiants are also deemed blue supergiants.
A giant star has a 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.
A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of a star.
Luminous blue variables (LBVs) are massive evolved stars that show unpredictable and sometimes dramatic variations in their spectra and brightness. They are also known as S Doradus variables after S Doradus, one of the brightest stars of the Large Magellanic Cloud. They are considered to be rare.
Mu Cephei, also known as Herschel's Garnet Star, Erakis, or HD 206936, is a red supergiant or hypergiant star in the constellation Cepheus. It appears garnet red and is located at the edge of the IC 1396 nebula. Since 1943, the spectrum of this star has served as a spectral standard by which other stars are classified.
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 20 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.
A yellow supergiant (YSG) is a star, generally of spectral type F or G, having a supergiant luminosity class. They are stars that have evolved away from the main sequence, expanding and becoming more luminous.
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 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 star formation, stability, and their expected demise as supernovae.
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K or lower. The appearance of the red giant is from yellow-white to reddish-orange, including the spectral types K and M, sometimes G, but also class S stars and most carbon stars.
R136a1 is one of the most massive and luminous stars known, at nearly 200 M☉ and nearly 4.7 million L☉, and is also one of the hottest, at around 46,000 K. It is a Wolf–Rayet star at the center of R136, the central concentration of stars of the large NGC 2070 open cluster in the Tarantula Nebula in the Large Magellanic Cloud. The cluster can be seen in the far southern celestial hemisphere with binoculars or a small telescope, at magnitude 7.25. R136a1 itself is 100 times fainter than the cluster and can only be resolved using speckle interferometry.
PZ Cassiopeiae is a red supergiant star located in the constellation of Cassiopeia, and a semi-regular variable star.
HD 179821 or V1427 Aquilae is either a post-red supergiant yellow hypergiant or a post-AGB yellow supergiant star in the constellation of Aquila, surrounded by a detached dust shell. It is a semi-regular variable nearing the end of its life.
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 kelvins (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 to be possibly one of the largest known stars 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 909 solar radii thus a volume of 750 million times that of the Sun. This estimate implies if it were placed at the center of the Solar System, its photosphere would extend past the orbit of Mars.
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, to a place called the blue giant branch.
A super-AGB star is a star with a mass intermediate between those that end their lives as a white dwarf and those that end with a core collapse supernova, and properties intermediate between asymptotic giant branch (AGB) stars and red supergiants. They have initial masses of 7.5–9.25 M☉ in stellar-evolutionary models, but have exhausted their core hydrogen and helium, left the main sequence, and expanded to become large, cool, and luminous.
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