Black hole (Illustration below) |
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If a neutron star has a mass in excess of 3 solar masses it will collapse further through what is called the event horizon to become a 'black hole'. Two types of black holes are now recognized. Stellar black holes detected as companions in binary star systems (with masses of the order of 10 solar masses) and huge accreting black holes thought to exist at the centers of galaxies with a mass of up to or more than a thousand million solar masses. Light cannot leave a black hole across the event horizon - defined as the radius for which the escape velocity is the velocity of light - but an accretion disc of matter can radiate from points outside the event horizon. |