Everyone knows Damon Knight's definition of science fiction, which appears in the rec.arts.sf.written FAQ:
"...[Science Fiction] means what we point to when we say it."
More definitions of SF here .

Simple:
    Hugo Gernsback: By "scientification,"... I mean the Jules Verne, H. G. Wells, and Edgar Allan Poe type of story---a charming romance intermingled with scientific fact and prosphetic vision

    Sam J. Lundwall: A simplified definition would be that the author of a "straight" science fiction story proceeds from (or alleges to proceed from) known facts, developed in a credible way...

Clear:
    Robert A. Heinlein: A handy short definition of almost all science fiction might read: realistic speculation about possible future events, based solidly on adequate knowledge of the real world, past and present, and on a thorough understanding of the nature and significance of the scientific method.
To make this definition cover all science fiction (instead of "almost all") it is necessary only to strike out the word "future." from: Science Fiction: its nature, faults and virtues, in The Science Fiction Novel, Advent, Chicago:1969

Science Fiction is speculative fiction in which the author takes as his first postulate the real world as we know it, including all established facts and natural laws. The result can be extremely fantastic in content, but it is not fantasy; it is legitimate--and often very tightly reasoned--speculation about the possibilities of the real world. This category excludes rocket ships that make U-turns, serpent men of Neptune that lust after human maidens, and stories by authors who flunked their Boy Scout merit badge tests in descriptive astronomy.
from: Ray Guns And Spaceships, in Expanded Universe, Ace, 1981

Classic:
    Sam Moskowitz: Science fiction is a branch of fantasy identifiable by the fact that it eases the "willing suspension of disbelief" on the part of its readers by utilizing an atmosphere of scientific credibility for its imaginative speculations in physical science, space, time, social science, and philosophy.

Adding science makes your work more salable. Editors commonly remark that they can’t get enough hard-SF. Link to guidelines. Listen to editorial comment.

Examples of recently published works, heavy on scientific content:

Novels:
Darwin's Radio by Greg Bear
Return to Mars by Ben Bova
Spherical Harmonic from Catherine Asaro’s Skolian Saga
Alien Taste
Infinity Beach by Jack McDevitt
Flashforward by Robert J. Sawyer
A Deepness in the Sky by Vinge, Vernor

Novella: "The Ice Dragon's Song" by Sparhawk, Bud
Novelette: "Mars is No Place for Children" by Mary A. Turzillo

Asimov's Magazine editor, Gardner Dozois, posted his recommended reading list. He has edited a new anthology, just out, called World-building.

Bibliographies by topic:

·     First Contact stories and novels, bibliography collected by Steven H. Silver (Programming Division Director for the 58th World Science Fiction Convention, Chicago 2000).

·     The Moon in science fiction, a bibliography of hundreds of fictional works involving the moon, listed alphabetically by author. Novel and story descriptions are included.

·     Mars in science fiction

·     Pluto in science fiction:

Stories, which include lots of science including astronomy, do well in terms of awards. See Laurie Mann's awardweb site.

Personal taste: I like to come away having learned something when I read fiction. Many science fiction readers get a healthy portion of all their science learning from reading fiction. Enhance scientific literacy among readers.

Scientific Literacy: The knowledge and understanding of scientific concepts and processes required for personal decision making, participation in civic and cultural affairs, and economic productivity. --National Education Standards (1996) National Academy Press Washington.

Especially enhance scientific literacy among among children:
Reading for the Future (RFF) Reading for the Future is a campaign aimed at using science fiction literature to help turn on kids to reading, excitement and wonder.

Why add astronomy to your fiction? All of the above reasons:

That's how we define the beast.
It's more salable.
Win awards.
Enhance scientific literacy.

plus:

It’s classy.
Make use of it's natural sense of wonder.
Easy way to work in symbolism.
Spice up your setting.
Establish intelligence and fine character in your protagonist.
Futurism. If we sketch out possible, promising futures, society may be influenced to follow where we've led. Read Robert Sawyer's thoughts on Interstellar Travel and The Future of the Solar System.