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. Astronomers cannot see black holes directly, so they infer their presence from effects on nearby matter, light and space-time — through accretion disks, orbiting stars, and ripples called gravitational waves. Two landmark observations, the 2015 LIGO gravitational-wave detection and the 2019 Event Horizon Telescope image of M87, turned the concept from theory into direct evidence.
Black holes form in more than one way, and astronomers sort them into size classes based on mass rather than a single origin story.
How black holes form
The best-understood route is stellar death. According to NASA's Imagine the Universe project, a stellar-mass black hole forms when a massive star runs out of nuclear fuel and its core collapses under its own gravity, a process that can accompany a supernova explosion. NASA states these stellar-mass black holes typically have masses about 5 to 20 times that of the Sun and are scattered throughout galaxies wherever massive stars have lived and died.
At the other end of the scale sit supermassive black holes, found at the center of nearly every large galaxy, with masses ranging from millions to billions of times the Sun's mass, per NASA. Exactly how these giants form and grow is described by NASA as an active area of research, though observations show a correlation between the size of a galaxy's central black hole and the size of the galaxy itself.
NASA also describes a proposed intermediate class, with masses between the stellar and supermassive ranges. The agency notes astronomers only recently began finding possible evidence for intermediate-mass black holes, after many earlier candidate objects turned out to be explained by other physical processes.
How astronomers detect something that emits no light
A black hole itself cannot be seen because it emits no light of its own. NASA's Imagine the Universe page explains that astronomers instead rely on indirect detection: they look for behavior in nearby objects that can only be explained by the presence of a very large, dense, invisible mass. That includes material being pulled in and heated into a glowing accretion disk, or stars visibly orbiting a point where no star can be seen.
One way to image that indirect evidence is radio interferometry. The Event Horizon Telescope (EHT) links radio observatories across the planet using a technique called very long baseline interferometry (VLBI), which, as explained by the physics outreach site Einstein-Online, combines simultaneous observations from multiple telescopes into data equivalent to a single telescope the size of Earth. The black hole does not appear directly; what the EHT records is radio emission from hot, swirling gas in the accretion disk, with a dark central region — the black hole's 'shadow' — created where light bends around the object under extreme gravity.
The first image: M87
On April 10, 2019, the EHT collaboration released the first-ever image built this way, showing the supermassive black hole at the center of the galaxy M87, located about 55 million light-years from Earth, according to MIT News, which reported on the release. MIT News reported the object's mass as roughly 6.5 billion times that of the Sun, with the imaged event horizon measuring just under 40 billion kilometers across. The observation campaign combined eight ground-based radio observatories and involved more than 200 researchers worldwide.
Hearing black holes: gravitational waves
A separate detection method does not use light at all. On September 14, 2015, at 5:51 a.m. EDT, the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first direct detection of gravitational waves — ripples in space-time predicted by Einstein's general relativity — according to MIT News. The signal came from two black holes, each about 30 times the mass of the Sun, merging roughly 1.3 billion light-years away. The collision converted energy equivalent to about three solar masses into gravitational waves, per MIT's report, briefly radiating more power than all the light in the visible universe. LIGO's twin 4-kilometer laser detectors in Louisiana and Washington State measured a change in arm length smaller than one ten-thousandth the diameter of a proton to register the signal.
LIGO has since been joined by the Virgo detector in Italy and KAGRA in Japan to form the LIGO-Virgo-KAGRA (LVK) Collaboration, which Caltech reports has recorded roughly 300 black hole mergers since 2015, including more than 200 during its fourth observing run alone. In November 2023, the collaboration observed the merger of the most massive black holes yet recorded by gravitational waves, an event designated GW231123, involving objects of roughly 100 and 140 solar masses that merged into a black hole of about 225 solar masses, Caltech reported.
Key facts at a glance
Black hole classes and how they were detected
Class / Event
Mass (solar masses)
Detection method
Stellar-mass black hole
~5–20
Indirect: effects on orbiting matter/stars (NASA)
Intermediate-mass (proposed)
Not precisely defined by NASA; few confirmed candidates
Indirect, limited confirmed candidates (NASA)
Supermassive black hole
Millions to billions
Indirect; imaged directly only for M87* and Sagittarius A* (NASA/EHT)
Black holes this massive are forbidden through standard stellar evolution models.
— Mark Hannam, Cardiff University, quoted by Caltech on the GW231123 merger
Together, the two methods — imaging the glowing material around a black hole with radio interferometry, and listening for the gravitational-wave 'chirp' of two black holes spiraling together — give astronomers independent, complementary ways to confirm an object is a black hole and to measure its mass, even though the black hole itself never emits anything that can be seen directly.
Questions
Can we ever actually see a black hole?
No — a black hole itself emits no light. What telescopes like the Event Horizon Telescope record is radio emission from the hot gas swirling around it (the accretion disk), with a dark 'shadow' where the black hole blocks and bends light, according to Einstein-Online and NASA.
How do black holes form?
Stellar-mass black holes form when a massive star exhausts its fuel and its core collapses, sometimes in a supernova, per NASA. How supermassive black holes at galaxy centers form is still an active area of research, NASA states.
What was the first direct image of a black hole?
On April 10, 2019, the Event Horizon Telescope collaboration released an image of the supermassive black hole in the galaxy M87, about 55 million light-years away, built from radio data gathered by eight observatories, as reported by MIT News.
What are gravitational waves and how do they relate to black holes?
Gravitational waves are ripples in space-time predicted by Einstein's general relativity; LIGO made the first direct detection on September 14, 2015, from two merging black holes about 30 solar masses each, located roughly 1.3 billion light-years away, according to MIT News.
How massive can black holes get?
Stellar-mass black holes are typically 5–20 times the Sun's mass (NASA), while supermassive black holes range from millions to billions of solar masses; the M87 black hole imaged in 2019 has a mass of about 6.5 billion Suns, per MIT News.
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