An extremely compact star which has collapsed by enormous gravitational pull to overcome electric and nuclear forces. Black holes are usually formed with stars much larger than the sun as they exhaust all their nuclear fuel and collapse into high density objects. The term black is used because not even light can escape their intense gravity. Material attracted to a black hole can gain enormous energy and radiate part of it before being swallowed up. It's generally believed that enormously massive black holes exist in the centre of ours and other galaxies.