Detailed Instructions: You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.
Q: Category: ASTRONOMY 
Clue: With a mass of 4.31 millions Suns, Sagittarius A* is thought to be a supermassive one of these in the Milky Way's center
A:
black hole