Is there a black hole at the centre of the Milky Way?

Yes. Sagittarius A* is a supermassive black hole of about 4.3 million solar masses at the galaxy's centre. Its existence is confirmed by decades of stellar orbit measurements and by the Event Horizon Telescope image of 2022.

How far away is Sagittarius A*?

It is about 26,000 light-years from Earth. Its light passes through the dusty disc of the Milky Way on the way to us, which is why it is studied in radio, infrared and X-ray light rather than visible light.

Could Sagittarius A* swallow Earth?

No. Earth and the Sun orbit far out in the galactic disc, and the black hole's gravity at that distance is negligible compared with the pull of the whole galaxy. It is not drawing the Solar System inwards.

Who won the Nobel Prize for discovering the Milky Way's black hole?

Reinhard Genzel and Andrea Ghez shared half of the 2020 Nobel Prize in Physics for discovering a supermassive compact object at the galaxy's centre. The other half went to Roger Penrose for his theoretical work on black hole formation.

Why is the Sagittarius A* image blurry?

The black hole is tiny on the sky, and the image is near the limit of what an Earth-sized radio telescope can resolve. Gas around it also changes in minutes, so the image is an average of many slightly different views.

The quiet giant at the heart of our galaxy

Sagittarius A*

Supermassive black hole · ≈ 26,000 ly

Sagittarius A* (pronounced 'A-star') is the supermassive black hole at the centre of the Milky Way. It holds about 4.3 million times the mass of the Sun, packed into a region smaller than the orbit of Mercury. It lies roughly 26,000 light-years from Earth in the direction of the constellation Sagittarius, hidden behind thick clouds of dust that block visible light but let radio waves, infrared and X-rays through.

For decades astronomers suspected a giant black hole lurked there. The proof came from watching individual stars whip around an invisible point at thousands of kilometres per second, work that earned a Nobel Prize in 2020. In 2022 the Event Horizon Telescope released the first image of its shadow, a dark centre ringed by glowing gas.

Key facts

Distance: ≈ 26,000 light-yearsAbout 8.2 kiloparsecs
Mass: ≈ 4.3 million M☉
Event horizon diameter: ≈ 25 million kmNon-spinning estimate; about 18× the Sun's width
Shadow size on the sky: ≈ 52 microarcsecondsRing measured by the EHT
Constellation: Sagittarius
Discovered: 1974As a compact radio source, by Bruce Balick and Robert Brown
First image: 12 May 2022Event Horizon Telescope
Type: Supermassive black hole

A black hole at the centre of the galaxy

A black hole is a region where gravity is so strong that nothing, not even light, can escape once it crosses a boundary called the event horizon. Sagittarius A* is a supermassive black hole, a class that weighs millions to billions of Suns and sits at the centres of most large galaxies. With about 4.3 million solar masses, it is modest by those standards, yet its gravity dominates the motion of stars within a few light-years of it.

If Sagittarius A* does not spin, its event horizon would be about 25 million km across, roughly 18 times the diameter of the Sun. Everything we see comes from just outside that boundary: hot, magnetised gas spiralling inwards and shining in radio waves, infrared light and X-rays.

Weighing it with stars

Starting in the 1990s, two teams, one led by Reinhard Genzel in Germany and one by Andrea Ghez in the United States, tracked stars close to the galactic centre using large infrared telescopes and adaptive optics, which correct for blurring by Earth's atmosphere. Over many years, they mapped the stars' orbits and found they all circled a single dark, compact point.

The stars' speeds and orbit sizes reveal how much mass sits at the focus. The answer, about 4 million Suns in a volume far too small to hold that many stars, left a black hole as the only reasonable explanation. Genzel and Ghez shared half of the 2020 Nobel Prize in Physics for this discovery.

The star S2

The best-studied of these stars, S2, completes an elongated orbit every 16 years. At its closest approach in May 2018 it passed about 120 times the Earth–Sun distance from the black hole, moving at about 2.5% of the speed of light. The GRAVITY instrument at ESO's Very Large Telescope saw its light stretched to longer wavelengths by the black hole's gravity, as general relativity predicts.

A rotating orbit

In 2020 the same team showed that S2's orbit slowly rotates, so each loop traces a slightly different ellipse, like the petals of a rosette. This effect, called Schwarzschild precession, matched Einstein's prediction and is the same phenomenon that shifts Mercury's orbit around the Sun, but far stronger.

The Event Horizon Telescope image

Sagittarius A* is huge, but so far away that its shadow appears about as large on the sky as a doughnut on the surface of the Moon would from Earth. Resolving it needed a telescope the size of the planet. The Event Horizon Telescope links radio dishes across several continents so they act together as one virtual instrument.

Its image, released on 12 May 2022 from observations made in 2017, shows a bright ring of radio emission about 52 microarcseconds across surrounding a dark centre. Imaging Sagittarius A* was harder than imaging M87*, because gas orbits this smaller black hole in minutes rather than days, so its appearance changed during each night of observation. In 2024 the collaboration released a polarised view revealing an ordered magnetic field around it.

A sleepy black hole

Compared with the blazing cores of active galaxies, Sagittarius A* is remarkably faint. It swallows only a tiny trickle of gas, and most of the material nearby never falls in. Even so, it is not entirely quiet. It flares in X-rays and infrared light about once a day, as clumps of gas heat up close to the horizon, and the flares can brighten it many times over within an hour.

There are signs that it was more active in the past. Echoes of an X-ray outburst a few centuries ago can be seen reflected off nearby gas clouds, and giant structures above and below the galactic plane, such as the Fermi bubbles seen in gamma rays, may record a much larger outburst millions of years ago.

A crowded neighbourhood

The region around Sagittarius A* is the densest stellar environment in the Milky Way. Within a few light-years lie a cluster of millions of stars, many of them old, alongside a surprising number of young, massive stars that formed only a few million years ago. How young stars formed in such a hostile place, where tides from the black hole should tear apart gas clouds, is still debated.

The black hole sits inside a larger radio source called Sagittarius A, which includes a supernova remnant and a spiral-shaped stream of hot gas. Astronomers expect that thousands of smaller, stellar-mass black holes also orbit in this central region. Most of them are invisible, but a handful have been spotted in X-rays as they feed on companion stars, hinting at a much larger hidden population.

Surprising facts about Sagittarius A*

Sagittarius A* is about 1,500 times less massive than M87*, yet it looks almost the same size on the sky because it is roughly 2,000 times closer.
Despite its enormous mass, the black hole would fit comfortably inside the orbit of Mercury.
The star S2 reaches speeds of almost 8,000 km/s at its closest point, enough to cross the Atlantic Ocean in under a second.
Light from Sagittarius A* that reaches us today left it around 26,000 years ago, long before the first farming villages.
Dust between us and the galactic centre is so thick that only about one trillionth of the visible light heading our way gets through.

Frequently asked questions

Is there a black hole at the centre of the Milky Way?: Yes. Sagittarius A* is a supermassive black hole of about 4.3 million solar masses at the galaxy's centre. Its existence is confirmed by decades of stellar orbit measurements and by the Event Horizon Telescope image of 2022.
How far away is Sagittarius A*?: It is about 26,000 light-years from Earth. Its light passes through the dusty disc of the Milky Way on the way to us, which is why it is studied in radio, infrared and X-ray light rather than visible light.
Could Sagittarius A* swallow Earth?: No. Earth and the Sun orbit far out in the galactic disc, and the black hole's gravity at that distance is negligible compared with the pull of the whole galaxy. It is not drawing the Solar System inwards.
Who won the Nobel Prize for discovering the Milky Way's black hole?: Reinhard Genzel and Andrea Ghez shared half of the 2020 Nobel Prize in Physics for discovering a supermassive compact object at the galaxy's centre. The other half went to Roger Penrose for his theoretical work on black hole formation.
Why is the Sagittarius A* image blurry?: The black hole is tiny on the sky, and the image is near the limit of what an Earth-sized radio telescope can resolve. Gas around it also changes in minutes, so the image is an average of many slightly different views.

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.

Sources