Black holes form when matter is compressed so densely that gravity creates an event horizon, a boundary beyond which light cannot escape. The best-established route to a stellar-mass black hole is the collapse of the core of a sufficiently massive star. Intermediate-mass black holes may form through repeated mergers or collisions, but their existence and origins are harder to confirm. Supermassive black holes can grow by consuming matter and merging, yet scientists are still working out how their first seeds formed.
These mass classes are useful guides, not sharply separated natural categories. NASA’s May 2026 report on Webb observations adds a notable development: evidence suggests that at least some early supermassive black holes may have formed already massive, challenging a simple account in which every one began as a small stellar remnant.
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How do black holes form?
A black hole forms when enough mass is concentrated within a small enough region that an event horizon develops. The event horizon is the boundary beyond which nothing, including light, can escape. It is not a solid surface; it marks the limit of what can send signals back out.
There is no single formation story for every black hole. The route depends on the object’s mass and surroundings. For stellar-mass black holes, the central process is the collapse of a massive star’s core. For intermediate-mass and supermassive black holes, scientists are considering several routes involving collapse, accretion, stellar encounters, and mergers. Some routes are better established than others.
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It also helps to separate formation from growth. Formation produces a black hole or its initial “seed.” Growth adds mass later, chiefly when matter falls in or when black holes merge. Those processes can continue over long periods and may change an object’s mass class.
What are the three main black-hole mass classes?
A common classification distinguishes stellar-mass, intermediate-mass, and supermassive black holes. The ranges below follow NASA Science’s “Types of Black Holes” overview. They are approximate: NASA notes that scientists continue to reassess where the boundaries belong, and its explainers use somewhat different divisions.
| Class | Approximate mass scale | What is best established about formation | Evidence and confidence |
|---|---|---|---|
| Stellar-mass | A few to hundreds of solar masses | Collapse of the core of a sufficiently massive star is the established basic route. Black-hole mergers can also create a more massive remnant. | The stellar-collapse pathway is comparatively well understood, though the outcome depends on the star and its remaining core. |
| Intermediate-mass | Around 100 to hundreds of thousands of solar masses in NASA’s general classification | Proposed routes include mergers of stellar-mass black holes and repeated stellar collisions or mergers in dense environments. | Candidates are difficult to confirm; evidence is indirect and varies by object. |
| Supermassive | Hundreds of thousands to billions of solar masses in NASA’s general classification | Possible seeds include remnants of early massive stars, direct-collapse objects, and objects formed through runaway collisions; later growth can involve accretion and mergers. | The existence of supermassive black holes is established, but the dominant seed pathway and full growth history remain unsettled. |
A solar mass is the mass of the Sun, used as a convenient unit for comparing astronomical objects. The table’s ranges should not be read as strict borders: NASA’s “Types of Black Holes” describes stellar remnants from a few to hundreds of solar masses, places intermediate-mass objects at around 100 to hundreds of thousands, and describes supermassive objects from hundreds of thousands to billions. Those intervals overlap at their approximate edges by design.
How do stellar-mass black holes form?
Core collapse is the established route
A massive star produces energy through nuclear reactions in its core. When it can no longer sustain the reactions that support it against gravity, the core collapses. If the remnant core is sufficiently massive, the collapse can produce a black hole. If the remnant is not massive enough, the outcome can instead be a neutron star.
NASA gives “around 20 times the Sun’s mass or more” as an approximate guide to the mass of a black-hole-forming star in one explainer. That is not a universal cutoff. The star’s evolution, how much mass it loses, and the mass of the core left at collapse all matter. It would therefore be inaccurate to say that every star above a fixed starting mass inevitably becomes a black hole.
What happens after the black hole forms?
A black hole can gain mass from a companion star if matter moves from that star into the black hole’s surroundings and eventually falls in. Black holes can also merge with other black holes. In its September 8, 2020 explainer “What Are Black Holes?”, NASA reported that the event GW190521 produced a remnant black hole weighing 142 solar masses. The example shows how mergers can create remnants heavier than many individual stellar black holes.
That remnant’s mass is an example, not a boundary that defines a separate class. Classification depends on approximate ranges, and a merger can change an object’s mass without changing the basic fact that it is a black hole.
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What is an intermediate-mass black hole?
An intermediate-mass black hole is a proposed middle category between the black holes commonly associated with individual stars and the supermassive black holes found at the centers of many galaxies. In NASA’s general classification, the category covers around 100 to hundreds of thousands of solar masses. Other NASA explainers draw the ranges differently, so “intermediate-mass” does not identify one universally agreed interval.
The category matters because it could help explain how black holes bridge the gap between stellar remnants and supermassive objects. But the central challenge is observational: it can be difficult to establish that an object is a black hole of intermediate mass rather than another kind of source or a black hole in a different mass range.
How might intermediate-mass black holes form?
Two broad ideas are under consideration. One is that stellar-mass black holes merge repeatedly, building a larger object. Another is that stars in a dense environment collide or merge repeatedly, creating a very massive star or remnant that can collapse into a black hole. These are proposed pathways, not a settled account of how the class forms.
Dense stellar environments are relevant to both ideas because close encounters and repeated mergers are more plausible where many stars and remnants occupy a compact region. Even so, a plausible route is not proof that it produced a particular candidate.
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Black holes cannot be seen directly by light escaping from inside their event horizons. Astronomers instead look for their effects on nearby matter and objects. NASA describes clues that include:
- Stellar motions: stars moving as if they are orbiting an unseen, concentrated mass.
- Compact X-ray emission: bright X-rays from hot material close to a candidate black hole.
These clues are indirect. An unseen mass or an X-ray source is not automatically a confirmed intermediate-mass black hole; the evidence must support that interpretation for the particular object. Candidate claims therefore need to be assessed individually rather than treated as equally conclusive detections.
How do supermassive black holes form?
Supermassive black holes weigh hundreds of thousands to billions of solar masses in NASA’s general classification. Their existence is established, but the origin of their first seeds is an active question. NASA describes two broad seed families, often called “light” and “heavy,” and other formation ideas are also discussed. These labels describe proposed starting points, not two confirmed, exclusive populations.
Light seeds: remnants of early massive stars
In the light-seed picture, the first generations of massive stars end their lives and leave black-hole remnants. Those relatively small seeds then need to gain mass through accretion, mergers, or both to become supermassive. The scenario connects the formation of the black-hole seed to the life cycle of early stars.
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The key question is whether a seed formed this way could grow quickly enough, under the conditions available in the early universe, to account for the supermassive black holes observed at early times. A seed’s existence alone does not explain its eventual mass; the subsequent growth history matters too.
Heavy seeds: direct collapse and other proposed routes
In heavy-seed scenarios, a black hole starts from a more massive precursor. NASA describes possible formation through the direct collapse of a very massive star or a massive gas cloud. Runaway collisions in dense stellar environments are another proposed route discussed by researchers.
A heavier starting point reduces the amount of later growth needed to reach supermassive scales, but that does not make the scenario a proven explanation. Different routes may operate in different environments, and a single pathway need not explain every supermassive black hole.
How supermassive black holes grow
Once a seed exists, it can gain mass by accreting gas and other matter and by merging with other black holes. The surrounding galaxy matters because it contains material and structures that can influence what reaches the black hole. NASA describes a connection between black-hole growth and the evolution of host galaxies, while noting that quantitative descriptions of that relationship remain incomplete.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsThat qualification is important: a relationship between a black hole and its host does not establish one simple cause-and-effect sequence for every galaxy. Black holes and galaxies evolve in complex environments, and the balance among available gas, accretion, mergers, and other conditions can differ.
What does new Webb evidence say about early black holes?
On May 27, 2026, NASA reported that Webb observations offered evidence that at least some early supermassive black holes were already enormous. The report said researchers found evidence for black holes that formed without a stellar-collapse phase and without a substantially more massive host galaxy feeding them. NASA described the result as a challenge to classical scenarios of black-hole formation.
Roberto Maiolino of the University of Cambridge, a researcher and co-author of the reported studies, called the result “a remarkable finding” and characterized it as “a paradigm shift, a total revisiting of the classical scenarios of how black holes form and grow.” Those words describe his interpretation of that specific result. The finding does not establish that all supermassive black holes formed this way, nor does it settle the broader question of which seed pathway dominates.
The significance is narrower and more useful than a claim that the old picture is simply disproved: observations are testing whether some early black holes could have started out much more massive than a stellar remnant. That possibility changes the range of plausible histories scientists must consider.
How do black holes grow, and what does “accretion” mean?
Accretion is the process by which matter moves into a black hole’s surroundings and ultimately adds to its mass. Matter can come from gas, a companion star, or other nearby material. Black holes can also gain mass through mergers with other black holes. Accretion and mergers are not competing explanations; both can contribute, and both can operate across different mass classes.
Why accretion can be bright
As matter falls toward a black hole, it can become extremely hot and emit radiation, including X-rays. That light comes from hot matter outside the event horizon, not from inside the horizon. The black hole itself does not send light out across the boundary that defines it.
This distinction also explains why astronomers can detect some black holes indirectly: the surrounding matter can be luminous even though the black hole is not. A bright source near a candidate is evidence about its environment, not a view of the black hole’s interior.
Why black holes do not swallow everything
A black hole is not a cosmic vacuum cleaner. Matter must come sufficiently close to be captured or lose enough orbital energy to move inward. Objects can orbit a black hole without falling into it, just as the presence of a massive object does not automatically pull every distant object off its path. NASA and ESA explain black-hole growth through nearby accreting matter, not through a universal process that pulls in everything in the universe.
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How the formation and growth routes compare
The comparison below separates the most established route to a class from hypotheses about its origin and the mechanisms that can add mass later. Accretion and mergers are growth processes; they should not be mistaken for proof of one particular seed history.
| Mass class | Formation picture | Growth routes | Main uncertainty |
|---|---|---|---|
| Stellar-mass | Collapse of a sufficiently massive stellar core | Accretion from a companion or nearby matter; mergers with other black holes | The collapse outcome depends on the star and remnant core, so an initial stellar mass alone does not give a universal result. |
| Intermediate-mass | Possible repeated black-hole mergers or repeated stellar collisions and mergers in dense environments | Accretion and further mergers | Candidate evidence is indirect, and the class boundaries and assembly history are not settled. |
| Supermassive | Light seeds from early massive stars; heavy seeds from direct collapse; runaway collisions also proposed | Accretion of gas and other matter; mergers with other black holes | The dominant seed route, the timing of growth, and the detailed relationship with host-galaxy evolution remain open questions. |
What scientists know—and what remains open
The strongest conclusions are not equally strong for every class. Stellar-core collapse is a well-established basic route to stellar-mass black holes, though not every massive star produces one. Black holes also gain mass through accretion and mergers. Supermassive black holes exist, but how their seeds formed and how they reached their observed masses remain active research questions. Intermediate-mass black holes occupy a useful proposed category, but identifying and confirming individual members remains challenging.
- Well established: a sufficiently massive stellar core can collapse into a black hole; matter and black-hole mergers can contribute to growth; hot accreting matter outside the event horizon can emit radiation.
- Still being tested: how common intermediate-mass black holes are, which proposed routes create them, and which supermassive seed pathways dominate.
- Newly sharpened question: NASA’s May 2026 Webb report raises the possibility that at least some early black holes began much more massive than a stellar-collapse remnant.
NASA’s “Types of Black Holes,” “What Are Black Holes?” (September 8, 2020), and its May 27, 2026 Webb report provide the class definitions, collapse example, merger remnant, and early-universe development discussed here. ESA’s black-hole overview provides further context on accretion and observation. NASA also offers a downloadable Black Hole Field Guide for readers seeking an educational follow-up.
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