"Astronomy is the observational and theoretical study of celestial bodies, of the intervening regions of Space and of the universe as a whole."
Above Quote from _Fact on File Dictionary Astronomy_ by Illingworth and is probably available in your library.

Astronomers are usually willing to help you with your homework assignments, but you will get faster and more complete help if you use a dictionary when the answer is easily obtained through locally available books.

Hope you get an "A".