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What does “black hole spin” mean, and how do astronomers measure it?

Stephen Smith, Charlotte, North Carolina
RELATED TOPICS: BLACK HOLES
A black hole’s spin is its event horizon’s rate of rotation, and astronomers measure it by studying X-rays from gas near the black hole.

Like most objects in the universe (planets, stars, asteroids, and other compact objects), black holes rotate. We refer to this property as “spin” or, more formally, its “angular momentum” about its axis of rotation. This concept can be difficult to visualize because a black hole is not a solid body, but it helps if you start by imagining a black hole as the end product of a star’s evolution. If a star begins its life with more than some critical mass (usually estimated at about 10 to 20 times the Sun’s mass), it will ultimately end up as a black hole.

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