A black hole is a region of space where gravity is so strong that nothing, not even light, can escape once it crosses the event horizon. Most form when a massive star runs out of nuclear fuel and its core collapses; supermassive versions grow at galaxy centers over billions of years. Direct images and gravitational-wave detections have confirmed their existence and tested Einstein's general relativity.
A black hole is defined by NASA as a region of space where gravity is strong enough that nothing, including light, can escape once it passes a boundary called the event horizon. Because no light escapes, black holes themselves cannot be seen directly; their presence is inferred from their gravitational effects on nearby matter and, since 2019, from direct images of the glowing material around them.
How a black hole forms
According to NASA, a star generates energy through nuclear fusion in its core, and the outward pressure from that energy balances the inward pull of the star's own gravity. When a star with more than 20 times the mass of the Sun exhausts its nuclear fuel, that balance breaks down. The core collapses under its own weight, triggering a supernova explosion that blasts away the star's outer layers. If the leftover core exceeds roughly three solar masses, NASA states it continues collapsing into a stellar-mass black hole.
Not every collapsing star produces a black hole. A core below about three solar masses typically becomes a neutron star instead, according to NASA's description of stellar evolution. Once a black hole exists, it can grow further by pulling in gas, dust, and even other stars or black holes that stray too close.
The three main types
NASA groups black holes into three broad categories by mass. Stellar-mass black holes, ranging from about three to several dozen solar masses, are scattered throughout the Milky Way and form from collapsing massive stars. Intermediate-mass black holes, with roughly 100 to more than 10,000 solar masses, are rarer and harder to confirm; NASA cites one of the most convincing examples identified in 2019. Supermassive black holes, containing anywhere from 100,000 to billions of solar masses, sit at the centers of most large galaxies, including the Milky Way.
Black hole types by mass (NASA)
Type
Typical mass
Location
Stellar-mass
3 to dozens of solar masses
Throughout the Milky Way
Intermediate-mass
100 to 10,000+ solar masses
Rare; confirmed examples limited
Supermassive
100,000 to billions of solar masses
Centers of large galaxies
Seeing the unseeable: direct images
Because light cannot escape a black hole, astronomers cannot photograph the object itself. Instead, the Event Horizon Telescope (EHT) — a global network of radio telescopes acting as one Earth-sized instrument, according to the U.S. National Science Foundation (NSF) — images the glowing gas that swirls around a black hole just outside its event horizon. On April 10, 2019, the EHT Collaboration released the first such image, of the supermassive black hole at the center of galaxy M87, located about 55 million light-years from Earth and measured at roughly 6.5 billion solar masses, per the NSF summary.
On May 12, 2022, the EHT Collaboration released a second image, this time of Sagittarius A*, the supermassive black hole at the center of our own Milky Way. The NSF describes it as the first direct visual evidence of that object, with a mass the NSF gives as roughly 4 million times the Sun's mass. A summary of the findings on cosmoquest.org similarly puts its mass at roughly four million solar masses and its distance at roughly 27,000 light-years from Earth. Both images show the same signature: a dark central shadow ringed by bright, lensed light from superheated orbiting gas.
Ripples in spacetime: gravitational waves
A separate line of evidence comes from gravitational waves — ripples in spacetime predicted by Albert Einstein's general theory of relativity in 1915. On September 14, 2015, the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in Hanford, Washington, and Livingston, Louisiana, registered a signal designated GW150914, according to a LIGO science summary. The signal came from the merger of two black holes with masses of about 36 and 29 times the Sun's mass, which combined into a single black hole of about 62 solar masses. The missing three solar masses were radiated away as gravitational-wave energy, LIGO states, from an event more than one billion light-years away.
LIGO reports that the detection reached a statistical significance of more than five sigma, meaning a false alarm of that strength would be expected only once in roughly 200,000 years of comparable data. The observed waveform matched predictions from general relativity, giving physicists a new way to study black holes that does not rely on light at all.
What black holes reveal about the universe
NASA's description of black hole anatomy identifies three key zones: the event horizon, the point of no return where escape velocity exceeds the speed of light; the accretion disk, the spiraling matter whose radiation lets astronomers detect otherwise invisible black holes; and the singularity at the core, where matter is compressed to extreme density and current physics, including general relativity, is expected to break down. NASA notes this is where quantum effects are thought to become important, meaning black holes mark the frontier where relativity and quantum mechanics must eventually meet.
Because they bend light, warp time, and anchor the structure of galaxies, black holes function as natural laboratories. Observations from direct imaging and from gravitational-wave detectors give scientists two independent ways to test whether gravity behaves as Einstein's theory predicts under the most extreme conditions known in the universe.
LIGO estimated that the peak gravitational-wave power radiated during the final moments of the black hole merger was more than ten times greater than the combined light power from all the stars and galaxies in the observable Universe.
— LIGO science summary, GW150914
Questions
What is the minimum mass needed to form a black hole?
NASA states that a collapsing stellar core needs to exceed roughly three solar masses to become a black hole; below that threshold it typically forms a neutron star instead.
Has a black hole ever been photographed?
Not directly, since light cannot escape one. The Event Horizon Telescope has imaged the glowing gas around two supermassive black holes: M87* in 2019 and Sagittarius A* in 2022, according to the NSF.
How far away is the black hole at the center of the Milky Way?
Sagittarius A* lies roughly 27,000 light-years from Earth and has a mass of about 4 million Suns, based on figures reported following the EHT's 2022 image.
How do gravitational waves help scientists study black holes?
Merging black holes produce ripples in spacetime that detectors like LIGO can register even though no light is involved; LIGO's first detection, GW150914 in September 2015, confirmed a merger of two black holes located more than a billion light-years away.
What is the difference between stellar-mass and supermassive black holes?
Per NASA, stellar-mass black holes form from single collapsing stars and weigh a few to dozens of solar masses, while supermassive black holes sit at galaxy centers and range from 100,000 to billions of solar masses.
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