One way to “measure mass” is to look at how long it takes to complete an orbit of a given size. You can do this around the Sun or around a black hole, and they will give the same answer if they “have the same mass.” Compared to the Sun, the gravity of the black hole is only extremely strong when you are close to the hole, seen here in this figure by comparing how close the lines are near each of the objects. [Image: S. Larson]
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