Black holes are objects so dense that not even light can escape their gravity, and even light cannot escape from inside a black hole. On the other hand, a black hole exerts the same force on something far away from it as any other object of the same mass would. For example, if our Sun was magically crushed until it was about 1 mile in size, it would become a black hole, but the Earth would remain in its same orbit.
Even back in Isaac Newton’s time, scientists speculated that such objects could exist, even though we now know they are more accurately described using Einstein’s General Theory of Relativity. Using this theory, black holes are fascinating objects, where space and time become so warped that time practically stops in the vicinity of a black hole.
The former type has measured masses ranging from 4 to 15 Suns, and is believed to be formed during supernova explosions. The after-effects are observed in some X-ray binaries known as black hole candidates. On the other hand, galaxy-mass black holes are found in Active Galactic Nuclei (AGN).
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These are thought to have the mass of about 10 to 100 billion Suns. The mass of one of these super massive black holes has recently been measured using radio astronomy. X-ray observations of iron in the accretion disks may actually be showing the effects of such a massive black hole as well.