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Supermassive black holes grow mainly by accreting matter, especially gas, and by merging with other black holes. They begin as smaller “seed” black holes, then gain mass as their galaxies assemble and change. Scientists have not yet determined which seed type was most common or how much growth came from accretion versus mergers.
Where do supermassive black holes start?
Models propose two broad kinds of seeds. They differ in their starting mass and in how they might form, but neither has been established as the universal origin of supermassive black holes.
| Candidate seed | Approximate starting mass | Proposed formation route |
|---|---|---|
| Remnant of a massive early star | About 100 times the Sun’s mass, as an example given by NASA Science | When a massive first-generation star dies, its remnant may become a black hole. |
| Heavy, or direct-collapse, seed | About 104–105 times the Sun’s mass, the approximate range in NASA Science’s overview | A massive gas cloud may collapse directly into a black hole rather than first forming ordinary stars. |
These are candidate starting points, not two confirmed populations with a known winner. Astronomers infer early black-hole histories from distant quasars and galaxies, then compare those observations with models; different seed masses and later growth histories can be difficult to distinguish.
How does accretion add mass?
Gas and dust drawn toward a black hole can collect in a hot, luminous flow around it. Some material loses energy as it spirals inward; material that crosses the event horizon—the boundary beyond which it cannot escape—adds to the black hole. Stars can also be consumed. The black hole itself does not shine from inside its horizon, but the energized material around it can emit radiation, making active feeding detectable. NASA Goddard’s explanation of massive black-hole growth describes accretion as a central growth route.
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Accretion need not proceed steadily. A NASA account of a study of early black holes describes feeding that switched on abruptly and lasted for short periods. That result supports the possibility of episodic growth, not a single cycle shared by every black hole. NASA’s report on early black-hole growth discusses this pattern.
How do mergers contribute?
Galaxies interact and merge as cosmic structure develops. When galaxies containing central black holes come together, the black holes may eventually form a binary and coalesce. The resulting black hole is more massive than either original one, although the overall share of supermassive-black-hole growth supplied by mergers is not established by the sources here.
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Galaxy mergers and gas accretion are not necessarily competing explanations. An interaction can help funnel gas toward a galactic nucleus, potentially connecting a galaxy merger with an accretion episode. That does not mean every episode of black-hole growth is caused by a merger.
Why were some black holes already huge so early?
Astronomers have observed billion-solar-mass black holes in quasars at redshifts greater than 7.5. A review of quasars at cosmic dawn frames the challenge this way: those objects had to form and grow in less than 700 million years. The Annual Reviews article on quasars and the intergalactic medium at cosmic dawn presents this as a constraint for models to explain, not proof of one particular seed or feeding mechanism.
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In broad terms, a heavier seed would require less subsequent growth to reach a given mass than a lighter one. But the observed early quasars alone do not settle which seed formed them or how much mass they gained through accretion and mergers. Those histories remain model-dependent.
How does black-hole activity affect its galaxy?
Active black holes release energy through radiation and mechanical outflows, which can affect gas in their surroundings. That influence is often called feedback. Its effects on the host galaxy are complex: the available evidence does not justify saying that feedback always shuts down star formation or that all black holes affect their galaxies in the same way.
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A Hubble deep-field study reported statistical evidence linking black-hole activity with galaxy assembly. It proposed a sequence in which activity was first obscured in dusty merging systems and later became visible after some dust cleared. This is an interpretation of that study’s observations, not a universal timeline for every merger or galaxy. NASA’s Hubble account of the study describes its findings.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What can reveal black-hole growth—and what remains unknown?
Electromagnetic observations, including the light from accretion flows and quasars, reveal active feeding. Gravitational waves offer a different way to study black-hole mergers: their signals can encode information about the merging objects’ masses and dynamics. The European Space Agency describes LISA as a future mission intended to study massive black-hole formation and interactions; its science page is a mission rationale, not a report of LISA detections. ESA’s LISA science survey also characterizes the broader evolutionary picture available today as circumstantial.
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The key open questions are the origins and relative abundance of the different seed types, the balance between accretion and mergers across a black hole’s history, and how closely black-hole growth tracks the growth of its host galaxy. Future observations can test these models, but no single growth story has yet been shown to explain every system.
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