If for no other reason I would go for a DSLR on performance grounds.
For Africa you may want to shoot multiple frames of an animal running or use a good telephoto lens for zooming in on the same creature.
For large landscape shots you my want a nice wide angle lens.
For Astronomy a DSLR should produce less noise when the ISO setting are changed for low light situations. Also you could get a monster lens.
The down side is the size and bulk of all the lenses, and more to get damaged/stolen.
Guess it all depends on what you want.
Maybe a simple compact will be easier to live with if you can accept average results.
Check out: http://www.dpreview.com
and have a look at all the specs of the cameras that take your fancy.