Red supergiants are stars with a supergiant luminosity class (Yerkes class I) 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.
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.
The luminosity differences between stars is most apparent at low temperatures, where giant stars are much brighter than main-sequence stars. Supergiant have the lowest surface gravities and hence are the largest and brightest at a particular temperature.
Specific to supergiants, the luminosity class is further divided into normal supergiants of class Ib and bright 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. More often the designation Ia-0 will be used, and more commonly still Ia+. These hypergiant spectral classifications are very rarely applied to red supergiants, although the term hypergiant is sometimes used for the most extended and unstable red supergiants.
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 5171A).
Red supergiants are cool and large. They have spectral types of K and M, hence temperatures below 4,100 K. They are typically several hundred to over a thousand times the radius of the Sun, 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☉. There is an upper limit to the luminosity of a red supergiant at around half a million L☉. Stars above this luminosity would be too unstable and simply don't form.
Red supergiants have masses between about 10 M☉ and 40 M☉. 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. 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.
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.
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.
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.
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. In addition to high resolution mapping of the circumstellar material around red supergiants, VLBI or VLBA observations of masers can be used to derive accurate parallaxes and distances to their sources. 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.
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.
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 though binary interaction. The cores of red supergiants are still rotating and the differential rotation rate can be very large.
Supergiant luminosity classes are easy to determine and apply to large numbers of stars, but they group a number of very different types of star 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.
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 categorise 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 in a different way, usually producing a planetary nebula and white dwarf. 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. 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 10 M☉ and 30 M☉. 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. 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.
Main-sequence stars, burning hydrogen in their cores, with masses between 10 and 30 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.
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.
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.
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.
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 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.
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.
The observed progenitors of type II-P supernovae all have temperatures between 3,500K and 4,400K and luminosities between 20,000 L☉ and 200,000 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.
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. For example, in the substantial Double Clusters in Perseus there is just a single red supergiant, S Persei, while the massive Hodge 301 cluster in the Tarantula Nebula contains three. Until the 21st century the largest number of red supergiants known in a single cluster was five in NGC 7419. 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) 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. Similar massive clusters have been found near the far end of the galactic bar, but not such large numbers of red supergiants.
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:
Other examples have become known on account of their enormous size, more than 1,000 R☉:
119 Tauri (also known as CE Tauri) is a red supergiant star in the constellation Taurus. It is a semiregular variable and its angular diameter has been measured at about 10 mas.BC
BC may refer to:
Before Christ, an epoch used in dating years prior to the estimated birth of Jesus in the Julian and Gregorian calendarsFailed supernova
A failed supernova is an astronomical event in time domain astronomy in which a star suddenly brightens as in the early stage of a supernova, but then does not increase to the massive flux of a supernova. They could be counted as a subcategory of supernova imposters. They have sometimes misleadingly been called unnovae.HD 143183
HD 143183 is a red supergiant star of spectral type M3Ia in constellation Norma. It is a member of the Norma OB1 association, at a distance of about 2 kiloparsecs. It is one of the most luminous red supergiants with a luminosity about 300,000 times greater than the Sun (L☉), and is as well one of the largest stars with a radius over a thousand times that of the Sun (R☉). It has an estimated mass loss rate of 5×10−5 M☉ per year. It is surrounded by a dozen early-type stars and a circumstellar nebula which extends 0.12 parsecs (0.39 ly).
It is possible that HD 143183 is a spectroscopic binary with an OB+ companion but this is considered doubtful. HD 143183 lies approximately 1' from the 10th-magnitude O-class bright giant CD-53 6363, the second-brightest star in the cluster.HD 160529
HD 160529 (V905 Scorpii) is a Luminous Blue Variable (LBV) star located in the constellation of Scorpius. With an apparent magnitude of around +6.8 cannot be seen with the naked eye except under very favourable conditions, but it's easy to see with binoculars or amateur telescopes.HD 96919
HD 96919, also known by its Bayer designation of z2 Carinae and the variable star designation of V371 Carinae, is a blue supergiant star in the constellation Carina. It lies near the Carina Nebula and at a comparable distance.
V371 Car is an α Cyg variable, erratically pulsating and changing brightness by a few hundredths of a magnitude. Periods of 10 - 80 days have been identified. It shows unusual emission lines in its spectrum, and high-velocity absorption (HVA) events, temporary spectral features that are thought to indicate localised regions of enhanced mass loss.HD 96919 is a B9 supergiant, possibly located 6,000 light years from Earth. It is considered to be a post red supergiant star, either evolving towards a Wolf-Rayet star or on a blue loop before returning to a cooler temperature.HV 11423
HV 11423 (PMMR 114 or 140 LI-SMC) is a red supergiant star in the Small Magellanic Cloud. It is about 200,000 light-years away towards the constellation of Tucana.IX Carinae
IX Carinae (IX Car) is a red supergiant and pulsating variable star of spectral type M2Iab in the constellation Carina. It is a member of the Carina OB1 association along the Carina Nebula. It is one of the largest stars with a radius of 920 R☉ (640,000,000 km; 4.3 au). If placed at the center of the Solar System, it would extend close to the orbit of Jupiter.
IX Car is a semiregular variable star with a maximum brightness range of magnitude 7.2 - 8.5 and a period of 408 or 4,400 days.KQ Puppis
KQ Puppis (KQ Pup) is a spectroscopic binary variable star in the constellation Puppis. A red supergiant star and a hot main sequence star orbit each other every 9,742 days. Its apparent magnitude varies between 4.82 and 5.17.
The KQ Puppis system consists of a fairly typical M2 supergiant, in orbit with a hotter less luminous star. The hotter star is surrounded by a disc of material being transferred from the cool supergiant. This type of binary is referred to a VV Cephei system, although in this case there are no eclipses of either star. A portion of the disc does appear to be eclipsed and this is detected as a strong drop in far-ultraviolet radiation for about a third of the orbit.The red supergiant primary star has been compared to Betelgeuse. It shows small amplitude irregular pulsations, and also some variation associated with the orbital motion. The nature of the secondary is less certain. The spectrum shows high excitation features that would indicate an early B or hotter spectral type, but these may be associated with the disc rather than that star itself. Other studies have found a spectrum similar to an A supergiant, but this is thought to be an artefact of a B-type shell star.KQ Puppis has been catalogued as an outlying member of the open cluster Messier 47 (NGC 2422) and would be the brightest member of that cluster. Membership is uncertain as it appears to be more distant than the other stars in the cluster.N6946-BH1
N6946-BH1 is a disappearing red supergiant star in another galaxy, NGC 6946, on the northern border of the constellation of Cygnus. The star was 25 times the mass of the sun, and was 20 million light years distant from Earth. In March through to May 2009 its bolometric luminosity increased to at least a million solar luminosities, but by 2015 it had disappeared from optical view. In the mid and near infrared an object is still visible, however, it is fading away with a brightness proportional to t−4/3. The brightening was insufficient to be a supernova, and is called a failed supernova.The star's coordinates were at RA 20h 35m 27.56s and Dec +60° 08′ 08.29″. The brightness of the star, given by its apparent magnitude in different colour bands on 2 July 2005 is given by R = 21, V = 21, B = 22, U = 23. Prior to the optical outburst the star was about 100,000 times as bright as the sun. After the outburst it was invisible in the visual band and has declined to 5000 times as bright as the sun in infrared radiation.
One hypothesis is that the core of the star collapsed to form a black hole. The collapsing matter formed a burst of neutrinos that lowered the total mass of the star by a fraction of a percent. This caused a shock wave that blasted out the star's envelope to make it brighter. After the idea that a black holes are usually formed after a supernova, N6946-BH1 has given evidence that, instead of following this process, the star may automatically collapse into a black hole.Observed type II supernovae do not originate from stars with initial masses greater than about 18 M☉, and the rate of large star formation appears to exceed the rate of supernovae. The expectation is that something else is happening to these extra large stars. Failed supernovae and black hole formation is one proposed explanation. If this event indeed reflected the formation of a black hole, it is the first time that black hole formation has been observed.Omicron1 Canis Majoris
Omicron1 Canis Majoris (ο1 CMa, ο1 Canis Majoris) is a red supergiant star in the constellation Canis Major. It is a variable star in the constellation of Canis Major.PZ Cassiopeiae
PZ Cassiopeiae is a red supergiant star located in the Cassiopeia constellation, and a semi-regular variable star.Photographic magnitude
Photographic magnitude (mph or mp ) is a measure of the relative brightness of a star or other astronomical object as imaged on a photographic film emulsion with a camera attached to a telescope. An object's apparent photographic magnitude depends on its intrinsic luminosity, its distance and any extinction of light by interstellar matter existing along the line of sight to the observer.
Photographic observations have now been superseded by electronic photometry such as CCD charge-couple device cameras that convert the incoming light into an electric current by the photoelectric effect. Determination of magnitude is made using a photometer.Supergiant (disambiguation)
A supergiant is a massive and luminous star, including:
Blue supergiant star, a hot supergiant
Yellow supergiant star, a supergiant with a temperature similar to the sun
Red supergiant star, a cool supergiantSupergiant may also refer to:
Supergiant Games, a video game development company
Super Giant, a Japanese superhero
Alicella gigantea, the supergiant Amphipod
Rising Pune Supergiant, a cricket team in the Indian Premier League
Type-cD galaxy, a supergiant elliptical galaxy
Supergiant, a fictional character in the Marvel UniverseTZ Cassiopeiae
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.
TZ Cassiopeiae was reported as being variable by Williamina Fleming and published posthumously in 1911. It is a slow irregular variable star with a possible period of 3,100 days. It is over 60,000 times the luminosity of the Sun, and it is 645 to 800 times larger than the Sun. It is a member of the Cas OB5 stellar association, together with the nearby red supergiant PZ Cassiopeiae.The initial mass of TZ Cassiopeiae has been estimated from its position relative to theoretical stellar evolutionary tracks to be around 15 M☉.TZ Cas is losing mass through a powerful stellar wind at two millionths of a solar mass each year. It is unclear whether this is sufficient to cause the star to lose its atmosphere and become a blue supergiant before the core exhausts its fuel and collapses as a supernova. Either as a red or blue supergiant, or a Wolf-Rayet star, it will inevitably end its life violently in a supernova explosion when the core collapse occurs.V354 Cephei
V354 Cephei is a red supergiant star located within the Milky Way. It is an irregular variable located approximately 9,000 light-years away from the Sun, and is currently considered one of the largest known stars and one of most luminous of its type. It has an estimated radius of between 689 and 1,520 solar radii (479,000,000 and 1.057×109 km; 3.20 and 7.07 au). If it were placed in the center of the Solar System, it would extend to between the orbits of Jupiter and Saturn.XX Persei
XX Persei (IRC +50052 / HIP 9582 / BD+54 444) is a semiregular variable red supergiant star in the constellation Perseus, between the Double Cluster and the border with Andromeda.
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