Black Holes Explained: How They Form, Work, and Were Finally Photographed

A black hole is defined by NASA as an astronomical object with a gravitational pull so strong that nothing, including light, can escape it. The point of no return around a black hole is called the event horizon: inside that boundary, the speed needed to escape exceeds the speed of light, so nothing that crosses it can come back out.

How a black hole forms

The best-understood route is stellar collapse. According to NASA, a star with more than roughly 20 times the mass of the Sun eventually exhausts the nuclear fuel in its core and collapses under its own gravity. That collapse triggers a supernova explosion that blows away the star's outer layers. If what remains of the crushed core still exceeds about three solar masses, NASA states that no known force can stop the collapse from continuing all the way to a black hole.

Once formed, a black hole can grow by pulling in surrounding gas, by stripping material from a companion star, or by merging with another black hole, NASA explains on its black-hole overview page.

Supermassive black holes, found at the centers of galaxies and ranging from about 100,000 to billions of solar masses according to NASA, are less well understood. NASA notes their origin is still an open question, since they appear to have existed from the earliest stages of galaxy formation.

Event horizon, singularity, and the parts of a black hole

NASA's description of black hole 'anatomy' lists several distinct zones. At the center sits the singularity, a point where matter is crushed to what NASA describes as infinite density; scientists are not certain whether this is a physical structure or a mathematical artifact of the equations. Surrounding it is the event horizon, the boundary of no escape. Further out, light from infalling matter can orbit the black hole multiple times before escaping, producing thin rings visible around the black hole's dark 'shadow' in direct images — a region NASA calls the photon sphere.

Matter falling toward a black hole typically forms a hot, glowing accretion disk before crossing the event horizon. In many systems, some of that material is instead redirected into jets of particles moving at nearly the speed of light; in supermassive black holes these jets can stretch hundreds of thousands of light-years, NASA says.

Proving black holes exist: Penrose, Genzel, and Ghez

For decades black holes were a mathematical consequence of Einstein's general relativity rather than a confirmed physical reality. The Royal Swedish Academy of Sciences awarded half of the 2020 Nobel Prize in Physics to Roger Penrose, whose 1965 proof — described in the Academy's own announcement, reported by ScienceDaily — showed that black hole formation is a robust and unavoidable prediction of general relativity once a sufficiently massive, compact body collapses, and that such collapse necessarily produces a singularity.

The other half of that prize went jointly to Reinhard Genzel and Andrea Ghez, who led separate teams tracking the orbits of stars at the center of the Milky Way for decades using some of the world's largest telescopes. Their work, also summarized in the Nobel announcement covered by ScienceDaily, found an invisible object of about four million solar masses concentrated in a region no larger than the solar system — strong evidence for the supermassive black hole now known as Sagittarius A*.

Detecting the undetectable: gravitational waves and direct images

On 14 September 2015, the twin Advanced LIGO detectors recorded a signal, designated GW150914, matching the predicted waveform of two black holes spiraling together and merging. According to the LIGO Scientific Collaboration's own science summary, the two black holes had masses of about 36 and 29 times the Sun, merging into a single black hole of roughly 62 solar masses more than a billion light-years away; about three solar masses were converted into gravitational-wave energy in a fraction of a second. The detection carried a statistical significance of more than 5 sigma, meaning a false alarm of that strength would be expected roughly once every 200,000 years of comparable data, LIGO reports.

Four years later, the Event Horizon Telescope (EHT) Collaboration released the first-ever image of a black hole's shadow, taken of M87*, the supermassive black hole at the center of the galaxy Messier 87. As NASA's Jet Propulsion Laboratory explains, the EHT links radio telescopes around the globe into a single virtual instrument using very-long-baseline interferometry, achieving resolution about 4,000 times sharper than the Hubble Space Telescope. The April 2017 observation campaign behind the image, released in 2019, measured M87* at roughly 6.5 billion solar masses, located about 53 million light-years away.

On 12 May 2022, the EHT Collaboration released a second landmark image: Sagittarius A*, the Milky Way's own central black hole. As described by AAS Nova, the underlying data were recorded in April 2017, meaning more than five years passed between the observation and the image's release. Sagittarius A* is about four million solar masses and 27,000 light-years away — meaning it appears roughly the same apparent size in the sky as the far more massive and distant M87*, since M87* is about 2,000 times farther away and about 1,500 times more massive. The measured ring and shadow sizes of both black holes matched the predictions of general relativity, according to AAS Nova, giving astronomers independent confirmation of Einstein's theory across vastly different black hole masses.

Why this matters for physics

Each of these results tests general relativity in a regime Einstein could not observe directly: matter and spacetime under extreme gravity. The gravitational-wave signal from GW150914 matched the waveform predicted by general relativity for an inspiral, merger, and subsequent 'ringdown' of the resulting black hole, according to LIGO's analysis. The EHT's measured ring diameters for both M87* and Sagittarius A* likewise matched general-relativity predictions, per AAS Nova, giving scientists two independent confirmation methods — gravitational waves and direct imaging — that converge on the same theoretical framework.