Stellar black hole

A stellar black hole is a black hole formed by the gravitational collapse of a massive star (20 or more solar masses, though the exact amount of mass needed has not been determined and may depend on many parameters) at the end of its lifetime. The process is observed as a supernova explosion or as a gamma ray burst. The largest known stellar black hole (as of 2007) is 15.65±1.45 solar masses. [Nature 449, 799-801 (18 October 2007)] Additionally, there is evidence that the IC 10 X-1 X-ray source is a stellar black hole with a probable mass of 24-33 solar masses. [Prestwich et al., The Astrophysical Journal, volume 669, part 2 (2007), pages L21–L24]

A black hole could exist of any mass in theory (general relativity). 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 ablack hole. The maximum mass of a neutron star is not well known, but is believed to be about 3 solar masses. The least massive stellar-mass black hole so far observed has an estimated mass of 3.8 solar masses. [cite web|url=http://www.nasa.gov/centers/goddard/news/topstory/2008/smallest_blackhole.html |title=NASA Scientists Identify Smallest Known Black Hole |date=2008-04-01]

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 fundamental 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 masses 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 degrees 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 show differential rotation, and 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 come from observations of compact X-ray sources (combining X-ray and optical data). 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.

tellar mass black hole candidates

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. These candidates are all 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. [J. Casares: "Observational evidence for stellar mass black holes." [http://arxiv.org/abs/astro-ph/0612312 Preprint] ] [M.R. Garcia et al.: "Resolved Jets and Long Period Black Hole Novae." [http://arxiv.org/abs/astro-ph/0302230 Preprint] ]

References

ee also

Stellar-mass black hole candidates:
*Cygnus X-1
*LMC X-3
*A 0620-00
*SS 433

External links and further reading

* [http://www.hubblesite.org/go/blackholes Black Holes: Gravity's Relentless Pull] Award-winning interactive multimedia Web site about the physics and astronomy of black holes from the Space Telescope Science Institute
* [http://mintaka.sdsu.edu/faculty/orosz/web/ Black hole diagrams]
* [http://www.citebase.org/cgi-bin/citations?id=oai:arXiv.org:astro-ph/0307307 Janusz Ziółkowski "Black Hole Candidates"]
* [http://newswise.com/articles/view/534422/ Heaviest Stellar Black Hole Discovered in Nearby Galaxy, Newswise, 17-Oct-2007]