black hole

Object in space whose gravity is so great that nothing can escape from it, not even light. It is thought to form when a massive star shrinks at the end of its life. A black hole sucks in more matter, including other stars, from the space around it. Matter that falls into a black hole is squeezed to infinite density at the centre of the hole. Black holes can be detected because gas falling towards them becomes so hot that it emits X-rays.

Black holes containing the mass of millions of stars are thought to lie at the centres of quasars. Satellites have detected X-rays from a number of objects that may be black holes, but only a small number of likely black holes have been identified in our galaxy.

Cygnus X-1, first discovered in 1964 by astronomers at the US Naval Research Laboratory, is an X-ray source in the constellation of Cygnus. A0620–00, in the constellation of Monoceros, is one of the best black-hole candidates in the galaxy, discovered in the 1980s by US astronomers Jeffrey McClintock of the Harvard-Smithsonian Center for Astrophysics and Ronald Remillard of the Massachusetts Institute of Technology. V404 Cygni, close to Cygnus X-1, is a possible black hole discovered in 1992. Nova Muscae, identified as a black hole in 1992 by McClintock, Remillard, and US astronomer Charles Bailyn of Yale University, New Haven, Connecticut, lies approximately 18,000 light years from Earth.

The Hubble Space Telescope discovered in 1997 evidence of a black hole 300 million times the mass of the Sun. It is located in the middle of galaxy M84 about 50 million light years from Earth. In March 2001, NASA scientists, using images from the orbiting Chandra X-Ray Observatory of X-ray emissions from space objects as they may have appeared 12 billion years ago, concluded that there may have been about 300 billion black holes when the universe was young.

In 2004 astronomers reported for the first time a star being absorbed by a black hole. Three space-based telescopes were used to reconstruct the event, which was first observed by the Röntgen Satellite (ROSAT) in 1992 as an intense X-ray flare from the centre of a galaxy; further data obtained in 2001 confirmed that the event had been caused by a black hole absorbing a star.

Astronomers also discovered a second black hole at the centre of our galaxy in 2004. The Gemini Observatory on Mauna Kea, Hawaii, was used to image an area of the galactic core known as IRS 13, where the astronomers discovered a cluster of seven stars in an area only 0.065 light years across. They used data from the Hubble Space Telescope to determine that the stars were orbiting an intermediate mass black hole that they designated IRS 13E – the first intermediate mass black hole found in our galaxy. IRS 13E rotates around Sagittarius A∗, the supermassive black hole at the centre of our galaxy.

Microscopic black holes may have been formed in the chaotic conditions of the Big Bang. The English physicist Stephen Hawking has shown that such tiny black holes could ‘evaporate’ and explode in a flash of energy.