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How do astrophysicists measure a black hole's mass?

Kirt Bhatt, Columbia, Maryland
If the black hole is sitting by itself out in space, you probably won't be able to see it at all, since all light falling on it will be absorbed. After all, that's why it's called "black." The way you learn anything about a black hole is by seeing how it interacts with ordinary matter and radiation around it.

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