A stellar black hole (or stellar-mass black hole) is a black hole formed by the gravitational collapse of a star. [1] They have masses ranging from about 5 to several tens of solar masses. [2] They are the remnants of supernova explosions, which may be observed as a type of gamma ray burst. These black holes are also referred to as collapsars.
By the no-hair theorem, a black hole can only have three fundamental properties: mass, electric charge, and angular momentum. The angular momentum of a stellar black hole is due to the conservation of angular momentum of the star or objects that produced it.
The gravitational collapse of a star is a natural process that can produce a black hole. It is inevitable at the end of the life of a massive star when all stellar energy sources are exhausted. If the mass of the collapsing part of the star is below the Tolman–Oppenheimer–Volkoff (TOV) limit for neutron-degenerate matter, the end product is a compact star – either a white dwarf (for masses below the Chandrasekhar limit) or a neutron star or a (hypothetical) quark star. If the collapsing star has a mass exceeding the TOV limit, the crush will continue until zero volume is achieved and a black hole is formed around that point in space.
The maximum mass that a neutron star can possess before further collapsing into a black hole is not fully understood. In 1939, it was estimated at 0.7 solar masses, called the TOV limit. In 1996, a different estimate put this upper mass in a range from 1.5 to 3 solar masses. [3] The maximum observed mass of neutron stars is about 2.14 M☉ for PSR J0740+6620 discovered in September, 2019. [4]
In the theory of general relativity, a black hole could exist of any mass. The lower the mass, the higher the density of matter has to be in order to form a black hole. (See, for example, the discussion in Schwarzschild radius, the radius of a black hole.) There are no known stellar processes that can produce black holes with mass less than a few times the mass of the Sun. If black holes that small exist, they are most likely primordial black holes. Until 2016, the largest known stellar black hole was 15.65±1.45 solar masses. [5] In September 2015, a rotating black hole of 62±4 solar masses was discovered by gravitational waves as it formed in a merger event of two smaller black holes. [6] As of June 2020 [update] , the binary system 2MASS J05215658+4359220 was reported [7] to host the smallest-mass black hole currently known to science, with a mass 3.3 solar masses and a diameter of only 19.5 kilometers.
There is observational evidence for two other types of black holes, which are much more massive than stellar black holes. They are intermediate-mass black holes (in the center of globular clusters) and supermassive black holes in the center of the Milky Way and other galaxies.
Stellar black holes in close binary systems are observable when the matter is transferred from a companion star to the black hole; the energy released in the fall toward the compact star is so large that the matter heats up to temperatures of several hundred million degrees and radiates in X-rays. The black hole, therefore, is observable in X-rays, whereas the companion star can be observed with optical telescopes. The energy release for black holes and neutron stars are of the same order of magnitude. Black holes and neutron stars are therefore often difficult to distinguish.
The derived masses come from observations of compact X-ray sources (combining X-ray and optical data). All identified neutron stars have a mass below 3.0 solar masses; none of the compact systems with a mass above 3.0 solar masses display the properties of a neutron star. The combination of these facts makes it more and more likely that the class of compact stars with a mass above 3.0 solar masses are in fact black holes.
Note that this proof of the existence of stellar black holes is not entirely observational but relies on theory: we can think of no other object for these massive compact systems in stellar binaries besides a black hole. A direct proof of the existence of a black hole would be if one actually observes the orbit of a particle (or a cloud of gas) that falls into the black hole.
The large distances above the galactic plane achieved by some binaries are the result of black hole natal kicks. The velocity distribution of black hole natal kicks seems similar to that of neutron star kick velocities. One might have expected that it would be the momenta that were the same with black holes receiving lower velocity than neutron stars due to their higher mass but that doesn't seem to be the case, [8] which may be due to the fall-back of asymmetrically expelled matter increasing the momentum of the resulting black hole. [9]
It is predicted by some models of stellar evolution that black holes with masses in two ranges cannot be directly formed by the gravitational collapse of a star. These are sometimes distinguished as the "lower" and "upper" mass gaps, roughly representing the ranges of 2 to 5 and 50 to 150 solar masses (M☉), respectively. [10] Another range given for the upper gap is 52 to 133 M☉. [11] 150 M☉ has been regarded as the upper mass limit for stars in the current era of the universe. [12]
A lower mass gap is suspected on the basis of a scarcity of observed candidates with masses within a few solar masses above the maximum possible neutron star mass. [10] The existence and theoretical basis for this possible gap are uncertain. [13] The situation may be complicated by the fact that any black holes found in this mass range may have been created via the merging of binary neutron star systems, rather than stellar collapse. [14] The LIGO/Virgo collaboration has reported three candidate events among their gravitational wave observations in run O3 with component masses that fall in this lower mass gap. There has also been reported an observation of a bright, rapidly rotating giant star in a binary system with an unseen companion emitting no light, including x-rays, but having a mass of 3.3+2.8
−0.7 solar masses. This is interpreted to suggest that there may be many such low-mass black holes that are not currently consuming any material and are hence undetectable via the usual x-ray signature. [15]
The upper mass gap is predicted by comprehensive models of late-stage stellar evolution. It is expected that with increasing mass, supermassive stars reach a stage where a pair-instability supernova occurs, during which pair production, the production of free electrons and positrons in the collision between atomic nuclei and energetic gamma rays, temporarily reduces the internal pressure supporting the star's core against gravitational collapse. [16] This pressure drop leads to a partial collapse, which in turn causes greatly accelerated burning in a runaway thermonuclear explosion, resulting in the star being blown completely apart without leaving a stellar remnant behind. [17]
Pair-instability supernovae can only happen in stars with a mass range from around 130 to 250 solar masses (M☉) (and low to moderate metallicity (low abundance of elements other than hydrogen and helium – a situation common in Population III stars)). However, this mass gap is expected to be extended down to about 45 solar masses by the process of pair-instability pulsational mass loss, before the occurrence of a "normal" supernova explosion and core collapse. [18] In nonrotating stars the lower bound of the upper mass gap may be as high as 60 M☉. [19] The possibility of direct collapse into black holes of stars with core mass > 133 M☉, requiring total stellar mass of > 260 M☉ has been considered, but there may be little chance of observing such a high-mass supernova remnant; i.e., the lower bound of the upper mass gap may represent a mass cutoff. [11]
Observations of the LB-1 system of a star and unseen companion were initially interpreted in terms of a black hole with a mass of about 70 solar masses, which would be excluded by the upper mass gap. However, further investigations have weakened this claim.
Black holes may also be found in the mass gap through mechanisms other than those involving a single star, such as the merger of black holes.
Our Milky Way galaxy contains several stellar-mass black hole candidates (BHCs) which are closer to us than the supermassive black hole in the galactic center region. Most of these candidates are members of X-ray binary systems in which the compact object draws matter from its partner via an accretion disk. The probable black holes in these pairs range from three to more than a dozen solar masses. [20] [21] [22]
| Name | Mass (solar masses) | Orbital period (days) | Distance from Earth (ly) | Celestial Coordinates [23] | |
|---|---|---|---|---|---|
| BHC | Companion | ||||
| Gaia BH3 | 32.70 ± 0.82 | 0.76 ± 0.05 | 4,253.1 ± 98.5 | 1926 | 19:39:19 +14:55:54 |
| Cyg X-1 | 21.2 ± 2.2 [24] | 40.6+7.7 −7.1 [24] | 5.6 | 6000...8000 | 19:58:22 +35:12:06 |
| GRS 1915+105/V1487 Aql | 14 ± 4.0 | ≈1 | 33.5 | 40000 | 19:15:12 +10:56:44 |
| V404 Cyg | 12 ± 2 | 6.0 | 6.5 | 7800 ± 460 [25] | 20:24:04 +33:52:03 |
| A0620-00/V616 Mon | 11 ± 2 | 2.6–2.8 | 0.33 | 3500 | 06:22:44 −00:20:45 |
| XTE J1650-500 | 9.7 ± 1.6 [26] | 5–10 | 0.32 [27] | 10763 | 16:50:01 −49:57:45 |
| Gaia BH1 | 9.62 ± 0.18 | 0.93 ± 0.05 | 185.59 ± 0.05 | 1560 | 17:28:41 −00:34:52 |
| XTE J1550-564/V381 Nor | 9.6 ± 1.2 | 6.0...7.5 | 1.5 | 17000 | 15:50:59 −56:28:36 |
| 4U 1543-475/IL Lupi | 9.4 ± 1.0 | 0.25 | 1.1 | 24000 | 15:47:09 −47:40:10 |
| Gaia BH2 | 8.94 ± 0.34 | 1.07 ± 0.19 | 1,276.7 ± 0.6 | 3800 | 13:50:17 −59:14:20 |
| MAXI J1305-704 [28] | 8.9+1.6 −1.0 | 0.43 ± 0.16 | 0.394 ± 0.004 | 24500 | 13:06:55 −70:27:05 |
| GS 1354-64 (BW Cir) [29] | 7.9 ± 0.5 | 1.1 ± 0.1 | 2.5445 | >81500 | 13:58:10 −64:44:06 |
| XTE J1859+226 (V406 Vul) [30] | 7.8 ± 1.9 | 0.55 ± 0.16 | 0.276 ± 0.003 | 18:58:42 +22:39:29 | |
| HD 130298 [31] | >7.7 ± 1.5 | 24.2 ± 3.8 | 14.60 | 7910 | 14:49:34 −56:25:38 |
| NGC 3201 #21859 [32] [33] | 7.68 ± 0.50 | 0.61 ± 0.05 | 2.2422 ± 0.0001 | 15700 | 10:17:39 −46:24:25 |
| GS 2000+25/QZ Vul | 7.5 ± 0.3 | 4.9...5.1 | 0.35 | 8800 | 20:02:50 +25:14:11 |
| XTE J1819-254/V4641 Sgr | 7.1 ± 0.3 | 5...8 | 2.82 | 24000...40000 [34] | 18:19:22 −25:24:25 |
| LB-1 (disputed) [35] | 7 ± 2 [35] | 1.5 ± 0.4 [35] | 78.7999 ± 0.0097 [35] | 15000 [36] | 06:11:49 +22:49:32 [37] |
| GRS 1124-683/Nova Muscae 1991/GU Mus | 7.0 ± 0.6 | 0.43 | 17000 | 11:26:27 −68:40:32 | |
| H 1705-25/Nova Ophiuchi 1977/V2107 Oph [38] | 6.95 ± 1.35 [39] | 0.34 ± 0.08 | 0.52125 | 17:08:15 −25:05:30 | |
| XTE J1118+480/KV UMa | 6.8 ± 0.4 | 6...6.5 | 0.17 | 6200 | 11:18:11 +48:02:13 |
| MAXI J1820+070 [40] | 6.75+0.64 −0.46 | 0.49 ± 0.1 | 0.68549 ± 0.00001 | 9800 | 18:20:22 +07:11:07 |
| GRO J1655-40/V1033 Sco | 6.3 ± 0.3 | 2.6...2.8 | 2.8 | 5000...11000 | 16:54:00 −39:50:45 |
| GX 339-4/V821 Ara | 5.8 | 5...6 | 1.75 | 15000 | 17:02:50 −48:47:23 |
| GRO J1719-24 | ≥4.9 | ≈1.6 | possibly 0.6 [41] | 8500 | 17:19:37 −25:01:03 |
| NGC 3201 #12560 [32] [33] | 4.53 ± 0.21 | 0.81 ± 0.05 | 167.01 ± 0.09 | 15700 | 10:17:37 −46:24:55 |
| GRS 1009-45 / Nova Velorum 1993/MM Velorum [42] | 4.3 ± 0.1 | 0.5...0.65 | 0.285206 ± 0.0000014 | 17200 | 10:13:36 −45:04:33 |
| GRO J0422+32/V518 Per | 4 ± 1 | 1.1 | 0.21 | 8500 | 04:21:43 +32:54:27 |
Candidates outside our galaxy come from gravitational wave detections:
| Name | BHC mass (solar masses) | Companion mass (solar masses ) | Orbital period (days) | Distance from Earth (light years) | Location [23] |
|---|---|---|---|---|---|
| GW190521 (155+17 −11) M☉ | 78+9 −5 [43] | 78+9 −5 [43] | |||
| GW150914 (62 ± 4) M☉ | 36 ± 4 | 29 ± 4 | . | 1.3 billion | |
| GW170104 (48.7 ± 5) M☉ | 31.2 ± 7 | 19.4 ± 6 | . | 1.4 billion | |
| GW170814 (53.2+3.2 −2.5) M☉ | 30.5+5.7 −3.0 | 25.3+2.8 −4.2 | 1.8 billion | ||
| GW190412 | 29.7 | 8.4 | 2.4 billion | ||
| GW190814 | 22.2–24.3 | 2.50–2.67 | |||
| GW151226 (21.8 ± 3.5) M☉ | 14.2 ± 6 | 7.5 ± 2.3 | . | 2.9 billion | |
| GW170608 | 12+7 −2 | 7 ± 2 | 1.1 billion |
Candidates outside our galaxy from X-ray binaries:
| Name | Host galaxy | BHC mass (solar masses) | Companion mass (solar masses ) | Orbital period (days) | Distance from Earth (light years) |
|---|---|---|---|---|---|
| IC 10 X-1 [44] | IC 10 | ≥23.1 ± 2.1 | ≥17 | 1.45175 | 2.15 million |
| NGC 300 X-1 [45] | NGC 300 | 17 ± 4 | 26+7 −5 | 1.3663375 | 6.5 million |
| M33 X-7 | Triangulum Galaxy | 15.65 ± 1.45 | 70 ± 6.9 | 3.45301 ± 0.00002 | 2.7 million |
| LMC X-1 [46] | Large Magellanic Cloud | 10.91 ± 1.41 | 31.79 ± 3.48 | 3.9094 ± 0.0008 | 180,000 [47] |
| LMC X-3 [48] | Large Magellanic Cloud | 6.98 ± 0.56 | 3.63 ± 0.57 | 1.704808 | 157,000 |
The disappearance of N6946-BH1 following a failed supernova in NGC 6946 may have resulted in the formation of a black hole. [49]
A black hole is a region of spacetime where gravity is so strong that nothing, including light and other electromagnetic waves, is capable of possessing enough energy to escape it. Einstein's theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of no escape is called the event horizon. A black hole has a great effect on the fate and circumstances of an object crossing it, but it has no locally detectable features according to general relativity. In many ways, a black hole acts like an ideal black body, as it reflects no light. Quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is of the order of billionths of a kelvin for stellar black holes, making it essentially impossible to observe directly.
A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses (M☉), possibly more if the star was especially metal-rich. Except for black holes, neutron stars are the smallest and densest known class of stellar objects. Neutron stars have a radius on the order of 10 kilometers (6 mi) and a mass of about 1.4 M☉. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei.
A supernova is a powerful and luminous explosion of a star. A supernova occurs during the last evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed to form a diffuse nebula. The peak optical luminosity of a supernova can be comparable to that of an entire galaxy before fading over several weeks or months.
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.
Cygnus X-1 (abbreviated Cyg X-1) is a galactic X-ray source in the constellation Cygnus and was the first such source widely accepted to be a black hole. It was discovered in 1965 during a rocket flight and is one of the strongest X-ray sources detectable from Earth, producing a peak X-ray flux density of 2.3×10−23 W/(m2⋅Hz) (2.3×103 jansky). It remains among the most studied astronomical objects in its class. The compact object is now estimated to have a mass about 21.2 times the mass of the Sun and has been shown to be too small to be any known kind of normal star or other likely object besides a black hole. If so, the radius of its event horizon has 300 km "as upper bound to the linear dimension of the source region" of occasional X-ray bursts lasting only for about 1 ms.
X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the donor, to the other component, called the accretor, which is either a neutron star or black hole. The infalling matter releases gravitational potential energy, up to 30 percent of its rest mass, as X-rays. The lifetime and the mass-transfer rate in an X-ray binary depends on the evolutionary status of the donor star, the mass ratio between the stellar components, and their orbital separation.
X-ray bursters are one class of X-ray binary stars exhibiting X-ray bursts, periodic and rapid increases in luminosity that peak in the X-ray region of the electromagnetic spectrum. These astrophysical systems are composed of an accreting neutron star and a main sequence companion 'donor' star. There are two types of X-ray bursts, designated I and II. Type I bursts are caused by thermonuclear runaway, while type II arise from the release of gravitational (potential) energy liberated through accretion. For type I (thermonuclear) bursts, the mass transferred from the donor star accumulates on the surface of the neutron star until it ignites and fuses in a burst, producing X-rays. The behaviour of X-ray bursters is similar to the behaviour of recurrent novae. In the latter case the compact object is a white dwarf that accretes hydrogen that finally undergoes explosive burning.
In astronomy, the term compact object refers collectively to white dwarfs, neutron stars, and black holes. It could also include exotic stars if such hypothetical, dense bodies are confirmed to exist. All compact objects have a high mass relative to their radius, giving them a very high density, compared to ordinary atomic matter.
A super-luminous supernova is a type of stellar explosion with a luminosity 10 or more times higher than that of standard supernovae. Like supernovae, SLSNe seem to be produced by several mechanisms, which is readily revealed by their light-curves and spectra. There are multiple models for what conditions may produce an SLSN, including core collapse in particularly massive stars, millisecond magnetars, interaction with circumstellar material, or pair-instability supernovae.
A supermassive black hole is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (M☉). Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, not even light. Observational evidence indicates that almost every large galaxy has a supermassive black hole at its center. For example, the Milky Way galaxy has a supermassive black hole at its center, corresponding to the radio source Sagittarius A*. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei (AGNs) and quasars.
In astrophysics, silicon burning is a very brief sequence of nuclear fusion reactions that occur in massive stars with a minimum of about 8–11 solar masses. Silicon burning is the final stage of fusion for massive stars that have run out of the fuels that power them for their long lives in the main sequence on the Hertzsprung–Russell diagram. It follows the previous stages of hydrogen, helium, carbon, neon and oxygen burning processes.
An intermediate-mass black hole (IMBH) is a class of black hole with mass in the range 102–105 solar masses: significantly more than stellar black holes but less than the 105–109 solar mass supermassive black holes. Several IMBH candidate objects have been discovered in the Milky Way galaxy and others nearby, based on indirect gas cloud velocity and accretion disk spectra observations of various evidentiary strength.
The Tolman–Oppenheimer–Volkoff limit is an upper bound to the mass of cold, non-rotating neutron stars, analogous to the Chandrasekhar limit for white dwarf stars. Stars more massive than the TOV limit collapse into a black hole. The original calculation in 1939, which neglected complications such as nuclear forces between neutrons, placed this limit at approximately 0.7 solar masses (M☉). Later, more refined analyses have resulted in larger values.
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, although SS 433 may be a possible source. 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.
A Type II supernova or SNII results from the rapid collapse and violent explosion of a massive star. A star must have at least eight times, but no more than 40 to 50 times, the mass of the Sun (M☉) to undergo this type of explosion. Type II supernovae are distinguished from other types of supernovae by the presence of hydrogen in their spectra. They are usually observed in the spiral arms of galaxies and in H II regions, but not in elliptical galaxies; those are generally composed of older, low-mass stars, with few of the young, very massive stars necessary to cause a supernova.
Gravitational-wave astronomy is an emerging field of science, concerning the observations of gravitational waves to collect relatively unique data and make inferences about objects such as neutron stars and black holes, events such as supernovae, and processes including those of the early universe shortly after the Big Bang.
A pair-instability supernova is a type of supernova predicted to occur when pair production, the production of free electrons and positrons in the collision between atomic nuclei and energetic gamma rays, temporarily reduces the internal radiation pressure supporting a supermassive star's core against gravitational collapse. This pressure drop leads to a partial collapse, which in turn causes greatly accelerated burning in a runaway thermonuclear explosion, resulting in the star being blown completely apart without leaving a stellar remnant behind.
Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface.
Gamma-ray burst progenitors are the types of celestial objects that can emit gamma-ray bursts (GRBs). GRBs show an extraordinary degree of diversity. They can last anywhere from a fraction of a second to many minutes. Bursts could have a single profile or oscillate wildly up and down in intensity, and their spectra are highly variable unlike other objects in space. The near complete lack of observational constraint led to a profusion of theories, including evaporating black holes, magnetic flares on white dwarfs, accretion of matter onto neutron stars, antimatter accretion, supernovae, hypernovae, and rapid extraction of rotational energy from supermassive black holes, among others.
A hypernova is a very energetic supernova which is believed to result from an extreme core-collapse scenario. In this case, a massive star collapses to form a rotating black hole emitting twin astrophysical jets and surrounded by an accretion disk. It is a type of stellar explosion that ejects material with an unusually high kinetic energy, an order of magnitude higher than most supernovae, with a luminosity at least 10 times greater. Hypernovae release so much of gamma rays they usually appear similar to a type Ic supernova, but with unusually broad spectral lines indicating an extremely high expansion velocity. Hypernovae are one of the mechanisms for producing long gamma ray bursts (GRBs), which range from 2 seconds to over a minute in duration. They have also been referred to as superluminous supernovae, though that classification also includes other types of extremely luminous stellar explosions that have different origins.
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