Black Holes, What They Are and How They Form

What Are Black Holes

A black hole is a mass packed into an extremely small volume. According to NASA, the gravity just below its apparent surface, the event horizon, is so strong that nothing escapes it, not even light. ESA describes it in similar terms, as an object so dense that its escape velocity outruns light itself: for comparison, Earth’s escape velocity is about 11 km/s, a tiny fraction of what it would take to leave a black hole behind.

NASA defines the event horizon as the boundary containing all the matter that makes up the black hole: past that boundary, escaping would require moving faster than light, which makes it impossible. Newton’s laws explain gravity well enough for everyday purposes, but close to a black hole they give way to Einstein’s general relativity, published in 1915, which laid the theoretical groundwork for these objects to exist at all. At the black hole’s center, its equations predict a singularity, a point where matter is crushed beyond anything known physics can describe.

How They Form

Stellar black holes come from the death of a star far more massive than the Sun. Once its nuclear fuel runs out, the star explodes as a supernova; if the leftover core exceeds roughly three solar masses, per ESA, it keeps collapsing under its own weight until it becomes a black hole. Below that threshold, the remnant stops shrinking earlier, ending up as a white dwarf or a neutron star instead.

Supermassive black holes are a different story. ESA describes them as objects found at the center of most galaxies, the Milky Way included, with masses of millions or even billions of times the Sun’s. Exactly how they form isn’t covered by the general-audience sources consulted for this Recap; what is documented is how common they are at galactic centers, and how precisely their masses have been measured in a few well-studied cases.

Real-world example: picture a star wandering too close to a supermassive black hole. Tidal forces stretch it like taffy until it shreds into a stream of gas, a tidal disruption event, which NASA estimates happens to any given supermassive black hole roughly once every 100,000 years.

How We Observe Them

For centuries, black holes stayed a theoretical guess. John Michell first proposed the idea in 1784, describing objects so dense that light couldn’t escape them; in 1915, Einstein supplied the theoretical foundation with general relativity. Direct proof didn’t arrive until 2015, when LIGO detected gravitational waves from the merger of two black holes weighing 29 and 36 solar masses, an event that produced a 62-solar-mass black hole and released roughly 3 solar masses’ worth of energy as gravitational radiation in a fraction of a second.

In 2019 came the first direct image, of the black hole at the center of galaxy M87, some 55 million light-years away, with a mass of 6.5 billion suns. In 2022 came the second, of Sagittarius A*, the black hole at the Milky Way’s center, with a mass of about 4 million suns and a distance of roughly 27,000 light-years. In both cases, the Event Horizon Telescope didn’t photograph the black hole itself, which by definition emits no light, but the dark shadow it casts against the glowing ring of gas surrounding it.

Among the records NASA catalogs, without a specific year attached in the source consulted: the closest known black hole, Gaia BH1, about 1,500 light-years away; the most distant, the quasar QSO J0313-1806, roughly 13 billion light-years out; the most massive, TON 618, at 66 billion solar masses; and the fastest spinning, GRS 1915+105, turning over more than 1,000 times a second.

Myths Worth Retiring

The most common myth is the cosmic vacuum cleaner: a black hole that sucks in everything nearby. NASA rejects that outright, its gravity behaves exactly like that of any other mass at the same distance. A planet orbiting far from a black hole would keep orbiting the same way if that mass belonged to an ordinary star of equal weight instead.