What is black hole in astronomy?

Answer 1

Black holes sound like they're straight out of a science fiction story: objects so dense that nothing in the universe can escape from their gravitational pull. But over the past few decades astronomers have been steadily building up evidence that black holes are not only real, but, in fact, quite prevalent in the universe.

It is now thought that almost all galaxies contain gigantic black holes in their centers, millions or even billions of times more massive than the Sun. Some of these beasts are among the most violent and energetic objects in the universe - active galactic nuclei and quasars, which shoot off jets even as they suck in surrounding gas - while others, often older ones like the black hole at the center of the Milky Way, are considerably more quiet feeders.

Galaxies are also thought to contain many examples of small black holes, with masses only a few times greater than that of the Sun. Astronomers have detected a handful of these in our galaxy, by observing the light emitted when they shred apart their companion star in a binary system. Several of these small black holes have been dubbed "microquasars" because they produce miniature jets akin to those of their larger cousins.

Answer 2

A black hole is a theoretical body that has a density of infinity and a volume of 0.

Answer 3

http://www.damtp.cam.ac.uk/user/gr/public/bh_home.html