# How Big Is TON 618? Mass, Event Horizon and Measurement Uncertainty

The black hole powering TON 618 is estimated to contain roughly 41–70 billion times the Sun’s mass. For a non-spinning black hole, that mass range corresponds to an event-horizon diameter of about 240–413 billion kilometres, or roughly 1,600–2,760 astronomical units.

Those are calculated sizes, not a directly photographed edge. TON 618 is a distant quasar, and its black-hole mass is inferred from the light and motion of rapidly moving gas. Different emission lines and calibrations produce different answers.

TON 618 at a glance

Property Best concise description
Object A hyperluminous quasar powered by an actively feeding supermassive black hole
Redshift About 2.219
Published mass estimates Roughly 4.07 × 10¹⁰ to 7 × 10¹⁰ solar masses
Calculated Schwarzschild radius About 800–1,380 AU
Calculated Schwarzschild diameter About 1,600–2,760 AU
Measurement status Indirect and model-dependent, not a direct dynamical measurement

One astronomical unit, or AU, is the average Earth–Sun distance: about 149.6 million kilometres.

Why are two different masses quoted for TON 618?

A widely repeated estimate places the black hole near 66–70 billion solar masses. It is based on a single-epoch virial method using the quasar’s luminosity and the width of its hydrogen-beta emission line.

A later catalogue analysis reported a lower value of about 40.7 billion solar masses. The difference does not mean the black hole physically shrank. It reflects a revised estimate based on how the broad-line region was measured and calibrated.

Both values depend on a model of gas too distant to resolve directly. The gas may not move in a simple spherical or randomly oriented distribution. Viewing angle, radiation pressure, line choice and the empirical relationship used to infer the region’s size all contribute uncertainty.

The accurate phrasing is therefore a range—not “TON 618 weighs exactly 66 billion Suns.”

How is TON 618’s event horizon calculated?

For a non-spinning black hole, the Schwarzschild radius is:

r = 2GM/c²

This equals approximately 2.95 kilometres for every solar mass. Applying it to the published mass range gives:

Assumed mass Schwarzschild radius Event-horizon diameter Light-crossing time across diameter
40.7 billion Suns About 120 billion km, or 800 AU About 240 billion km, or 1,600 AU About 9 days
66 billion Suns About 195 billion km, or 1,300 AU About 390 billion km, or 2,600 AU About 15 days
70 billion Suns About 207 billion km, or 1,380 AU About 413 billion km, or 2,760 AU About 16 days

These figures are transparent calculations rounded to match the uncertainty of the mass estimates. A rotating black hole has a different horizon geometry, so the Schwarzschild values are useful scale estimates rather than observations of TON 618’s spin.

What are astronomers actually seeing?

TON 618’s black hole emits no light from inside its event horizon. Astronomers see the quasar: an intensely bright region of hot gas outside the black hole.

Spectra split that light into wavelengths. Broad emission lines reveal gas moving at thousands of kilometres per second. Combining a velocity estimate with an inferred size for the line-emitting region allows astronomers to calculate the central mass using gravity.

This is called a single-epoch virial estimate. It makes very distant black holes measurable, but it is less direct than tracking individual stars around a nearby black hole or modelling resolved stellar motions across a galaxy’s centre.

How far away is TON 618?

TON 618 has a redshift of about 2.219. Its light has travelled for roughly 10.8 billion years, so astronomers observe the quasar as it appeared when the universe was much younger.

You may also see a distance near 18 billion light-years. That is a present-day comoving-distance estimate that accounts for cosmic expansion. Light-travel distance and comoving distance answer different questions, so they should not be presented as interchangeable.

Is TON 618 the largest known black hole?

It is one of the most massive black holes with a published quasar-based estimate, but there is no simple permanent record holder. Some other candidates have very large indirect estimates, while objects such as the central black hole in Holm 15A have strong direct dynamical measurements.

The newly revised SciQuest comparison of TON 618, Holm 15A and the Phoenix cluster explains why measurement quality matters more than ranking headline numbers.

The short answer

TON 618 probably contains a black hole of tens of billions of solar masses. Depending on the adopted published estimate, a non-spinning equivalent would have an event-horizon diameter of roughly 1,600–2,760 AU. Both the mass and derived size should be presented as model-dependent ranges.

Update note: This article was substantially revised and recalculated on August 23, 2026, to present published mass estimates as a range and distinguish radius from diameter.

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