Expert: Courtney Seligman - 7/25/2011

Question
sir,
   I wont to know what is the difference between astronomy,astrophysics and cosmology? and what is the behavior of blackhole if it has no food?

Answer
Astronomy is the entire field of astronomy. It involves anything that has to do with the sky and its "contents" -- observations, theory, appearance, physical reality, history, the future, you name it.

Astrophysics is that part of astronomy which deals with the theory of how astronomical bodies and systems of bodies "work". Usually, that means how stars and stellar systems work, but it includes any theoretical study, including how planetary systems form, how galaxies evolve, and so on. It does not include observational astronomy, except for using observational results to test theories, or suggest problems with existing theories.

Cosmology is that part of astrophysics which deals with the origin and evolution of the Universe. Strictly speaking, it deals with only the origin and evolution of our Universe, but some cosmologists work on metaphysical questions, such as whether there could be other Universes, and what (if there are) their properties might be.

I presume your last question means what does a black hole do if nothing is falling into it. In that case, it moves through space in the same way that any other object of its mass and motion would, absorbing any light that falls into it, bending the path of any light that passes close (but not too close) to it, but otherwise completely dark, silent and (usually) undetectable. Stephen Hawking suggests that if such a black hole is very small (smaller than an atom), it may emit small amounts of radiation through quantum tunneling, and eventually "evaporate". But "normal"-sized black holes would be essentially permanent things, and just keep going and going, for the rest of eternity. Of course, saying that it moves through space assumes that it has some motion relative to the other objects which make up its galaxy or galaxy cluster. Supermassive black holes are usually near the center of such structures, and have little or no movement relative to their surroundings. So they would just "sit" there, instead of moving along. But they would still (in the absence of "food") be completely dark, and nearly undetectable.