Black holes trap all light, making them invisible. Yet, astronomers detect and measure them through effects on nearby matter, radiation, and spacetime.

BulletsIn

• Black holes invisible, no light escapes event horizon
• Gas, dust spiral in, form hot accretion disc, emit strong radiation
• Bright emissions allow detection billions of light years away
• Star motions near galaxy centres reveal black hole mass
• Faster orbiting stars = heavier black hole
• Velocity dispersion used for precise size estimates
• Reverberation mapping tracks light delays from accretion disc
• X-ray telescopes detect intense glow from falling matter
• Motion of nearby stars proved Sagittarius A in Milky Way
• Gravitational waves from collisions reveal merging black holes