Stellar mass

Stellar mass is a phrase that is used by astronomers to describe the mass of a star. It is usually enumerated in terms of the Sun's mass as a proportion of a solar mass (M). Hence, the bright star Sirius has around 2.02 M.[1] A star's mass will vary over its lifetime as additional mass becomes accreted, such as from a companion star, or mass is ejected with the stellar wind or pulsational behavior.


Stars are sometimes grouped by mass based upon their evolutionary behavior as they approach the end of their nuclear fusion lifetimes.

Very-low-mass stars with masses below 0.5 M do not enter the asymptotic giant branch (AGB) but evolve directly into white dwarfs. (At least in theory; the lifetimes of such stars are long enough—longer than the age of the universe to date—that none has yet had time to evolve to this point and be observed.)

Low-mass stars with a mass below about 1.8–2.2 M (depending on composition) do enter the AGB, where they develop a degenerate helium core.

Intermediate-mass stars undergo helium fusion and develop a degenerate carbon–oxygen core.

Massive stars have a minimum mass of 7–10 M, but this may be as low as 5–6 M. These stars undergo carbon fusion, with their lives ending in a core-collapse supernova explosion.[2] Black holes created as a result of a stellar collapse are termed stellar-mass black holes.

The combination of the radius and the mass of a star determines the surface gravity. Giant stars have a much lower surface gravity than main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.[3]


One of the most massive stars known is Eta Carinae,[4] with 100–150 M; its lifespan is very short—only several million years at most. A study of the Arches Cluster suggests that 150 M is the upper limit for stars in the current era of the universe.[5] The reason for this limit is not precisely known, but it is partially due to the Eddington luminosity which defines the maximum amount of luminosity that can pass through the atmosphere of a star without ejecting the gases into space. However, a star named R136a1 in the RMC 136a star cluster has been measured at 265 M, putting this limit into question.[6] A study has determined that stars larger than 150 M in R136 were created through the collision and merger of massive stars in close binary systems, providing a way to sidestep the 150 M limit.[7]

The first stars to form after the Big Bang may have been larger, up to 300 M or more,[8] due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive, population III stars is long extinct, however, and currently only theoretical.

With a mass only 93 times that of Jupiter (MJ), or .09 M, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core.[9] For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 MJ.[10][11] When the metallicity is very low, however, a recent study of the faintest stars found that the minimum star size seems to be about 8.3% of the solar mass, or about 87 MJ.[11][12] Smaller bodies are called brown dwarfs, which occupy a poorly defined grey area between stars and gas giants.


In the present day, the Sun is losing mass from the emission of electromagnetic energy and by the ejection of matter with the solar wind. It is expelling about (2–3)×1014 M per year.[13] The mass loss rate will increase when the Sun enters the red giant stage, climbing to (7–9)×10−14 M y−1 when it reaches the tip of the red-giant branch. This will rise to 10−6 M y−1 on the asymptotic giant branch, before peaking at a rate of 10−5 to 10−4 M y−1 as the Sun generates a planetary nebula. By the time the Sun becomes a degenerate white dwarf, it will have lost 46% of its starting mass.[14]


  1. ^ Liebert, J.; et al. (2005), "The Age and Progenitor Mass of Sirius B", The Astrophysical Journal, 630 (1): L69–L72, arXiv:astro-ph/0507523, Bibcode:2005ApJ...630L..69L, doi:10.1086/462419.
  2. ^ Kwok, Sun (2000), The origin and evolution of planetary nebulae, Cambridge astrophysics series, 33, Cambridge University Press, pp. 103–104, ISBN 0-521-62313-8.
  3. ^ Unsöld, Albrecht (2001), The New Cosmos (5th ed.), New York: Springer, pp. 180–185, 215–216, ISBN 3540678778.
  4. ^ Smith, Nathan (1998), "The Behemoth Eta Carinae: A Repeat Offender", Mercury Magazine, Astronomical Society of the Pacific, 27: 20, retrieved 2006-08-13.
  5. ^ "NASA's Hubble Weighs in on the Heaviest Stars in the Galaxy", NASA News, March 3, 2005, retrieved 2006-08-04.
  6. ^ Stars Just Got Bigger, European Southern Observatory, July 21, 2010, retrieved 2010-07-24.
  7. ^, "Mystery of the 'Monster Stars' Solved: It Was a Monster Mash", Natalie Wolchover, 7 August 2012
  8. ^ Ferreting Out The First Stars, Harvard-Smithsonian Center for Astrophysics, September 22, 2005, retrieved 2006-09-05.
  9. ^ Weighing the Smallest Stars, ESO, January 1, 2005, retrieved 2006-08-13.
  10. ^ Boss, Alan (April 3, 2001), Are They Planets or What?, Carnegie Institution of Washington, archived from the original on 2006-09-28, retrieved 2006-06-08.
  11. ^ a b Shiga, David (August 17, 2006), "Mass cut-off between stars and brown dwarfs revealed", New Scientist, archived from the original on 2006-11-14, retrieved 2006-08-23.
  12. ^ Hubble glimpses faintest stars, BBC, August 18, 2006, retrieved 2006-08-22.
  13. ^ Carroll, Bradley W.; Ostlie, Dale A. (1995), An Introduction to Modern Astrophysics (revised 2nd ed.), Benjamin Cummings, p. 409, ISBN 0201547309.
  14. ^ Schröder, K.-P.; Connon Smith, Robert (2008), "Distant future of the Sun and Earth revisited", Monthly Notices of the Royal Astronomical Society, 386 (1): 155–163, arXiv:0801.4031, Bibcode:2008MNRAS.386..155S, doi:10.1111/j.1365-2966.2008.13022.x
AFGL 2298

AFGL 2298, also known as IRAS 18576+0341, is a luminous blue variable star (LBV) located in the constellation Aquila, very close to the galactic plane. Its distance is not well known; it may be anywhere between 23,000 and 42,000 light years (7,000 to 13,000 parsecs) away from the Earth. Despite being extremely luminous, it is extremely reddened by interstellar extinction, so its apparent magnitude is brighter for longer-wavelength passbands; in fact, in visual wavelengths it is completely undetectable.AFGL 2298 has an absolute bolometric magnitude of −11.25, making it one of the most luminous stars known. Indeed, many of the hottest and most luminous stars known are luminous blue variables and other early-type stars. However, like all LBVs, AFGL 2298 is highly variable and the bolometric magnitude refers to its peak luminosity. Its status as an LBV was confirmed in 2003.Like most extremely massive stars, AFGL 2298 is undergoing mass loss. For example, in 2005 it was estimated to be losing 3.7×10−5 solar masses each year, although the rate of mass loss itself varies frequently and dramatically. The stellar mass is currently being ejected as a nebula around the star (similar to AG Carinae), which was imaged by the Very Large Telescope in 2010. The nebula was found to be fairly circular, and the properties of the dust appeared to be constant throughout the entire nebula.

BOSS Great Wall

The BOSS Great Wall is a supercluster complex that was identified, using the Baryon Oscillation Spectroscopic Survey (BOSS) of the Sloan Digital Sky Survey (SDSS), in early 2016. It was discovered by a research team from several institutions, consisting of: Hiedi Lietzen, Elmo Tempel, Lauri Juhan Liivamägi, Antonio Montero-Dorta, Maret Einasto, Alina Streblyanska, Claudia Maraston, Jose Alberto Rubiño-Martín and Enn Saar. The BOSS Great Wall is one of the largest superstructures in the observable universe.

The large complex has a mean redshift of z ~ 0.47 (z times Hubble length ≈ 6800 million light years). It consists of two elongated superclusters, two large superclusters, and several smaller superclusters as well. The elongated superclusters form galaxy walls, with the larger of the two having a diameter of 186/h Mpc (supercluster A in the figure); the second wall's being 173/h Mpc (supercluster B). The other two main superclusters are moderately large, having diameters of 91/h Mps and 64/h Mpc (superclusters D and C, respectively).The superstructure is roughly 1 billion light years in diameter, and has a total mass approximately 10,000 times the Milky Way galaxy. It contains at least 830 visible galaxies (represented in the figure within their respective superclusters), as well as many others that are not visible (dark galaxies). The researchers used Minkowski functionals to verify the structure's overall shape and size; the first three quantifying the thickness, width, and length followed by the fourth determining the structure's overall curvature. The research team compared the luminosities and stellar masses within the superstructure to known high stellar mass galaxies within the SDSS's 7th data release, DR7. This allowed the team to scale the data using known values, from local superclusters, to determine the overall morphology of the BOSS Great Wall. It is currently debated amongst astronomers if the BOSS Great Wall is the largest structure in the universe, due to the intricacies of its shape and overall size. The question of whether the supercluster complex is moving together or being slowly separated by the expanding universe is a key factor to this discussion. Nevertheless, when compared to several other chain structures, such as the Sloan Great Wall, the BOSS Great Wall's superclusters are far richer, containing more dense, high stellar mass galaxies. The BOSS Great Wall's discovery, and the data gained therein, should prove very beneficial for astronomers who study the overall structure of the cosmic web.

Binary black hole

A binary black hole (BBH) is a system consisting of two black holes in close orbit around each other. Like black holes themselves, binary black holes are often divided into stellar binary black holes, formed either as remnants of high-mass binary star systems or by dynamic processes and mutual capture, and binary supermassive black holes believed to be a result of galactic mergers.

For many years, proving the existence of binary black holes was made difficult because of the nature of black holes themselves, and the limited means of detection available. However, in the event that a pair of black holes were to merge, an immense amount of energy should be given off as gravitational waves, with distinctive waveforms that can be calculated using general relativity. Therefore, during the late 20th and early 21st century, binary black holes became of great interest scientifically as a potential source of such waves, and a means by which gravitational waves could be proven to exist. Binary black hole mergers would be one of the strongest known sources of gravitational waves in the Universe, and thus offer a good chance of directly detecting such waves. As the orbiting black holes give off these waves, the orbit decays, and the orbital period decreases. This stage is called binary black hole inspiral. The black holes will merge once they are close enough. Once merged, the single hole settles down to a stable form, via a stage called ringdown, where any distortion in the shape is dissipated as more gravitational waves. In the final fraction of a second the black holes can reach extremely high velocity, and the gravitational wave amplitude reaches its peak.

The existence of stellar-mass binary black holes (and gravitational waves themselves) were finally confirmed when LIGO detected GW150914 (detected September 2015, announced February 2016), a distinctive gravitational wave signature of two merging stellar-mass black holes of around 30 solar masses each, occurring about 1.3 billion light years away. In its final 20 ms of spiraling inward and merging, GW150914 released around 3 solar masses as gravitational energy, peaking at a rate of 3.6×1049 watts — more than the combined power of all light radiated by all the stars in the observable universe put together. Supermassive binary black hole candidates have been found but as yet, not categorically proven.

IGR J17091-3624

IGR J17091-3624 (also IGR J17091) is a stellar mass black hole 28,000 light-years away. It lies in the constellation Scorpius in the Milky Way galaxy.

List of exoplanet extremes

The following are lists of extremes among the known exoplanets. The properties listed here are those for which values are known reliably.

M33 X-7

M33 X-7 is a black hole binary system in the galaxy M33. The system is made up of a stellar-mass black hole and a companion star. M33 X-7 is the largest known stellar black hole with an estimated mass of 15.65 times that of the Sun (M☉).[7] The total mass of the system is estimated to be around 85.7 M☉, which would make it the most massive black hole binary system.


M60-UCD1 is an ultracompact dwarf galaxy. It is 54 million light years from Earth, close to Messier 60 (M60, NGC 4649) in the Virgo Cluster. Half of its stellar mass is in the central sphere 160 light years in diameter.


MOA-2007-BLG-192L is a low-mass red dwarf star or brown dwarf, approximately 3,000 light-years away in the constellation of Sagittarius. It is estimated to have a mass approximately 6% of the Sun's. In 2008, an Earth-sized extrasolar planet was announced to be orbiting this object.

Messier 49

Messier 49 (also known as M 49 or NGC 4472) is an elliptical galaxy located about 56 million light-years away in the equatorial constellation of Virgo. This galaxy was discovered by French astronomer Charles Messier on February 16, 1777.

As an elliptical galaxy, Messier 49 has the physical form of a radio galaxy, but it only has the radio emission of a normal galaxy. From the detected radio emission, the core region has roughly 1053 erg (1046 J or 1022 YJ) of synchrotron energy. The nucleus of this galaxy is emitting X-rays, suggesting the likely presence of a supermassive black hole with an estimated mass of 5.65 × 108 solar masses, or 565 million times the mass of the Sun. X-ray emissions shows a structure to the north of Messier 49 that resembles a bow shock. To the southwest of the core, the luminous outline of the galaxy can be traced out to a distance of 260 kpc. The only supernova event observed within this galaxy is SN 1969Q, discovered in June 1969.This galaxy has a large collection of globular clusters, estimated at about 5,900. However, this count is far exceeded by the 13,450 globular clusters orbiting the supergiant elliptical galaxy Messier 87. On average, the globular clusters of M 49 are about 10 billion years old. Between 2000 and 2009, strong evidence for a stellar mass black hole was discovered in an M 49 cluster. A second candidate was announced in 2011.Messier 49 was the first member of the Virgo Cluster of galaxies to be discovered. It is the most luminous member of that cluster and more luminous than any galaxy closer to the Earth. This galaxy forms part of the smaller Virgo B subcluster located 4.5° away from the dynamic center of the Virgo Cluster, centered on Messier 87. Messier 49 is gravitationally interacting with the dwarf irregular galaxy UGC 7636. The dwarf shows a trail of debris spanning roughly 1 × 5 arcminutes, which corresponds to a physical dimension of 6 × 30 kpc.

Orbital decay

In orbital mechanics, decay is a gradual decrease of the distance between two orbiting bodies at their closest approach (the periapsis) over many orbital periods. These orbiting bodies can be a planet and its satellite, a star and any object orbiting it, or components of any binary system. Orbits do not decay without some friction-like mechanism which transfers energy from the orbital motion. This can be any of a number of mechanical, gravitational, or electromagnetic effects. For bodies in a low Earth orbit, the most significant effect is the atmospheric drag.

If left unchecked, the decay eventually results in termination of the orbit when the smaller object strikes the surface of the primary; or for objects where the primary has an atmosphere, the smaller object burns, explodes, or otherwise breaks up in the larger object's atmosphere; or for objects where the primary is a star, ends with incineration by the star's radiation (such as for comets), and so on.

Collisions of stellar-mass objects usually produce cataclysmic effects, such as gamma-ray bursts.

Due to atmospheric drag, the lowest altitude above the Earth at which an object in a circular orbit can complete at least one full revolution without propulsion is approximately 150 km (90 mi).

Stellar black hole

A stellar black hole (or stellar-mass black hole) is a black hole formed by the gravitational collapse of a star. They have masses ranging from about 5 to several tens of solar masses. The process is observed as a hypernova explosion or as a gamma ray burst. These black holes are also referred to as collapsars.

Stellar mass loss

Stellar mass loss is a phenomenon observed in some massive stars. It occurs when a triggering event causes the ejection of a large portion of the star's mass. Stellar mass loss can also occur when a star gradually loses material to a binary companion or into interstellar space.

Swift J1745-26

Swift J1745-26 is a stellar-mass black hole located a few degrees from the center of the Milky Way galaxy toward the constellation Sagittarius. It was discovered by NASA's Swift satellite on September 16, 2012 due to the detection of an X-ray nova. The pattern of X-rays from the nova indicated that the central object was a black hole. Its name arises from the coordinates of its sky position. While astronomers do not know its precise distance, they think the object resides about 20,000 to 30,000 light-years away in the galaxy's inner region. Ground-based observatories have detected infrared and radio emissions from Swift J1745-26, but thick clouds of obscuring dust have prevented astronomers from catching Swift J1745-26 in visible light.Swift J1745-26 must be a member of a low-mass X-ray binary (LMXB) system, which includes a normal, sun-like star.

Tolman–Oppenheimer–Volkoff limit

The Tolman–Oppenheimer–Volkoff limit (or TOV limit) is an upper bound to the mass of cold, nonrotating neutron stars, analogous to the Chandrasekhar limit for white dwarf stars. Observations of GW170817, the first gravitational wave event due to merging neutron stars (which are thought to have collapsed into a black hole within a few seconds after merging), suggest that the limit is close to 2.17 solar masses. A neutron star in a binary pair (PSR J2215+5135) has been measured to have a mass close to or slightly above this limit, 2.27+0.17−0.15 M☉. Earlier theoretical work placed the limit at approximately 1.5 to 3.0 solar masses, corresponding to an original stellar mass of 15 to 20 solar masses. In the case of a rigidly spinning neutron star, the mass limit is thought to increase by up to 18-20%.

Tom Bolton (astronomer)

Charles Thomas Bolton (born 1943) is an American astronomer who was one of the first astronomers to present strong evidence of the existence of a stellar-mass black hole.

Ultraluminous X-ray source

An ultraluminous X-ray source (ULX) is an astronomical source of X-rays that is less luminous than an active galactic nucleus but is more consistently luminous than any known stellar process (over 1039 erg/s, or 1032 watts), assuming that it radiates isotropically (the same in all directions). Typically there is about one ULX per galaxy in galaxies which host them, but some galaxies contain many. The Milky Way has not been shown to contain a ULX. The main interest in ULXs stems from their luminosity exceeding the Eddington luminosity of neutron stars and even stellar black holes. It is not known what powers ULXs; models include beamed emission of stellar mass objects, accreting intermediate-mass black holes, and super-Eddington emission.

WR 7

WR 7 (HD 56925) is a Wolf–Rayet star in the constellation of Canis Major. It lies at the centre of a complex bubble of gas which is shocked and partially ionised by the star's radiation and winds.

The distance is uncertain, with estimates between 3.5 kiloparsecs (11,410 light-years) and 6.9 kiloparsecs (22,500 light-years). Assuming a distance of 4.8 kiloparsecs (15,600 light-years), this star is calculated to be 280,000 times brighter than our Sun] 16 times more massive, and 1.41 times larger with a surface temperature of 112,000 K.

Stars of its kind are characterised by a rapid loss of stellar mass, driven by chemically enriched high-speed stellar winds. It is estimated that it loses mass at the rate of 7x10−5 solar masses each year through winds of 1,545 km/s.The ring nebula NGC 2359 is excited by the ionising radiation of WR7. It is also known as Thor's Helmet or the Duck Nebula. The ring is approximately 4pc across and prominent at wavelengths from radio to X-ray.

XTE J1650-500

XTE J1650-500 is a binary system containing a stellar-mass black hole candidate and 2000–2001 transient binary X-ray source located in the constellation Ara.

In 2008, it was claimed that this black hole had a mass of 3.8±0.5 solar masses, which would have been the smallest found for any black hole; smaller than GRO 1655-40, the then known smallest of 6.3 MSun. However, this claim was subsequently retracted; the more likely mass is 5–10 solar masses.

The binary period of the black hole and its companion is 0.32 days.

Zombie star

A zombie star is a hypothetical result of a Type Iax supernova which leaves behind a remnant star, rather than completely dispersing the stellar mass. Type Iax supernovae are similar to Type Ia, but have a lower ejection velocity and lower luminosity. Scientists think that Type Iax supernovae occur at a rate between 5 and 30 percent of the Ia supernova rate. Thirty supernovae have been identified in this category.In a binary system consisting of a white dwarf and a companion star, the white dwarf strips away material from its companion. Normally the white dwarf would eventually reach a critical mass, and fusion reactions would make it explode and completely dissipate it, but in a Type Iax supernova, only half of the dwarf's mass is lost.

Star systems
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