ROME — Some of the universe's biggest black holes may have been born in the most crowded nurseries imaginable. New research based on observations from NASA's James Webb Space Telescope (JWST) suggests that dense clusters of young stars in the early universe could have collapsed, star by star, into "heavy seed" black holes — the missing starting points for the supermassive black holes that now sit at the centres of galaxies, including our own Milky Way.
The study, led by Viola Bocchi of Sapienza University of Rome and published in the journal Astronomy and Astrophysics, does not claim that Webb has directly photographed one of these intermediate-mass black holes. Instead, it shows that two groups of ancient star clusters Webb has already found — the "Cosmic Gems" arc and the "Firefly Sparkle" galaxy — have exactly the density, mass and chemical makeup needed for runaway stellar collisions to build black holes hundreds to thousands of times the mass of the Sun.
Illustration: The James Webb Space Telescope observing a distant gravitationally lensed galaxy with dense young star clusters in the early universe.
What Happened
Reporting on the research on November 3, 2026, phys.org summarised the central finding: the young massive clusters found by Webb in the Cosmic Gems arc and the Firefly Sparkle galaxy are, in the authors' words, "ideally suited to form intermediate-mass black holes (IMBHs) via collision-based models." The authors estimate typical resulting black holes in the range of about 100 to about 4,000 solar masses, formed with an efficiency of a few percent — meaning a few percent of a cluster's mass could end up in its central black hole.
The two systems give strikingly different answers, and that difference is itself revealing. The Cosmic Gems clusters are extremely compact, with a half-mass radius of roughly one parsec — the radius containing half of a cluster's mass — and they could produce black holes of 1,600 to 2,700 solar masses. The Firefly Sparkle clusters are larger and less massive, with half-mass radii of about four to six parsecs, and would produce smaller black holes of roughly 180 to 810 solar masses, near the lower boundary of the intermediate-mass range.
"Our results suggest that these dense, metal-poor clusters are viable factories for heavy seeds that are capable of growing into the supermassive black holes observed in the early universe," the authors conclude, according to the report.
One reason the result matters now is timing. Webb has found supermassive black holes shining surprisingly early in cosmic history, some only a few hundred million years after the Big Bang. Growing such giants from small stellar-mass seeds through steady feeding and mergers alone is difficult in the time available. A "heavy seed" — a black hole born already weighing thousands of suns — gives the growth process a major head start, and the new work identifies a concrete place where such seeds could form.
Background
Astronomers broadly accept that supermassive black holes, some weighing billions of suns, grow over cosmic time through a hierarchy of mergers and gas accretion. What remains unsettled is how they started. The conventional seeds are stellar-mass black holes, formed when massive stars of roughly 8 to 50 solar masses end their lives in supernova explosions or collapse directly, or when two neutron stars merge. Between those stellar-mass objects and supermassive giants lies the intermediate-mass range, roughly 100 to 100,000 solar masses — a class for which observational evidence remains thin but persistent.
Two earlier lines of evidence illustrate both the promise and the difficulty. A 2024 study of Omega Centauri, a globular cluster on the outskirts of the Milky Way, argued that fast-moving stars at its centre could only be explained by an intermediate-mass black hole of about 8,200 solar masses. Separately, a 2020 gravitational-wave detection provided what was described as the only direct evidence of an intermediate-mass black hole, when two black holes of 85 and 66 solar masses merged into one of about 142 solar masses.
Webb changed the search by revealing populations of dense stellar systems at very high redshifts — that is, seen as they were when the universe was very young. The Cosmic Gems arc, first discovered by the Hubble Space Telescope during its Reionization Lensing Cluster Survey and identified as a powerful source of radiation from the epoch of reionization, was revisited by Webb in 2024. Webb found five young massive clusters there, the first discovery of such ancient clusters, in a galaxy whose light left when the universe was only about 460 million years old, at a redshift of about 10.2.
The Firefly Sparkle is another gravitationally lensed early galaxy, imaged as it was only about 600 million years after the Big Bang, at a redshift of about 8.3. Webb found ten young massive clusters in it. Gravitational lensing — the bending of light by a massive foreground object — stretches and magnifies these distant galaxies, letting Webb resolve clusters that would otherwise blur together. The clusters in both systems have masses of roughly 100,000 to 10 million solar masses and half-mass radii of roughly 0.4 to 15 parsecs.
Illustration: A dense young star cluster in the early universe where massive stars collide at the centre, forming a growing black hole seed.
Crucially, these clusters were also sources of ionising radiation — the energetic light that helped reionise the early universe — and the new research argues they may have been black hole factories at the same time.
Why It Matters
The mechanism at the heart of the study is runaway collision. In an extremely dense cluster, massive stars sink toward the centre and collide repeatedly, building an ever-heavier object that eventually collapses into a black hole. Because these clusters are so dense, the researchers could not run full N-body simulations — step-by-step calculations tracking every star — and instead used a model based on runaway collisions and mass loss through stellar winds, validating the results against direct N-body simulations of compact clusters.
Chemistry plays a decisive role. The clusters have very low metallicity — the proportion of elements heavier than hydrogen and helium — at or below about 2 percent of the Sun's value. In the authors' words, "the low metallicity is a critical factor for retaining the seed mass despite stellar winds." Young clusters are metal-poor because few stars have yet lived and died in them to enrich the gas, and low metallicity means weaker stellar winds, so growing objects lose less mass as they collide and merge.
That combination — extreme compactness plus low metallicity — is why the Cosmic Gems clusters outperform the Firefly Sparkle ones. As the authors put it, "the extreme compactness of the Cosmic Gems clusters facilitates the formation of black hole seeds with high masses of 1,600–2,700 solar masses." The Firefly Sparkle clusters, with larger radii for typically lower total masses of 100,000 to one million solar masses, form less massive cores, limiting the runaway process.
The lower end of the new mass range is also significant. At roughly 100 solar masses, it is comparable to black holes that could form from the universe's hypothetical first stars, the extremely massive Population III stars. But the authors note that "the upper bound extends significantly higher, allowing the most massive clusters to produce objects firmly in the 'heavy seed' regime," adding that "forming such heavy seeds is highly beneficial to form supermassive black holes at high redshift."
The findings may also help explain one of Webb's most puzzling discoveries: the so-called "little red dots," compact red objects in the early universe whose spectra are often interpreted as powered by supermassive black holes. "Our results suggest that dense massive star clusters could provide possible massive seeds from which the supermassive black holes in the little red dots may have grown," the researchers write.
Illustration: A supermassive black hole at the centre of a spiral galaxy, grown over billions of years from a much smaller seed.
What Happens Next
The authors are candid about the caveats, and they matter. Webb has not observed any intermediate-mass black holes in these clusters; the study demonstrates a plausible formation environment, not a detection. Many properties of the clusters remain unknown, including their rotation and internal structure, which could change the outcomes and lower the resulting black hole masses. The initial mass function — the distribution of stellar masses at birth — is also not known, although the authors note it could be top-heavy, favouring massive stars, which would help rather than hinder heavy-seed formation, as would possible primordial mass segregation, where the heaviest stars are born near the centre.
What comes next is therefore observational as much as theoretical. Astronomers will dig deeper into these young massive clusters, trying to fill in the unknowns about their structure and stellar populations, and searching for indirect signatures of hidden intermediate-mass black holes — such as unusually fast-moving stars, as claimed in Omega Centauri, or gravitational-wave signals from mergers. Future Webb observations and next-generation observatories may also test whether the "little red dots" really are powered by black holes grown from such seeds, or by dense star clusters themselves.
For now, the study offers a satisfying symmetry: the same crowded, metal-poor nurseries that lit up the early universe may also have seeded its greatest gravitational monsters. If the model holds, the path from a cluster of young stars to a billion-sun black hole at a galaxy's heart begins not with a single spectacular explosion, but with a slow, crowded pile-up in the dark — hundreds of millions of years before our Sun existed.
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FAQ
What is a "heavy seed" black hole?
A heavy seed is a black hole born already weighing hundreds to thousands of times the mass of the Sun, rather than starting as a small stellar-mass black hole of a few tens of solar masses. Because it starts larger, it has a head start in growing into a supermassive black hole, which helps explain how such giants could exist only a few hundred million years after the Big Bang.
What are intermediate-mass black holes, and why are they hard to find?
Intermediate-mass black holes weigh roughly 100 to 100,000 solar masses, between stellar-mass black holes and supermassive ones. Evidence for them remains thin: a 2024 study pointed to an about 8,200-solar-mass object in Omega Centauri from fast-moving stars, and a 2020 gravitational-wave merger produced a roughly 142-solar-mass black hole, but no intermediate-mass black hole has been directly imaged in an early-universe cluster.
What are the Cosmic Gems and the Firefly Sparkle?
They are two gravitationally lensed galaxies from the early universe observed by the James Webb Space Telescope. The Cosmic Gems arc, seen when the universe was about 460 million years old, contains five young massive clusters; the Firefly Sparkle, seen about 600 million years after the Big Bang, contains ten. Both are dense, metal-poor star clusters that the new study identifies as possible birthplaces of intermediate-mass black holes.
Did Webb actually see a black hole seed in these clusters?
No. Webb observed the star clusters themselves, not any black holes within them. The new research uses a runaway-collision model, validated against simulations, to estimate what masses of black holes those clusters could produce. It shows a plausible formation environment, not a direct detection.
How does this relate to Webb's "little red dots"?
Little red dots are compact red objects in the early universe whose light is often interpreted as coming from supermassive black holes. The researchers suggest that dense massive star clusters like those in the Cosmic Gems and Firefly Sparkle could have provided the massive seeds from which those black holes grew.
Sources
- phys.org, "JWST may have found where heavy black hole seeds formed," November 3, 2026 (reporting on Bocchi et al.).
- Viola Bocchi et al., "Formation of intermediate-mass black holes in young massive clusters detected with JWST: Analytic mass estimates," Astronomy and Astrophysics (2026), DOI: 10.1051/0004-6361/202659622.
- NASA / ESA / CSA / STScI, James Webb Space Telescope observations of the Cosmic Gems arc and Firefly Sparkle galaxy, as cited in the phys.org report.
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