What is a black hole, and how do we know they exist?

A black hole is a region of space where gravity is so strong that light cannot escape. NASA describes black holes as huge concentrations of matter packed into very small spaces, not as literal holes. Because they give off no light of their own, astronomers detect them by the effects they have on their surroundings, and since 2019 by a direct image of the glow around one. [1] [2]

The boundary of a black hole is called the event horizon. NASA explains that it is not a solid surface like Earth's. It is a boundary containing all the matter that makes up the black hole, and just beneath it gravity is strong enough that nothing can escape. Some black holes form when a star dies and its matter is compressed into a tiny space. Stellar-mass black holes are at least a few times the mass of the Sun, while supermassive ones exceed 1 million Suns. NASA also says there is much still unknown, such as what matter looks like inside an event horizon. [1] [2] [3]

The main method is watching what nearby objects do. If a star accelerates in orbit around an unseen object, its motion reveals the object's mass. When that mass is too compact and large for any other explanation, astronomers conclude it is a black hole. Tracking the orbits of several stars near the center of the Milky Way showed that our galaxy houses a supermassive black hole, a discovery that won the 2020 Nobel Prize. [3] [1]

Gas gives a second clue. Matter pulled from a companion star forms a swirling accretion disk, where particles collide and heat to millions of degrees, producing X-rays that telescopes can detect. Hubble observations since the early 1990s indicate supermassive black holes at the heart of nearly all large galaxies. [3]

Massive accelerating objects create gravitational waves, ripples in space-time that detectors can register. Black holes also bend light from more distant objects, an effect called gravitational lensing that can reveal isolated black holes. In April 2019 the Event Horizon Telescope, a network of telescopes around the world, produced a direct image of the supermassive black hole in galaxy Messier 87, showing a dark shadow against the glow of its accretion disk. Live Science puts the black hole's mass at 6.5 billion Suns. [1] [4]

Individual cases can be uncertain. Cygnus X-1 was identified as a black hole candidate in 1971, and Stephen Hawking conceded a bet with Kip Thorne about it in 1990. BBC Science Focus notes that its identification has since become less certain because the companion star is so large that it is hard to pin down the mass of the compact object. The magazine adds that evidence for many candidates remains indirect and relies on theoretical assumptions about the maximum size of a neutron star. [5]