How big is TON 618? Mass, event horizon and distance

The black hole powering TON 618 is estimated to contain roughly 41–66 billion times the Sun’s mass, or about 8–13 × 10⁴⁰ kg. For a non-spinning black hole, that mass range corresponds to an event-horizon diameter of about 240–390 billion kilometres—roughly 1,600–2,600 astronomical units (AU), or 0.025–0.041 light-years.

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 calibrations produce different answers.

Key facts about TON 618

• Mass: about 6.6 × 10¹⁰ solar masses (2004 analysis) or about 4.1 × 10¹⁰ (later recalculation), both from the same type of indirect method.
• Event-horizon diameter: about 1,600–2,600 AU, or roughly 27–43 times the diameter of Neptune’s orbit.
• Brightness: its measured luminosity at a wavelength of 5100 Å alone is about 2 × 10⁴⁰ watts (10⁴⁷·³ erg/s).
• Distance: redshift 2.219; its light has travelled for about 10.8 billion years.
• Measurement status: indirect and model-dependent, with a typical uncertainty of about a factor of four.

TON 618 at a glance

Source note: redshift and masses from Shemmer et al. (2004) and Ge et al. (2019); the 7 × 10¹⁰ figure is quoted by Carr et al. (2021). Sizes calculated by SciQuest as shown below. One AU, the average Earth–Sun distance, is 149,597,870.7 km.

Why are two different masses quoted for TON 618?

The widely repeated estimate of about 66 billion solar masses comes from a 2004 study by Shemmer and colleagues. They measured the width of TON 618’s hydrogen-beta emission line—about 10,500 km/s—and its luminosity at 5100 Å, then applied a single-epoch virial method calibrated on galaxies weighed by reverberation mapping. Their table lists log(M/M☉) = 10.82, or about 6.6 × 10¹⁰ solar masses.

A 2019 catalogue by Xue Ge and colleagues lists a lower value, log(M/M☉) = 10.61, or about 4.07 × 10¹⁰ solar masses. Their table draws on the same group’s earlier recalculation of hydrogen-beta masses with a host-galaxy correction. The difference does not mean the black hole shrank. It reflects a different calibration and correction applied to the same kind of measurement.

Both values depend on a model of gas too distant to resolve. The gas may not move in a simple, randomly oriented way, and viewing angle, radiation pressure and the empirical size–brightness relation all add uncertainty. Single-epoch masses typically carry an absolute uncertainty of about a factor of four. TON 618 is also roughly 200 times more luminous than the brightest active galaxies in the reverberation-mapped sample used to calibrate that relation, a limitation Shemmer and colleagues flagged.

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

With the Sun’s gravitational parameter GM☉ = 1.327 × 10²⁰ m³/s² and c = 299,792,458 m/s, this equals about 2.95 kilometres for every solar mass. Applying it to the published mass range gives:

Calculated by SciQuest and rounded to match the uncertainty of the mass estimates. A rotating black hole has a smaller horizon, so these are scale estimates rather than observations of TON 618’s spin.

What is the diameter of TON 618 in light-years and AU?

Worked for the 6.6 × 10¹⁰ estimate:

1. Radius: 2.953 km × 6.6 × 10¹⁰ ≈ 1.95 × 10¹¹ km.
2. Diameter: 2 × 1.95 × 10¹¹ km ≈ 3.90 × 10¹¹ km.
3. In AU: 3.90 × 10¹¹ km ÷ 1.496 × 10⁸ km per AU ≈ 2,610 AU.
4. In light-years: one light-year is 299,792.458 km/s × 31,557,600 s ≈ 9.461 × 10¹² km, so 3.90 × 10¹¹ km ÷ 9.461 × 10¹² km ≈ 0.041 light-years.

The same steps for 4.07 × 10¹⁰ solar masses give about 1,610 AU, or 0.025 light-years. So TON 618’s event horizon is roughly 1,600–2,600 AU, or 0.025–0.041 light-years, across—about 9 to 15 light-days.

For a familiar scale: Neptune orbits the Sun at about 30 AU, so its orbit is about 60 AU across. TON 618’s horizon would span roughly 27–43 Neptune orbits. By contrast, the horizon of Sagittarius A*, the Milky Way’s 4.3-million-solar-mass black hole, would be only about 0.17 AU across—about 9,500 to 15,000 times smaller.

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 lets astronomers calculate the central mass from gravity.

This single-epoch virial method 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 2.219. Using Planck 2018 cosmological parameters, its light has travelled for about 10.8 billion years, so astronomers observe the quasar as it was when the universe was about 3 billion years old.

You may also see a distance near 18 billion light-years. That is the present-day (comoving) distance, which accounts for cosmic expansion while the light was in transit; the same parameters give about 18.3 billion light-years. 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 has one of the largest published mass estimates, but there is no confirmed record holder. Holm 15A’s black hole, at (4.0 ± 0.8) × 10¹⁰ solar masses, was weighed directly from stellar motions, and lensing has measured black holes of about 3–4 × 10¹⁰ in the Abell 1201 and Cosmic Horseshoe galaxies. The Phoenix A black hole has larger popular figures but no direct measurement.

Our guide to the biggest black hole in the universe ranks the candidates by evidence, and our comparison of TON 618, Holmberg 15A and Phoenix A explains why measurement quality matters more than headline numbers.

The short answer

TON 618 probably contains a black hole of tens of billions of solar masses. Depending on the adopted estimate, a non-spinning equivalent would have an event-horizon diameter of roughly 1,600–2,600 AU (0.025–0.041 light-years). Both the mass and the derived size should be presented as model-dependent ranges.

Related reading

• Phoenix A black hole: mass estimates, distance and size
• What is at the centre of the Milky Way? Sagittarius A* explained
• Can anything escape from a black hole?

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. On October 12, 2026, it was updated to add the light-year diameter calculation and scale comparisons, attribute each mass estimate to its source and method, and link to SciQuest’s guide to the biggest black holes. It was revised with AI assistance and checked against the sources listed below.

Sources

• Shemmer et al., Near-infrared spectroscopy of high-redshift active galactic nuclei. I. A metallicity–accretion rate relationship, The Astrophysical Journal 614, 547 (2004)
• Ge et al., The blueshift of the C IV broad emission line in QSOs, The Astronomical Journal 157, 148 (2019); table data via CDS VizieR J/AJ/157/148
• Vestergaard and Peterson, Determining central black hole masses in distant active galaxies and quasars. II., The Astrophysical Journal 641, 689 (2006)
• Carr, Kühnel and Visinelli, Constraints on stupendously large black holes, Monthly Notices of the Royal Astronomical Society 501, 2029 (2021)
• Mehrgan et al., A 40 billion solar mass black hole in the extreme core of Holm 15A, The Astrophysical Journal 887, 195 (2019)
• GRAVITY Collaboration, The mass distribution in the Galactic Centre from interferometric astrometry of multiple stellar orbits, Astronomy & Astrophysics (2022)
• Planck Collaboration, Planck 2018 results. VI. Cosmological parameters, Astronomy & Astrophysics 641, A6 (2020)
• NASA NSSDCA, Sun Fact Sheet and Planetary Fact Sheet
• NIST, Speed of light in vacuum; IAU, 2012 Resolution B2 on the astronomical unit