Asked by: Michael Thorne, Havant
Although it seems very likely that most spiral galaxies (and possibly all types of galaxies) contain a black hole, these are not the only places where these gravitational beasts reside.
By their very nature, black holes are not directly observable, and astronomers rely on their effects on the environment to detect them. The disc of material orbiting a black hole can become extremely hot and emit huge quantities of radiation (mainly X-rays), which may be detected by telescopes. Many of the Universe’s more energetic phenomena have therefore been attributed to the accretion of matter by black holes. These include discs that eject powerful beams of X-rays.
Astronomers have also discovered isolated, stellar-mass black holes adrift among the stars in our Milky Way. These have been found indirectly by measuring how their extreme gravity bends the light of a more distant star behind them.