Stellar vs. Supermassive Black Holes: The Mass Gap Mystery

Black holes are not one thing. The term covers objects that range from a few solar masses — city-sized remnants of collapsed stars — to monsters ten billion times heavier than the Sun that anchor entire galaxies. Between the two categories lies a near-empty zone, a forbidden range of masses that stellar physics says cannot easily be produced. In May 2020, a gravitational wave event called GW190521 delivered a black hole squarely inside that forbidden zone, formed from two progenitors that were themselves already in the gap. Astronomers are still sorting out what this means.

Key takeaways

• Stellar black holes form from the collapse of massive stars and typically range from about 3 to 65 solar masses.
• Supermassive black holes range from roughly 1 million to 40 billion solar masses and reside in the centers of virtually all large galaxies.
• The mass gap — the range from approximately 65 to a few hundred solar masses — is where pair-instability supernovae theory predicts black holes cannot easily form through single-star evolution.
• Intermediate-mass black holes (IMBHs) — defined as those between roughly 100 and 100,000 solar masses — are the least understood class, with only indirect evidence accumulated before GW190521.
• GW190521, detected by LIGO and Virgo on May 21, 2020, produced a remnant black hole of approximately 142 solar masses from two progenitors of ~85 and ~66 solar masses — both in or near the pair-instability gap.
• The fastest-growing supermassive black holes in the early universe reached billion solar masses within the first billion years of the Big Bang, a formation timeline that challenges standard accretion models.

How Stellar Black Holes Form

When a star with a mass between roughly 20 and 130 times the Sun exhausts its nuclear fuel, the iron core that results cannot sustain fusion. The core collapses in milliseconds. If the collapsing mass is large enough and the supernova explosion does not completely unbind the star, a black hole forms from the remnant.

The lower mass limit for a stellar black hole is set by the boundary between neutron stars and black holes. A neutron star's maximum stable mass — the Tolman–Oppenheimer–Volkoff limit — is approximately 2 to 2.5 solar masses. Above that, neutron degeneracy pressure cannot halt collapse, and a black hole forms. This creates a theoretical gap between about 2.5 and 5 solar masses where neither neutron stars nor black holes are predicted to commonly form — the "lower mass gap," a separate puzzle from the pair-instability gap described below.

Most stellar black holes formed through single-star evolution are expected to be 5 to 20 solar masses. The highest-mass stellar black holes observed in X-ray binaries — where a black hole accretes material from a companion star — reach roughly 20 to 30 solar masses. Beyond that, the physics becomes more complex.

X-ray binary — a system where a compact object (black hole or neutron star) accretes material from a companion star, producing intense X-ray emission. These systems are the primary way astronomers detect and mass-measure stellar black holes in the Milky Way.

Source: NASA HEASARC — Catalog of confirmed black holes in X-ray binary systems, mass measurements via orbital dynamics.

The Pair-Instability Ceiling

The upper mass limit for stellar black holes is set by a process called pair-instability: a stellar nuclear physics phenomenon that prevents very massive stars from collapsing cleanly into black holes.

In stars with core masses between roughly 40 and 65 solar masses (corresponding to initial stellar masses of roughly 130 to 250 solar masses), the core temperature becomes so extreme that high-energy photons spontaneously convert into electron-positron pairs. This pair production absorbs energy that was previously providing thermal pressure, causing a sudden pressure drop. The core begins to collapse.

The collapse triggers explosive oxygen and silicon burning so energetic that it completely disrupts the star in a pair-instability supernova — leaving no remnant at all. No black hole forms. The star simply blows itself apart. At even higher core masses (above roughly 130 solar masses), pair instability leads instead to a direct collapse to a black hole before the explosion can fully develop.

The result is a theoretical ceiling: stars in a certain mass range cannot leave behind black holes. The "pair-instability mass gap" spans roughly 65 to a few hundred solar masses. Black holes in this range are not impossible — they just cannot be the direct product of a single dying star.

Pair-instability supernova — an explosion triggered when photon-to-electron-positron pair production causes a sudden pressure loss in a massive star's core, igniting runaway thermonuclear burning that completely disrupts the star, leaving no remnant.

Source: Woosley, S.E., Astrophysical Journal (2017) — Pair-instability supernovae mass limits, stellar evolution models.

Supermassive Black Holes: A Completely Different Beast

At the other extreme of the mass spectrum sit supermassive black holes (SMBHs), objects whose formation is not constrained by stellar nuclear physics — because they do not form from individual stars in the way stellar black holes do.

The Milky Way's central black hole, Sagittarius A*, masses about 4 million solar masses. M87*, imaged by the Event Horizon Telescope in 2019, is approximately 6.5 billion solar masses. The largest known SMBHs — objects like TON 618 or Holm 15A* — are estimated at 40 to 66 billion solar masses.

These objects reside at the centers of virtually all large galaxies and appear to have co-evolved with their host galaxies over cosmic time. The mass of a galactic SMBH correlates tightly with the velocity dispersion of the stars in the galaxy's central bulge — an empirical relationship suggesting the black hole and galaxy grew together, regulated by feedback from the black hole's activity.

The size scales are worth sitting with for a moment. A stellar black hole of 10 solar masses has a Schwarzschild radius (event horizon radius) of about 30 km. M87*, at 6.5 billion solar masses, has a Schwarzschild radius of about 19 billion km — larger than the orbit of Pluto around the Sun.

Source: Event Horizon Telescope Collaboration (2019), Astrophysical Journal Letters — M87* mass and first image of black hole shadow.

The Formation Problem: How Did Supermassive Black Holes Get So Massive So Fast?

The deepest puzzle in black hole astrophysics is not the pair-instability gap. It is how supermassive black holes reached billion-solar-mass scales within the first billion years after the Big Bang.

Observations of high-redshift quasars — galaxies with extremely active central black holes visible across the observable universe — show SMBHs with masses of one to ten billion solar masses at redshifts z > 6, meaning they existed when the universe was less than 1 billion years old. ULAS J1120+0641, discovered in 2011, has a black hole of approximately 2 billion solar masses at redshift z = 7.09 — when the universe was roughly 770 million years old.

Growing a black hole to that mass from a small stellar seed requires sustained accretion close to or beyond the Eddington limit — the maximum rate at which a black hole can accrete before radiation pressure from the infalling matter blows away its own food supply. Even at the Eddington limit continuously from the universe's very early moments, a stellar-mass black hole seed cannot reach a billion solar masses in 770 million years.

Three proposed solutions are under active study. First, massive seeds: if the first black holes formed not from the collapse of ordinary stars but from the direct collapse of massive metal-free gas clouds at redshifts z > 10, they could start with initial masses of 10,000 to 100,000 solar masses. Second, super-Eddington accretion: some theoretical models allow brief periods of accretion substantially above the Eddington limit, accelerating growth. Third, hierarchical mergers: dense early-universe environments could drive frequent black hole mergers, growing SMBHs faster than accretion alone allows.

JWST has begun detecting black holes at even earlier cosmic epochs — candidates at z > 10 — that are more massive than current models predict. The tension between observed early-universe black hole masses and the time available to grow them remains unresolved.

Source: Royal Society Proceedings A, Volonteri et al. (2016) — Supermassive black holes in the early universe: formation mechanisms.

Source: Mortlock et al., Nature (2011) — Discovery of quasar ULAS J1120+0641 at z = 7.09, black hole mass ~2 × 10⁹ solar masses.

The Mass Gap: GW190521 Changes the Conversation

On May 21, 2020, LIGO and Virgo detected a gravitational wave signal lasting only about 0.1 seconds — a brief, deep chirp unlike most previously detected binary mergers. The analysis, published in September 2020, revealed a merger of two black holes with masses approximately 85 and 66 solar masses, producing a remnant of approximately 142 solar masses (the remaining mass was radiated away as gravitational wave energy).

Both progenitors were at or above the theoretical pair-instability mass ceiling. The remnant sat squarely in the intermediate-mass range. No previous unambiguous gravitational wave detection had produced a black hole of 142 solar masses.

Source: LIGO Scientific Collaboration and Virgo Collaboration (2020), Physical Review Letters — GW190521: binary black hole merger exceeding the pair-instability mass gap.

How did objects of 85 and 66 solar masses form? Several models are on the table. Hierarchical mergers — in which previous black hole mergers produced these intermediate-mass progenitors — require dense environments such as globular cluster cores or active galactic nucleus disks, where black holes are common enough to find merger partners frequently. Low-metallicity stellar collapse — stars with very few heavy elements retain more mass during their evolution and may avoid the most catastrophic pair-instability disruption, collapsing into more massive black holes than metal-rich stars. Primordial black holes — a speculative but not ruled-out class of black holes formed in the very early universe before stars existed — are another candidate.

The signal's short duration and lack of clearly detected inspiral phases also left room for alternative interpretations, including a merger of a black hole with a neutron star, or a more exotic object. The community broadly accepts the binary black hole interpretation, but with acknowledged uncertainty.

The Lower Mass Gap: Why No Black Hole Has Been Found Between 3 and 5 Solar Masses

The pair-instability gap at 65 to several hundred solar masses is not the only vacancy in the black hole mass spectrum. A second, lower gap exists between the maximum stable mass of neutron stars — roughly 2 to 2.5 solar masses, set by the Tolman–Oppenheimer–Volkoff limit — and the lightest confirmed stellar black holes, which cluster around 5 solar masses. The range from approximately 3 to 5 solar masses contains almost nothing.

Stellar evolution models predict that collapsing stars should be able to produce compact remnants across a continuous range of masses. The abrupt absence of objects between about 3 and 5 solar masses is therefore unexpected. Two broad explanations have been proposed. In one, the supernova explosion mechanism is so efficient in this mass range that it ejects enough material to prevent black hole formation entirely, leaving only a neutron star. In the other, compact objects do form in this range but are systematically fainter and harder to find — we are looking at a detection bias rather than a true gap.

The detection question is partly technical. Most known stellar black holes were identified through X-ray binary systems, where the black hole accretes material from a companion star and glows in X-rays. X-ray binaries preferentially contain more massive black holes, because more massive black holes can accrete at higher rates and be detected at greater distances. A 3- to 5-solar-mass black hole in a quiet binary — not currently accreting at high rates — would be nearly invisible in X-ray surveys.

Lower mass gap — the near-absence of compact objects with masses between roughly 2.5 and 5 solar masses, intermediate between the maximum neutron star mass and the lightest observed stellar black holes. Distinct from the pair-instability (upper) mass gap which lies above 65 solar masses.

The gravitational wave event GW190814, detected by LIGO and Virgo in August 2019, punched directly into this question. The merger involved a 23-solar-mass black hole and a secondary object with a mass of approximately 2.6 solar masses — precisely in the lower mass gap. The analysis, published in 2020, could not definitively determine whether the secondary was the heaviest neutron star ever measured or the lightest black hole ever measured. The object's mass exceeds the current best estimate for the maximum neutron star mass, but not by enough to rule out an extremely stiff nuclear equation of state. It remains genuinely ambiguous.

Source: Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), Astrophysical Journal Letters (2020) — GW190814 discovery paper; secondary component mass 2.59 +0.08/-0.09 solar masses, described as either the heaviest neutron star or lightest black hole yet detected.

LIGO's fourth observing run (O4), which began in May 2023 and has been detecting events at significantly improved sensitivity due to hardware upgrades, has accumulated additional low-mass-ratio events. The population of compact binary mergers from O4 is being analyzed for objects near the lower mass gap boundary. If a statistically significant number of events cluster near 2.5 to 5 solar masses, it would suggest the gap is a detection artifact. If the gap persists in the gravitational wave population — which is not biased toward X-ray luminosity — it points to a real astrophysical absence.

The pair-instability upper gap and the lower mass gap together mean that the compact object mass spectrum has two distinct forbidden zones framing the range of ordinary stellar black holes. Filling both, or confirming both as real absences, will require the next generation of gravitational wave detectors. The Einstein Telescope and Cosmic Explorer, planned for the 2030s with sensitivity roughly 10 times greater than current LIGO, will detect compact binary mergers across the observable universe and accumulate the statistics needed to map both gaps in detail.

Source: Farr et al., Astrophysical Journal (2011) — Statistical analysis of black hole X-ray binary masses showing the lower mass gap; argued the gap is astrophysical rather than a selection artifact.

Intermediate-Mass Black Holes: The Missing Middle

GW190521's remnant at 142 solar masses belongs to a class called intermediate-mass black holes (IMBHs): objects between roughly 100 and 100,000 solar masses. IMBHs are theoretically important because they could be the seeds from which supermassive black holes grew, and they might occupy the cores of globular clusters and dwarf galaxies. But they are observationally elusive.

Before GW190521, the evidence for IMBHs was entirely indirect and contested. A small number of ultraluminous X-ray sources — X-ray binaries too luminous to be powered by a stellar black hole at the Eddington limit — have been interpreted as IMBHs. The globular cluster NGC 1277 and a few others show central kinematics consistent with a massive central object. HLX-1 in galaxy ESO 243-49 is a particularly strong IMBH candidate, with an estimated mass around 20,000 solar masses.

GW190521 provided the first direct evidence that objects in the mass gap actually exist and can participate in mergers. It does not explain how they formed, but it confirms they are there.

Source: Farrell et al., Nature (2009) — HLX-1 discovery and IMBH mass estimate.

Comparing the Black Hole Classes

Property	Stellar BH	Intermediate-mass BH	Supermassive BH
Mass range	~3–65 M☉	~100–100,000 M☉	~10⁶–10¹⁰ M☉
Formation pathway	Stellar collapse / supernova	Uncertain (mergers? direct collapse?)	Uncertain (seeds + accretion + mergers)
Location	Throughout galaxies	Globular clusters? Dwarf galaxy centers?	Galactic centers
Detection method	X-ray binaries, GW	Indirect (X-ray, kinematics), GW	Gas kinematics, reverberation mapping, EHT
Event horizon radius	~9–200 km	~300 km – 300,000 km	~3 × 10⁶ – 3 × 10¹⁰ km
Confirmed examples	Hundreds	Handful of candidates	Millions (inferred)

What GW190521 Implies Going Forward

LIGO's third observing run produced GW190521. The fourth and fifth runs, using upgraded detectors with improved sensitivity, are probing ever-more-distant mergers and lower-mass-ratio events. The merger history of black holes across the pair-instability gap is being assembled event by event.

The population statistics matter enormously. If IMBHs are common — if the pair-instability gap is regularly populated through hierarchical mergers in globular clusters or AGN disks — that has implications for how supermassive black holes assembled over cosmic time. A steady supply of 100–1,000 solar mass objects merging repeatedly could seed billion-solar-mass SMBHs faster than accretion alone allows.

Pulsar timing arrays, including the NANOGrav collaboration, have been detecting a stochastic gravitational wave background — a low-frequency hum from the accumulated mergers of supermassive black hole binaries across cosmic history. Correlating that background with the stellar-mass and intermediate-mass merger rates from LIGO will eventually allow a continuous mass spectrum of black hole mergers to be measured.

For more on the gravitational wave observations underlying this picture, see gravitational waves explained and for context on the extreme end of the stellar remnant spectrum, see neutron stars.

The Takeaway

Black holes span a mass range of ten billion to one — from a few solar masses to tens of billions — but the range between roughly 65 and a few hundred solar masses has been the dark matter of the black hole world: theoretically expected to be sparse, observationally nearly absent. GW190521 confirmed that objects in this mass range exist and merge. The formation question remains open. Supermassive black holes at the other end of the spectrum present a separate formation puzzle: how does the universe grow billion-solar-mass objects in less than a billion years? JWST is finding increasingly early and massive black holes that tighten this constraint. The mass gap and the early-universe formation problem may have the same answer, or completely different ones. Either way, the black hole mass spectrum is the most physically rich unsolved problem in stellar astrophysics.

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Read next:

• Gravitational Waves Explained: Listening to the Universe
• Neutron Stars: The Most Extreme Objects in the Universe
• Black Holes Explained: How They Work

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Q: What is the black hole mass gap?
A: The mass gap refers to the range between roughly 65 and a few hundred solar masses where pair-instability supernovae theory predicts black holes cannot easily form through single stellar collapse. Stars in this mass range either explode completely or collapse directly; neither produces a gap-mass black hole from a single star.

Q: What is GW190521?
A: A gravitational wave event detected by LIGO and Virgo on May 21, 2020. Two black holes of approximately 85 and 66 solar masses merged to produce a remnant of about 142 solar masses — the first confirmed observation of a black hole firmly in the intermediate-mass range and the first confirmed progenitors straddling the pair-instability gap.

Q: How do supermassive black holes form?
A: Not fully understood. The leading proposals are massive seeds from the direct collapse of metal-free gas clouds in the early universe, super-Eddington accretion periods, and hierarchical mergers. All three may operate simultaneously. The challenge is that observed billion-solar-mass black holes at z > 6 existed within the first billion years of the universe, leaving little time for growth from small stellar seeds.

Q: What is an intermediate-mass black hole?
A: A black hole with mass between roughly 100 and 100,000 solar masses — between stellar black holes and supermassive black holes. They are theoretically expected in globular cluster cores and as seeds of supermassive black holes, but are observationally rare. GW190521 produced the first confirmed detection in this class.

Q: Why can't stars make black holes above ~65 solar masses?
A: Due to pair-instability supernovae. In stars above a certain mass, core temperatures become high enough for photons to spontaneously produce electron-positron pairs. This pair production absorbs thermal pressure, triggering a collapse that ignites explosive thermonuclear burning powerful enough to completely destroy the star before a black hole can form.

Q: How big is Sagittarius A compared to a stellar black hole?*
A: Sagittarius A* has a mass of roughly 4 million solar masses and a Schwarzschild radius of about 12 million km. A 10-solar-mass stellar black hole has a Schwarzschild radius of about 30 km. The size difference is roughly a factor of 400,000.