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This seems like a question that would be in the Relativity FAQ, but I didn't see it.
Briefly: I've seen the claim made that there is plenty of observational evidence for the existence of black holes. But I don't understand how, from the outside, one can tell the difference between a black hole and a very massive star that has not yet collapsed into a black hole. What are the key differences that are observable from far away?
Briefly: I've seen the claim made that there is plenty of observational evidence for the existence of black holes. But I don't understand how, from the outside, one can tell the difference between a black hole and a very massive star that has not yet collapsed into a black hole. What are the key differences that are observable from far away?