Stellar black holes

GRO J0422 +32 is an X-ray nova that is part of a binary star system in which one of the components is a massive black hole between 3.66 and 4.97 solar masses.  The secondary star is a red dwarf star of spectral class M1V apparent magnitude 13.2 and 0.116 mass times that of the Sun and has an orbital period, around the black hole of 0.212 days.



Is about 8000 light years from our solar system and is located in the constellation Perseus.  It was discovered during an outburst on August 5, 1992 by Compton Gamma Ray Observatory and it is one of the less massive black holes discovered, not much higher than the maximum limit of neutron stars by 2.7 times the solar mass.  Our Galaxy, the Milky Way, contains several potential candidates for the role of stellar mass black holes, places much closer to us than the supermassive black hole in the Galactic Center, believed to be responsible for the radio source Sagittarius A.  Each candidate is part of a binary X, where the compact object subtracts matter with fellow. The range of masses of these black holes ranges from a minimum of 3 to a little more than a dozen solar masses.

A stellar black hole (or stellar-mass black hole) is a black hole formed by the gravitational collapse of a massive star (20 or more solar masses, although it is not known exactly, because of various parameters, depends on the minimum mass that should have the star) at the end of its evolution. The training process is completed with the explosion of a supernova or gamma ray burst. The most massive stellar black hole known until now (2007) is 1.45 ± 15.65 m☉, although there is evidence that the black hole in the IC 10 X spring X-1 has a higher mass, estimated at 24-33 m☉.  In theory, a black hole could exist of any mass, according to the theory of general relativity.  The smaller the mass, the greater must be the density of matter because they come to form a black hole.  Currently the astrophysicists are inclined to believe that there may be black holes with masses less than a few times that of the Sun; if there were, it would be of primordial black holes.



The gravitational collapse of a massive star, inevitable at the end of its existence because it fails the power source (i.e. nuclear fusion reactions) that counteracts gravity, is one of the processes by which most frequently you create these objects.  If the mass of the star is less than a given limit, rather than forming a black hole is created a degenerate star (white dwarf or neutron star).  The maximum mass that can be reached by a white dwarf is 1.44 m☉ (Chandrasekhar mass), while the mass limit from a neutron star is still not known exactly, but should be around 3 m☉. It is believed, however, that there is a limit similar to Chandrasekhar, who called the Oppenheimer-Volkoff limit and would correspond to 3.8 m☉.  The mass of the least massive black hole so far observed is close to this limit.  Stellar black holes are the “lightest” of this object class; in fact there were discovered several other types of black holes far heavier: they are the intermediate-mass black holes, which are located in the center of globular clusters, and the supermassive black holes, which are at the core of all galaxies, like the milky way, including Active Galaxies.  Every black hole has only three fundamental characteristics: mass, electric charge and angular momentum (spin). The spin of a stellar black hole is due to the conservation of angular momentum of the star from which the compact celestial body originated.  Some of the recently discovered black holes lie within close binary systems in which are gravitationally bound to another star, which are nearer to subtract matter.

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