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How can astronomers detect a black hole that gives off no light?
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Star
Stars in the open cluster NGC 290, as seen by the Hubble Space Telescope.
Because a black hole has an event horizon from which not even light can escape, it cannot be seen directly. Instead, astronomers detect them by observing the effects of their intense gravitational fields on nearby matter and visible objects.123
Key methods of detection include:
- Binary Star Systems: If a black hole is paired with a visible star, astronomers can observe the oscillatory motion of the visible companion to derive the mass of the invisible member. Furthermore, the black hole may pull gas from its companion; as this matter spirals inward, it becomes intensely heated and radiates high-energy X-rays from the periphery of the black hole before disappearing. A well-known example of this is Cygnus X-1.1245
- Stellar Orbits: At the centers of galaxies, such as the Milky Way, astronomers track the orbits of stars moving around an invisible, massive point, such as Sagittarius A*, to confirm the presence of a supermassive black hole.2