If your talking about proper astrophotography as in camera body fitted to a telescope rather than as Mathew said star trails etc then forget the large megapixel count cameras as what your looking for is the ability to grab as much precious light as possible and a large sensor with less but larger pixels does the job much better than a large sensor crammed with smaller pixels.
Canon have been the main stay of those using DSLR's for many years as they have produced many astro variants of their prosumer DSLR's and even have clip in astro filters designed to fit them such as UHC & Ha etc that makes life very easy.
Of course a dedicated cooled astro camera will do an even better job, especially when it comes to feint objects/long exposures.