Science regular Alan Gilmour - Astronomers were measuring the speeds that a binary pair of stars were orbiting each other when they noticed that one of the stars had an extra wobble. It was in a small orbit around an unseen companion. A clue to the existence of a black hole. The star and dark companion were orbiting each other in 40 days. Knowing the approximate mass of the star, the astronomers calculated that the invisible companion was about four times the mass or weight of the Sun.
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