What Is the Biggest Black Hole?
Black holes come in several size ranges, from stellar remnants to supermassive objects at galaxy centers.
The biggest known black holes are not measured by width alone, but by mass, and the current record holders keep shifting as astronomers find better data.
If you are asking what is the biggest black hole, the answer depends on whether you mean the most massive black hole confirmed so far, the largest by event horizon size, or the most extreme candidate reported in the literature.
That distinction matters because black holes are invisible directly, so scientists infer their scale from the behavior of nearby gas, stars, and light.
How Scientists Define “Biggest”
A black hole’s mass is usually expressed in solar masses, where one solar mass equals the mass of the Sun.
More mass generally means a larger event horizon, but spin also affects the size of that boundary, so a fast-spinning black hole can have a different horizon shape from a non-rotating one of the same mass.
When astronomers discuss the biggest black holes, they usually focus on one of these measures:
- Mass: the standard way to compare black holes.
- Event horizon size: the boundary beyond which nothing escapes.
- Observational certainty: how confidently the object has been measured.
Because black holes do not emit light, the most reliable estimates come from spectroscopy, stellar dynamics, and gas motion near the center of a galaxy or quasar.
The Largest Confirmed Black Holes Known
Among the most discussed contenders is TON 618, a distant quasar whose central black hole has been estimated at tens of billions of solar masses.
Many published estimates place it near 66 billion solar masses, although values can vary depending on the method used to interpret its light and motion.
Another famous giant is the black hole in the center of the galaxy cluster Holm 15A.
Earlier studies suggested an extremely large mass, making it one of the biggest black hole candidates ever reported.
However, some later analyses questioned how much of the central light belongs to the black hole environment versus the surrounding galaxy structure, showing why the title can be uncertain.
Other notable supermassive black holes include those in:
- M87*, the black hole imaged by the Event Horizon Telescope in 2019, with a mass of about 6.5 billion solar masses.
- Sagittarius A*, the Milky Way’s central black hole, with about 4 million solar masses.
- NGC 4889 and other giant elliptical galaxies, which may host black holes reaching into the tens of billions of solar masses.
Why TON 618 Is Often Mentioned First
TON 618 is frequently cited because it combines extreme mass with a bright quasar signal that makes it detectable across vast cosmic distances.
Quasars are powered by matter falling into supermassive black holes, and their brilliance can reveal systems that would otherwise be nearly impossible to study.
The mass estimate for TON 618 comes from analyzing the broad emission lines in its quasar spectrum and modeling the velocity of gas close to the black hole.
This is not a direct measurement, so uncertainty remains, but the object still stands out as one of the most massive black holes ever described in peer-reviewed astronomy.
How Do Astronomers Measure a Black Hole’s Mass?
Different techniques are used depending on the black hole’s environment and distance.
Nearby black holes can be studied by tracking stars orbiting the center, while distant quasars require indirect methods based on light signatures.
Stellar Orbits
In the Milky Way, astronomers measured Sagittarius A* by watching stars move rapidly around an invisible central mass.
Their orbital speeds and paths reveal the gravity at work, allowing a precise mass estimate.
Gas Dynamics
Gas orbiting a black hole forms a disk and emits radiation at different wavelengths.
Spectral measurements can show how fast this gas is moving, which helps infer the mass of the central object.
Reverberation Mapping
For active galactic nuclei and quasars, astronomers sometimes use reverberation mapping.
This method compares variations in brightness from the accretion disk and surrounding gas clouds to estimate the size of the emitting region and the black hole’s mass.
Gravitational Lensing and Imaging
Large black holes can distort light from background sources, and in rare cases they can be imaged indirectly through their surrounding shadow and accretion flow.
The Event Horizon Telescope used this approach for M87* and Sagittarius A*.
Why the Biggest Black Hole Title Is Hard to Pin Down
The record for the biggest black hole changes because the universe is full of extreme objects, and measurement errors are common at cosmic distances.
A mass estimate can shift when better instruments, updated models, or new interpretations of the host galaxy become available.
There are also practical limits:
- Very distant black holes are harder to observe clearly.
- Bright galaxies can hide the central signal.
- Different estimation methods can produce different results.
- Spin, orientation, and dust can affect observations.
For that reason, astronomers often describe the “largest known” black hole rather than a permanently fixed record holder.
What Makes Supermassive Black Holes Grow So Large?
Supermassive black holes likely grow through a combination of gas accretion, galaxy mergers, and long periods of steady feeding from their surroundings.
In the early universe, some black holes appear to have grown unusually fast, which remains an active research question in astrophysics.
Possible growth pathways include:
- Rapid accretion from dense gas in young galaxies.
- Black hole mergers after galaxy collisions.
- Direct collapse of massive gas clouds under special conditions.
The existence of enormous black holes like TON 618 raises questions about how such objects formed so early in cosmic history, especially when the universe was still relatively young.
Are There Bigger Black Holes Waiting to Be Found?
Yes.
Astronomy is still uncovering more massive and more distant black holes as surveys improve.
Instruments such as the James Webb Space Telescope, large ground-based observatories, and next-generation interferometers are expanding the census of active galaxies and quasars.
It is entirely possible that a larger black hole already exists in the observable universe but has not been measured well enough to replace current candidates.
As detection methods improve, the answer to what is the biggest black hole may change again.
Key Facts to Remember
- The biggest black holes are measured mainly by mass, not by appearance.
- TON 618 is one of the best-known contenders, often estimated at around 66 billion solar masses.
- Other giants, including the black holes in M87* and several massive elliptical galaxies, are also among the largest known.
- Mass estimates are indirect and can be revised as data improves.
- The current record holder may change as astronomers discover new quasars and refine their models.