Star S2's orbit16.1 yearsclosest approach about 120 AU, in 2018[1]
About the image. Drawn with a bright accretion disk for clarity. The real Sagittarius A* is a quiet black hole; the 2022 Event Horizon Telescope image shows a faint, glowing ring around its shadow.
At the centre of the Milky Way, about 27 000 light-years away, sits a black hole of about 4.3 million solar masses. It is called Sagittarius A*, after the constellation it lies in.
Weighed by its stars
Dust hides the galactic centre from ordinary telescopes, but infrared light gets through. From the 1990s, two teams, one led by Reinhard Genzel and one by Andrea Ghez, tracked individual stars there for decades. One of them, S2, swings around an invisible point every 16 years. At its closest, in 2018, it passed about 120 AU from it, moving at 7 650 kilometres per second. Only a black hole fits an orbit like that around so much invisible mass. The 2018 passage also showed the stretching of light predicted by general relativity. Genzel and Ghez shared the 2020 Nobel Prize in Physics for the discovery.
The picture
On 12 May 2022 the Event Horizon Telescope collaboration, a network of radio dishes spanning the planet, released the first image of Sagittarius A*: a dim ring of glowing gas around a dark centre, made from data taken in 2017.
Smaller than Mercury’s orbit
For all its mass, the event horizon is small: its radius works out to about 0.085 AU, less than a quarter of the way to Mercury’s orbit. Compared with TON 618, it is a lightweight.
To scale · radius in AUSgr A* event horizon: 0.085 · Mercury's orbit: 0.39Event horizon from the 4.3-million-Sun mass. Mercury's orbit: 0.387 AU.
How long to get there?
Sagittarius A* is about 26 996 light-years away.
Light299 792 km/s26 996 years
Breakthrough Starshotplanned light sail, 20% of light speed134 980 years
Parker Solar Probefastest spacecraft so far, 192 km/s42.2 million years
Voyager 1leaving the Solar System, 17 km/s476.1 million years
At Voyager 1's speed the trip would take about 5 950 871 human lifetimes of 80 years. Bars are on a logarithmic scale. Speeds: NASA, Breakthrough Initiatives.
Gallery
Artist's impressions rendered from the same model as the view at the top of this page.
Timeline
2002The orbit of the star S2 around an invisible mass is reported.
2018S2's closest approach confirms the gravitational redshift of general relativity.
2020Reinhard Genzel and Andrea Ghez share the Nobel Prize in Physics.
2022The Event Horizon Telescope releases the first image.
2023Mass refined to 4.30 million Suns.
Questions people ask
Is there a black hole at the centre of the Milky Way?
Yes. Sagittarius A* weighs about 4.3 million Suns. Stars orbiting it, followed for decades, proved it is there.
Is Sagittarius A* dangerous to Earth?
No. It is about 27 000 light-years away.
When was Sagittarius A* first photographed?
The Event Horizon Telescope image was released on 12 May 2022.