All of the major makers are producing some great cameras. I use Nikon full frame gear, but there is nothing wrong with Canon, Sony and Pentax. Nikon and Pentax have cameras specific for astro photography, perhaps the others do to.
I'd start by finding a lens that meets your needs, for the format (full frame, crop sensor...) you select. A fast, wide angle for aurora and landscape work, and a fast prime telephoto for wildlife. BTW, lenses for full frame work nicely on crop sensor, but not vice versa. A couple of examples would be the Nikkor 14-24/2.8 for landscape, and Nikkor 400/2.8 for wildlife. The reason I prefer a smaller telephoto is for me, the light is more important than reach. On high quality lenses, you can gain reach by adding a good teleconverter.
Now, both of the lenses mentioned above are really expensive, so there are others that will do a good job. For landscape, the Tokina 16-28/2.8, and the Sigmas that Jackie mentioned are good as well. Both are made with Nikon and Canon lens mounts.
As Lighthawk mentions below, a good support system is just as important as the camera and lens. Read Thom Hogan's "Support 101" essay for GREAT advice on the topic. The Cliff Notes version is to buy quality gear the first time, or you'll end up spending more in the long run. I'm on the road, but I'll try and add a link later. I like Really Right Stuff legs and ballhead, with a Wimberley gimbal head for wildlife. I also have a collection of cheap legs and heads that I tried to "get by" with.
http://bythom.com/support.htm