I've had this question for a really long time, but never found a good answer. It's a physics question, not astronomy, but quantum physics is often a part of the show.
In quantum physics, things remain in an undetermined state until they are observed. However, I've never heard a more meaningful answer about what an observer is than "something which causes quantum states to become determinate".
Example answers that I've heard in various pop-physics venues:
1) A human brain (if a person doesn't see it, it doesn't count)
2) A living brain (cat, dog, etc)
3) A camera (even if the film is never developed)
4) A large molecule
So, in practical terms, what is an observer? And, of course, how do we know?