Black Holes Explained: How They Form and What Science Has Confirmed

NASA defines a black hole as an area of such immense gravity that nothing, not even light, can escape from it. The visible boundary around this region is called the event horizon: the point where the speed needed to escape the pull of gravity exceeds the speed of light, according to NASA's official black-hole overview.

Inside that boundary, according to NASA's educational site for the public, the remaining matter from a collapsed star is thought to compress into an infinitely small point called a singularity. Despite its tiny size, its gravitational pull governs everything within the event horizon.

How a stellar black hole forms

NASA states that a stellar-mass black hole forms when a star with more than about 20 times the mass of the Sun exhausts the nuclear fuel in its core and collapses under its own weight. The collapse triggers a supernova that blows off the star's outer layers, while the core keeps falling inward. Once the collapsing core exceeds roughly three solar masses, NASA notes, the collapse into a black hole becomes inevitable because no known force can stop it.

A star normally holds its shape because the outward pressure from nuclear fusion balances the inward pull of its own gravity. When the fuel runs out, that balance breaks down, and gravity compresses the core until it forms a black hole, per the same NASA explanation.

Not all black holes are the same size

NASA's overview of black holes distinguishes several categories by mass. Stellar-mass black holes range from about three to several dozen solar masses and are scattered throughout the Milky Way. Supermassive black holes, found at the centers of most large galaxies including our own, weigh between 100,000 and billions of solar masses.

A third category, intermediate-mass black holes weighing roughly 100 to over 10,000 solar masses, had long been suspected and had only a handful of candidates identified through indirect evidence, according to NASA. NASA describes a 2019 gravitational-wave detection, covered below, as the most convincing example of this class to date.

Proof that does not rely on seeing light

Because black holes do not emit light, scientists confirm their existence indirectly. NASA points to several methods: detecting X-rays and radio waves from matter heated to extreme temperatures near the event horizon, observing particle jets moving near the speed of light that emit radio waves, X-rays and gamma rays, and detecting gravitational waves — ripples in spacetime first measured in 2015 from an event called GW150914, when two black holes spiraled into each other and merged, according to NASA.

The gravitational-wave method later produced the first confirmed intermediate-mass black hole. The LIGO Scientific Collaboration and the Virgo Collaboration announced on 2 September 2020 that they had detected a signal, designated GW190521, from an event that occurred on 21 May 2019, according to LIGO's official detection page. Two black holes with masses of about 85 and 66 solar masses merged into a single remnant of 142 solar masses — the first clear detection of an intermediate-mass black hole, LIGO states.

LIGO also notes the merger is scientifically notable because the progenitor masses fall inside the so-called pair-instability mass gap, a range (roughly 65 to 120 solar masses) where standard stellar-collapse theory does not predict black holes should form directly, suggesting the objects may themselves have resulted from earlier mergers.

Seeing a black hole's shadow

Beyond indirect detection, astronomers have produced a direct image. The Event Horizon Telescope, a global network of radio observatories, released the first image of a black hole in 2019 — the one at the center of galaxy M87, estimated at 6.5 billion solar masses and about 55 million light-years away, according to NASA's black-hole overview.

The image shows a bright ring of glowing gas surrounding a dark central "shadow," the silhouette cast by the black hole's event horizon against the light from infalling material.

What remains uncertain

According to NASA, the origin of supermassive black holes remains poorly understood, even though evidence shows they already existed in the very early history of galaxies.