A black hole is a region of space-time containing enough matter to create a gravitational field sufficiently strong to prevent light escaping from it. This idea was first suggested by John Mitchell in 1783, but a detailed understanding of the properties of black holes only came with the discovery of General Relativity. It is now known that black holes can have only three properties: mass, angular momentum and electric charge.
A 5000 light year long jet emerging from a 2.4x109 solar-mass black hole at the centre of the giant elliptical galaxy M87. (HST)
Although black holes are themselves invisible, they are the most efficient sources of energy known in the universe, capable of converting almost 50% of the rest-mass that falls into them into energy. In contrast, hydrogen fusion in stars only converts 1% of rest mass into energy.
As matter falls towards a black hole, the principle of the conservation of angular momentum demands that its velocity of rotation increases. Thus the matter forms a rapidly rotating accretion disc around the black hole. Within the accretion disc there are many collisions that raise the temperature so high that energy is released in the form of intense X-rays.
The accretion disc surrounding a black hole.
Black holes have been found in some double-star systems and in the centres of galaxies. Those in double-star systems are probably about 10 times the mass of the Sun and the radius of the event horizon is about 30km. They are the remnants of more massive stars that eventually exhausted their nuclear fuel and were crushed into a black hole by the overwhelming force of gravity.
Quasars are the most luminous objects in the universe and belong to a class of galaxies that have black holes, typically 109 times the mass of the Sun, at their centres. Our own galaxy has a black hole at its centre. It is quiet at the moment as there is no matter falling into it.
Black holes in the nuclei of galaxies are responsible for a wide range of exotic high-energy astrophysical processes, including jets such as that seen in the nearby galaxy M87, as well as intense radio, X-rays and γ-rays. These observations enable Einstein’s theory to be tested in strong gravitational fields.
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