What is the biggest black hole in the Milky Way?

The biggest overall is Sagittarius A*, the supermassive black hole at the centre, with about 4.3 million solar masses. Among stellar-mass black holes, made by collapsing stars, the most massive known as of 2025 is Gaia BH3 at about 33 solar masses.

How was Gaia BH3 discovered?

ESA's Gaia spacecraft saw a star wobbling in an 11.6-year orbit around an invisible companion. Measuring that orbit showed the companion weighs about 33 solar masses, too heavy to be anything but a black hole.

What is the closest black hole to Earth?

As of 2025, the closest known is Gaia BH1, about 1,560 light-years away. Gaia BH3, at about 1,900 light-years, is the second closest known.

How do stellar-mass black holes form?

They form when the core of a massive star collapses after its nuclear fuel runs out. If the core is heavy enough, no force can stop the collapse, and it becomes a black hole a few to a few tens of times the Sun's mass.

Is Gaia BH3 dangerous to Earth?

No. At about 1,900 light-years away it has no measurable effect on the Solar System. Its gravity only matters to objects close to it, such as its companion star.

A sleeping stellar giant hidden in our galaxy

Gaia BH3

Dormant stellar black hole · ≈ 1,900 ly

Gaia BH3 is a black hole about 33 times the mass of the Sun, lying roughly 1,900 light-years from Earth in the constellation Aquila. Announced in April 2024, it is the most massive stellar-mass black hole known in the Milky Way. It is completely dark: astronomers found it only because an old giant star orbiting it moves in a telltale loop across the sky, tracked by the European Space Agency's Gaia spacecraft.

Black holes like Gaia BH3 are the collapsed remains of massive stars. The galaxy may hold as many as 100 million of them, yet only a few dozen have been identified, mostly because they are feeding on a companion. Gaia BH3 is a dormant one, giving a rare look at a black hole that is simply sitting quietly in space.

Key facts

Mass: ≈ 33 M☉Measured as 32.7 ± 0.8 solar masses
Distance: ≈ 1,900 light-yearsAbout 590 parsecs
Constellation: Aquila
Companion: Old, metal-poor giant starAbout 0.76 solar masses
Orbital period: ≈ 11.6 yearsHighly elongated orbit
Announced: April 2024From early Gaia Data Release 4 data
State: DormantNo detected X-ray emission
Type: Stellar-mass black hole

What is a stellar-mass black hole?

Stellar-mass black holes are the most common kind of black hole. They typically weigh between about 5 and a few tens of solar masses, and form when a massive star, usually one born with more than about 20 times the Sun's mass, runs out of nuclear fuel. Its core can no longer support itself and collapses under its own gravity, sometimes in a supernova explosion and sometimes almost silently.

Such a black hole is tiny. A 33-solar-mass black hole like Gaia BH3 would have an event horizon only about 200 km across if it does not spin, smaller than many countries. Alone in space, it gives off no light at all, which makes the great majority of these objects invisible to telescopes.

X-ray binaries

Most known stellar black holes were found because they have a close companion star. Gas pulled off the companion forms a disc that spirals inwards and heats to millions of degrees, shining brightly in X-rays. These systems, called X-ray binaries, act as signposts that let astronomers spot and weigh black holes across the galaxy.

Cygnus X-1

Discovered as an X-ray source in 1964, Cygnus X-1 was the first object widely accepted as a black hole. It lies about 7,200 light-years away and orbits a blue supergiant star every 5.6 days. A 2021 study raised its mass to about 21 solar masses. Stephen Hawking famously bet that it was not a black hole, and conceded in 1990.

Found by a wobbling star

Gaia measures the positions of more than a billion stars with extraordinary precision. While checking early data for the mission's fourth data release, scientists noticed a star in Aquila tracing a small ellipse across the sky every 11.6 years. The size of that motion, combined with the star's distance and follow-up measurements of its speed from ground-based telescopes, including ESO's Very Large Telescope, revealed that it orbits an unseen companion of about 33 solar masses.

Nothing that heavy and that dark could be an ordinary star, which would be easily visible. The only explanation that fits is a black hole. The result was so significant that the Gaia collaboration published it in April 2024, ahead of the full data release, so other astronomers could begin studying it.

An ancient companion

The companion is an old giant star of about three quarters of the Sun's mass. It contains very few elements heavier than hydrogen and helium, a sign that it formed early in the galaxy's history, when the gas had not yet been enriched by many generations of stars. Its motion links it to a stellar stream called ED-2, the remains of a small star cluster that the Milky Way absorbed long ago.

This matters because theory predicts that massive stars poor in heavy elements lose less mass through stellar winds during their lives, leaving larger black holes behind. Gaia BH3's companion supports that idea, suggesting the star that collapsed into the black hole was similarly pristine.

Dormant black holes

The black hole and its companion never come close enough for gas to flow between them, so there is no hot disc and no X-ray glow. Before Gaia, such dormant black holes were almost impossible to find. Gaia BH3 is the third discovered by the mission, after Gaia BH1, about 1,560 light-years away and the closest black hole known as of 2025, and Gaia BH2, about 3,800 light-years away. Both of those weigh around 9 solar masses.

Black holes of more than 30 solar masses had already been detected far away, through the gravitational waves produced when pairs of them merge. Gaia BH3 shows that such heavy black holes also exist much closer to home, in our own galaxy. Finding more dormant systems like it will help astronomers learn how many black holes the Milky Way contains and how they are spread across it.

Surprising facts about Gaia BH3

Gaia BH3 was discovered without detecting any light from the black hole itself, only from the motion of its companion star.
Despite weighing 33 Suns, its event horizon would be only about 200 km across.
Its companion star is one of the older stars in the galaxy, part of a stream left over from a cluster the Milky Way swallowed.
Astronomers estimate the Milky Way may contain as many as 100 million stellar-mass black holes, but only a few dozen have been confirmed.
Black holes of similar mass were first detected in 2015 through gravitational waves from mergers billions of light-years away.

Frequently asked questions

What is the biggest black hole in the Milky Way?: The biggest overall is Sagittarius A*, the supermassive black hole at the centre, with about 4.3 million solar masses. Among stellar-mass black holes, made by collapsing stars, the most massive known as of 2025 is Gaia BH3 at about 33 solar masses.
How was Gaia BH3 discovered?: ESA's Gaia spacecraft saw a star wobbling in an 11.6-year orbit around an invisible companion. Measuring that orbit showed the companion weighs about 33 solar masses, too heavy to be anything but a black hole.
What is the closest black hole to Earth?: As of 2025, the closest known is Gaia BH1, about 1,560 light-years away. Gaia BH3, at about 1,900 light-years, is the second closest known.
How do stellar-mass black holes form?: They form when the core of a massive star collapses after its nuclear fuel runs out. If the core is heavy enough, no force can stop the collapse, and it becomes a black hole a few to a few tens of times the Sun's mass.
Is Gaia BH3 dangerous to Earth?: No. At about 1,900 light-years away it has no measurable effect on the Solar System. Its gravity only matters to objects close to it, such as its companion star.

Keep exploring

The 3D views in Ember’s Universe are scientific visualisations: physically based where possible, compressed or exaggerated where the real scales would hide everything.

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