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Stellar black hole
A stellar black hole is a black hole formed by the collapse of a massive star at the end of its lifetime. The process is observed as a supernova explosion or as a Gamma ray Burst. Such a black hole has a mass of at least 3 to 5 solar masses. The most massive known stellar black hole (as of 2001) is 14 solar masses.
A black hole could exist of any mass in theory (general relativity). However, are they also formed in nature? The lower the mass, the higher the density of matter has to be in order to form a black hole (see e.g. the discussion in Schwarzschild radius, the radius of a black hole). There are no known processes that can produce black holes with mass less than a few times the mass of the Sun. If they exist, they are most likely primordial black holes.
The collapse of a star is a natural process to produce a black hole. It is inevitable at the end of the life of a star, when all stellar energy sources are exhausted. If the mass of the collapsing part of the star is below a certain critical value, the end product is a compact star, either a white dwarf or a neutron star. Both these stars have a maximum mass. So if the collapsing star has a mass exceeding this limit, the collapse will continue forever (catastrophic gravitational collapse) and form a black hole. The maximum mass of a neutron star is not well known, but is believed to be between 3 and 5 solar masses.
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 centre of globular clusters) and supermassive black holes in the centre of the milky way and Active Galaxies.
A black hole can only have three properties: mass, electric charge and angular momentum (spin). It is believed that black holes formed in nature all have spin, but no definite observation on the spin have been performed. The spin of a stellar black hole is due to the conservation of angular momentum of the star that produced it.
The observed mass of stellar black holes
in X-ray compact binary systems
Stellar black holes in close binary systems are observable when matter is transferred from a companion star to the black hole. The energy release in the fall toward the compact star is so large that the matter heats up to temperatures of several hundred million degree and radiates in X-rays (X-ray astronomy). 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 often difficult to distinguish.
However, neutron stars may have additional properties. They rotate, can have a magnetic field and exhibit localized explosions (thermonuclear bursts). Whenever such properties are observed, the compact object in the binary system is revealed as a neutron star.
The derived masses from observations of compact x-ray sources (combining x-ray and optical data) is shown in the figure. All identified neutron stars have a mass below 3 to 5 solar masses. None of the compact systems with a mass above 5 solar masses reveals the properties of a neutron star. The combination of these facts make it more and more likely that the class of compact stars with a mass above 5 solar masses are in fact black holes.
Note that this proof of 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 than 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 blob of gas) that falls into the black hole.
Classification of black holes by type:
- Schwarzschild or static black hole
- rotating or Kerr black hole
- charged black hole or Newman black hole and Kerr-Newman black hole
A classification of black holes by mass:
- micro black hole and extra-dimensional black hole
- primordial black hole, a hypothetical leftover of the Big Bang
- stellar black hole, which could either be a static black hole or a rotating black hole
- Intermediate-mass black hole
- supermassive black hole, which could also either be a static black hole or a rotating black hole
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