http://www.nmm.ac.uk/visit/exhibitions/astronomy-photographer-of-the-year/The three main competition categories are:
Earth and Space: This is for photos that include landscape, people and other 'Earthly' things. Your picture should also include an astronomical subject – for example the stars, the Moon, or near-Earth phenomena such as aurora
Our Solar System: This is for photos of our Sun and its family of planets, moons, asteroids and comets
Deep Space: This is for photos of anything beyond our Solar System, including stars, nebulae and galaxies
entry is closed, but check out the entrants here (warning-possibility for vast time wastage here!):
http://www.flickr.com/groups/astrophoto/