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Short answer

A black hole is an object with gravity so strong that light cannot escape. Since black holes give off no light, astronomers find them by their effects on nearby gas, stars and light. [S1]

How scientists detect them

• Accretion disks: surrounding gas and dust glow, including in X-rays. [S1]
• Star orbits: stars circling an unseen mass revealed the black hole at the Milky Way's center. [S1]
• Gravitational waves from merging black holes. [S1]
• Gravitational lensing: bent light from objects behind them. [S1]

Three sizes

Stellar-mass (left when a very massive star collapses), intermediate-mass, and supermassive (in the centers of most large galaxies). [S2] The Milky Way's, Sagittarius A*, is about 4 million times the Sun's mass. [S2]

Myth

Black holes do not suck everything in. From far enough away their gravity acts like any object of the same mass. [S1]

Claims

• id: C1 claim: Black holes do not emit or reflect light, so they are effectively invisible. source: S1 evidence: "Black holes don’t emit or reflect light, making them effectively invisible to telescopes."
• id: C2 claim: Black holes do not suck in other matter. source: S1 evidence: "Black holes don’t suck in other matter."
• id: C3 claim: Astronomers divide black holes into stellar-mass, supermassive and intermediate-mass. source: S2 evidence: "stellar-mass, supermassive, and intermediate-mass"
• id: C4 claim: Almost every large galaxy has a supermassive black hole at its center. source: S2 evidence: "Almost every large galaxy, including our Milky Way, has a supermassive black hole"

Sources

• [S1] Black Holes. NASA. https://science.nasa.gov/universe/black-holes/ . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Types. NASA. https://science.nasa.gov/universe/black-holes/types/ . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_f98vpm80,
  author = {{iinventedlight}},
  title = {{Black holes: what they are and how we find them}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/black-holes-basics/},
  note = {Permanent ID iil:note:f98vpm80. License: CC-BY-4.0.},
}

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