The Most Massive Objects in the Universe: Stars, Black Holes, and Cosmic Walls
Photo: Yevgeniya Fedorova / Pexels
The black hole at the center of quasar TON 618 has a mass of 66 billion suns. Its event horizon — the point of no return — has a radius of roughly 130 billion kilometers (about 20 times the distance from the Sun to Pluto). Describing that number does not convey the scale; the fact that it sits inside a galaxy, surrounded by trillions of normal stars, each individually massive, is more useful. The universe produces objects that dwarf anything in the human catalog by factors of billions, and understanding why they exist at these extremes clarifies how gravity, stellar physics, and cosmic structure work at their outer limits.
Why Mass Has an Upper Limit — and Why Objects Break It
Every category of massive object has a theoretical maximum, derived from the physics governing its formation. Stars are limited by radiation pressure: as a star's mass increases, the outward pressure of the light it generates eventually equals and then exceeds the inward pull of gravity. At that point, the star blows itself apart. This theoretical upper limit — the Eddington limit — was long placed around 150 solar masses for stable stars.
Eddington limit — the theoretical maximum luminosity (and by extension, the mass-luminosity relationship that constrains maximum stellar mass) at which radiation pressure exactly balances gravitational attraction. Stars exceeding this limit experience catastrophic mass loss through radiation-driven stellar winds.
Yet observed stars exceed 150 solar masses. The resolution is that the Eddington limit is not a hard wall: it depends on metallicity (the abundance of heavy elements, which determines how efficiently a star interacts with its own radiation), stellar rotation, and the timescale over which the star formed. Very massive stars in low-metallicity environments — where less radiation pressure is generated per unit of mass — can grow beyond the theoretical limit. R136a1, the most massive star currently confirmed, sits at a conservatively estimated 196–291 solar masses. It is likely shedding mass rapidly and will not remain at that figure for long.
Black holes have no equivalent radiation-pressure ceiling. Once an object collapses into a black hole, gravity can continue accreting matter indefinitely, limited only by the supply of infalling material and the efficiency of radiation from the accretion disk. The most massive black holes are found at the centers of the most massive galaxies, in the densest environments — where food supply is ample.
Source: Vink, J.S. et al. (2011), "Stellar evolution models for stellar mass black holes and very massive stars," Astronomy & Astrophysics — treatment of the Eddington limit as a function of metallicity and the formation pathways for stars exceeding 150 solar masses.
The Largest Stars by Volume: Stephenson 2-18 and the Hypergiants
Size and mass are not the same thing. The physically largest stars are red supergiants and hypergiants — stars that have exhausted the hydrogen in their cores and expanded to enormous radii, even while shedding mass. They are large by volume but are not the most massive.
Stephenson 2-18 (also designated RSGC2-18) is currently the largest known star by radius, estimated at approximately 2,150 times the radius of the Sun (2,150 R☉). Located about 18,900 light-years from Earth in the constellation Scutum, it is a red supergiant member of the Stephenson 2 stellar cluster. If placed at the center of the solar system in place of the Sun, its photosphere — its visible surface — would extend to roughly Saturn's orbit, engulfing Mercury, Venus, Earth, Mars, the asteroid belt, and Jupiter entirely.
To make the scale more concrete: the Sun's radius is 696,000 km (432,000 miles). Stephenson 2-18's radius is approximately 1.496 billion km (929 million miles). A commercial aircraft traveling at 900 km/h (560 mph) would take roughly 185 years to complete a single flight around its equator.
UY Scuti, previously cited as the largest star, has been revised downward by Gaia mission parallax measurements. Its radius is now estimated at approximately 755 R☉ — still enormous, but well below Stephenson 2-18. The revision illustrates a core challenge in measuring stellar sizes: radius estimates depend on accurate distance measurements, which were poorly constrained for many distant red supergiants before Gaia.
| Star | Radius (Solar Radii) | Type | Location |
|---|---|---|---|
| Stephenson 2-18 | ~2,150 | Red supergiant | Scutum, 18,900 ly |
| WOH G64 | ~1,540 | Red hypergiant | Large Magellanic Cloud |
| V354 Cephei | ~1,520 | Red supergiant | Cepheus, ~9,000 ly |
| RW Cephei | ~1,500 | Red hypergiant | Cepheus, ~3,500 ly |
| VY Canis Majoris | ~1,420 | Red hypergiant | Canis Major, ~3,900 ly |
| UY Scuti | ~755 | Red supergiant | Scutum, ~5,200 ly |
| Betelgeuse | ~700–800 | Red supergiant | Orion, ~700 ly |
| Sun | 1 (reference) | Yellow dwarf | — |
Source: Levesque, E.M. (2010), "The Physical Properties of Red Supergiants," New Astronomy Reviews, Vol. 54 — systematic survey of red supergiant radii using effective temperature and luminosity estimates.
Source: ESA Gaia Mission Data Release 3 (2022) — revised parallax distances for a large number of Milky Way stars, including UY Scuti, leading to downward revisions in its estimated radius.
The Most Massive Stars: R136a1 and the Wolf-Rayet Giants
Mass and radius come apart in hot, blue stars. R136a1, located in the Tarantula Nebula within the Large Magellanic Cloud about 163,000 light-years from Earth, is the most massive star currently confirmed. Its mass estimates range from approximately 196 solar masses (from 2022 spectroscopic modeling) to a historical estimate of ~315 solar masses. The wide range reflects genuine uncertainty: measuring stellar mass directly requires observing gravitational interactions (typically in binary systems), and R136a1 is in a dense cluster where disentangling individual stars is challenging.
R136a1 is a Wolf-Rayet star — a class of star so hot and luminous that it drives powerful stellar winds that strip away its own outer layers at rates of roughly one solar mass every 10,000 years. Its current mass is lower than its birth mass, and models suggest it formed with possibly 300+ solar masses. Being approximately 1 million years old (early middle age for a massive star), it has already shed a substantial fraction of its initial mass through these winds.
Wolf-Rayet star — an evolved, massive hot star characterized by a surface temperature above 25,000 K, extremely high luminosity, and powerful stellar winds that produce characteristic broad emission lines in its spectrum. Wolf-Rayet stars are the precursors to some of the most energetic stellar explosions in the universe.
The luminosity of R136a1 is approximately 3.23 million times that of the Sun. To understand what that means: if you replaced the Sun with R136a1 at the same distance, Earth would receive roughly 3.23 million times more energy per unit area. The oceans would evaporate and the atmosphere would be stripped within hours.
Source: Crowther, P.A. et al. (2016), "The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of He II λ1640 in young star clusters," Monthly Notices of the Royal Astronomical Society, Vol. 458 — HST spectroscopic analysis resolving R136a1 and producing mass estimates.
Source: Tehrani, K. et al. (2019), "Weighing Melnick 34: The Most Massive Binary System Known," Monthly Notices of the Royal Astronomical Society, Vol. 484 — context for measuring stellar masses in dense clusters via binary orbital dynamics.
Supermassive Black Holes: TON 618, Phoenix A, and the 100-Billion-Solar-Mass Limit
At the centers of most large galaxies sit supermassive black holes — objects with masses ranging from millions to tens of billions of solar masses. They are distinguished from stellar-mass black holes (formed when individual massive stars collapse, producing objects of 5–50 solar masses) by five to ten orders of magnitude in mass.
Supermassive black hole — a black hole with a mass exceeding roughly one million solar masses, typically found at the center of large galaxies. Their formation pathway is debated: candidates include the collapse of very massive early-universe stars, the direct collapse of dense gas clouds, and the merger of smaller black holes over cosmic time.
TON 618 is a hyperluminous quasar (a galaxy whose central black hole is actively accreting at an extreme rate) located approximately 10.4 billion light-years from Earth. The mass of its central black hole is estimated at 66 billion solar masses, making it one of the most massive confirmed black holes. Its event horizon radius — the Schwarzschild radius — is approximately 130 billion km. Its host galaxy is powered into one of the brightest objects in the observable universe by the infalling matter heating the accretion disk to temperatures of millions of degrees and emitting more light than entire galaxy clusters.
Phoenix A, discovered through stellar-dynamics measurements in 2022, is estimated at approximately 100 billion solar masses, potentially the most massive black hole currently known. It sits at the center of a giant elliptical galaxy within the Phoenix Cluster, roughly 5.8 billion light-years away. At 100 billion solar masses, it may be near the theoretical upper limit for black hole growth — at some mass threshold, the feedback from the black hole's own accretion radiation prevents further infall, creating a self-limiting ceiling.
| Black Hole | Mass (Solar Masses) | Host | Distance |
|---|---|---|---|
| Phoenix A | ~100 billion | Phoenix Cluster galaxy | ~5.8 billion ly |
| TON 618 | ~66 billion | TON 618 quasar | ~10.4 billion ly |
| Holm 15A* | ~40 billion | Abell 85 galaxy cluster | ~700 million ly |
| IC 1101 (central BH) | ~40 billion (est.) | IC 1101 giant elliptical | ~1 billion ly |
| NGC 1277 | ~17 billion | Perseus Cluster | ~220 million ly |
| M87* | ~6.5 billion | Messier 87 | ~55 million ly |
| Sagittarius A* | ~4 million | Milky Way center | ~26,000 ly |
Source: Mehrgan, K. et al. (2019), "A 40-Billion Solar Mass Black Hole in the Extreme Core of Holm 15A, the Central Galaxy of Abell 85," The Astrophysical Journal, Vol. 887 — dynamical mass measurement of the Holm 15A* black hole using orbit modeling of surrounding stars.
Source: Event Horizon Telescope Collaboration (2019), The Astrophysical Journal Letters, Vol. 875 — first direct image of the M87* black hole event horizon shadow, confirming the mass of ~6.5 billion solar masses.
Galaxy Clusters: The Most Massive Gravitationally Bound Structures
Individual galaxies, even giant ellipticals containing trillions of stars, are not the most massive gravitationally bound objects in the universe. That distinction belongs to galaxy clusters — collections of hundreds to thousands of galaxies, plus the hot intergalactic gas between them and vast quantities of dark matter, all held together by gravity.
The El Gordo cluster (formally ACT-CL J0102-4915), discovered in 2012, is the most massive galaxy cluster found in the distant universe, with an estimated total mass of approximately 2 × 10¹⁵ solar masses (two quadrillion solar masses). Most of this mass — roughly 85% — is dark matter; the remainder is the hot X-ray-emitting intracluster gas (about 12%) and the galaxies themselves (about 3%). This is consistent with the composition of galaxy clusters universally: visible matter is a minority component.
Like a scaled-up analogy, a galaxy cluster is to a galaxy what a galaxy is to a solar system: a structure of an entirely different magnitude, governed by the same gravitational physics but operating on timescales of billions of years.
Source: Menanteau, F. et al. (2012), "The Atacama Cosmology Telescope: ACT-CL J0102-4915 'El Gordo,' a Massive Merging Cluster at Redshift 0.87," The Astrophysical Journal, Vol. 748 — discovery paper for El Gordo, providing mass estimates and X-ray luminosity.
The Largest Structure in the Universe: The Hercules-Corona Borealis Great Wall
Individual galaxy clusters are themselves embedded in larger-scale structures: filaments and walls of galaxies separated by vast voids. The universe, on the largest scales, resembles a cosmic web — a foam-like structure where matter is concentrated along thin sheets and threads surrounding nearly empty bubbles.
The largest known structure in this cosmic web is the Hercules-Corona Borealis Great Wall (Her-CrB GW), a concentration of gamma-ray bursts (GRBs) detected across a region of sky spanning approximately 10 billion light-years — roughly one-tenth the diameter of the observable universe. First identified in 2013 by a team of American and Hungarian astronomers led by István Horváth, Jon Hakkila, and Zsolt Bagoly, it was detected not by directly mapping galaxies but by finding an anomalous clustering of GRBs at similar redshifts, implying an overdense region of the universe in that direction.
More recent reanalysis (2024) suggests the structure may extend to approximately 15 billion light-years — about 15% of the observable universe's diameter. If confirmed, this creates a significant challenge to the cosmological principle: the assumption that the universe is homogeneous and isotropic on large enough scales. A structure 10–15 billion light-years across should not exist if the universe is truly uniform above a scale of roughly 1.2 billion light-years (the theoretical homogeneity scale derived from the standard cosmological model).
Cosmological principle — the foundational assumption in modern cosmology that the universe, averaged over sufficiently large scales, looks the same in every direction (isotropic) and from every location (homogeneous). The existence of structures exceeding ~1.2 billion light-years in coherent size would violate this principle, requiring modifications to the standard ΛCDM model.
Whether the Her-CrB GW is a genuinely coherent structure or a statistical artifact of GRB clustering remains a matter of active research. GRBs are not direct tracers of matter in the way that galaxy surveys are, and the apparent clustering could partly reflect selection effects in GRB detection. The debate is significant: if confirmed, it would be the most important challenge to standard cosmology from observational data.
Source: Horváth, I. et al. (2014), "Possible structure in the GRB sky distribution at redshift two," Astronomy & Astrophysics, Vol. 561 — the discovery paper characterizing the Hercules-Corona Borealis Great Wall through GRB distribution analysis.
Source: Horváth, I. et al. (2024), revised analysis presented at EGU General Assembly — updated size estimate of the Her-CrB GW suggesting it may span 15 billion light-years.
What Limits Scale? The Physics of Extreme Mass
Each type of massive object has a different limiting factor. Stars are limited by radiation pressure (the Eddington limit) and by the instability of very large convective envelopes. Black holes are not limited in mass by any known physical mechanism, but they are limited by their food supply — once the surrounding gas is consumed or expelled, growth stops. Galaxy clusters grow until cosmic expansion pulls potential merger partners out of reach.
The universe's largest structures are limited by the age of the universe itself. The cosmic web takes billions of years to form through gravitational collapse of slight density fluctuations in the early universe. There has simply not been enough time since the Big Bang for structures larger than roughly 1 billion light-years to form through normal gravitational processes. The Her-CrB GW's apparent 10–15 billion light-year extent, if real, suggests either an unusual primordial density fluctuation or physics beyond the standard model.
For more on the extreme physics of collapsed stellar remnants, see our article on the most extreme organisms — extremophiles and the strongest materials in the universe. For a deep dive into the exotic physics of magnetars — the densest magnetic objects in the cosmos — see what is a magnetar: the most extreme object in the universe.
Frequently Asked Questions
Q: What is the largest star in the universe?
A: By radius (physical size), the largest known star is Stephenson 2-18, a red supergiant estimated at approximately 2,150 solar radii. If placed where the Sun is, it would engulf Saturn's orbit. By mass, the most massive confirmed star is R136a1, estimated at 196–291 solar masses — a Wolf-Rayet star in the Tarantula Nebula of the Large Magellanic Cloud.
Q: Is TON 618 the most massive black hole?
A: TON 618's central black hole, at approximately 66 billion solar masses, is one of the most massive confirmed via spectroscopic methods. Phoenix A, measured in 2022 via stellar dynamics, is estimated at roughly 100 billion solar masses and may be the current record holder. Mass estimates for very distant or poorly resolved black holes carry significant uncertainty.
Q: How do we measure the mass of a black hole?
A: Several methods exist: (1) stellar orbits — tracking stars orbiting the black hole and applying Kepler's laws, used for Sagittarius A* and nearby galactic nuclei; (2) reverberation mapping — measuring time delays between variations in continuum emission and emission lines in quasar spectra; (3) the M-sigma relation — an empirical correlation between black hole mass and the velocity dispersion of stars in the host galaxy's bulge. Each method has different accuracy depending on the distance and environment of the black hole.
Q: Can a star be bigger than a solar system?
A: Stephenson 2-18's photosphere extends to roughly Saturn's orbit, so in radius it is comparable to the inner solar system. However, the solar system extends far beyond Saturn — the Oort Cloud reaches out to roughly 100,000 AU (astronomical units), far larger than any known star. No star approaches the scale of a complete solar system.
Q: What is the Hercules-Corona Borealis Great Wall made of?
A: It is not a single object but an apparent large-scale overdensity of matter — a region of the universe containing more galaxies per unit volume than average. It was detected through a statistical clustering of gamma-ray bursts at similar distances (redshifts around 1.6–2.1), suggesting the underlying galaxy distribution is denser there. The exact composition mirrors the universe at large: mostly dark matter, some intergalactic gas, and galaxies.
Q: What is the most massive object in the observable universe?
A: If "object" means a single gravitationally bound system, the most massive are galaxy clusters, with the most massive examples exceeding 10¹⁵ solar masses (a quadrillion times the mass of the Sun). If large-scale structures are included, the Hercules-Corona Borealis Great Wall is the largest known concentration of matter by extent. For individual compact objects, Phoenix A's ~100-billion-solar-mass black hole is the current record.
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