Sagittarius A*

At a glance

Key points

The Event Horizon Telescope imaged its shadow in 2022, about the size of a doughnut on the Moon as seen from Earth.

• About 4.3 million Suns
• Imaged by the EHT in 2022
• Looks like a doughnut on the Moon from Earth

Finding it

Bruce Balick and Robert Brown picked up a tiny, bright radio source at the Galactic Centre in February 1974, using an interferometer at Green Bank, West Virginia; Brown later gave it the name Sgr A*. Dust in the plane of the Milky Way blocks almost all of its visible light, so it is studied in radio waves, infrared and X-rays. It lies in the constellation Sagittarius, close to the borders with Scorpius and Ophiuchus.

Weighed by stars

Stars such as S2 race around Sgr A* on tight orbits. Tracking them for decades pinned down its mass to about 1%, work recognised by the 2020 Nobel Prize in Physics.

A test of Einstein

When S2 swung past in May 2018, moving at almost 3% of the speed of light, the GRAVITY instrument on ESO's Very Large Telescope Interferometer saw its light stretched to longer wavelengths by the black hole's gravity, exactly as general relativity predicts and unlike the Newtonian expectation. Later tracking showed S2's orbit slowly rotating, the same relativistic effect that shifts Mercury's orbit, but far larger.

A flickering target

Because gas circles it in minutes, Sgr A* changed during each night of observation, making its image far harder to assemble than M87*'s.

Two black holes compared

Sgr A* is more than a thousand times smaller and less massive than M87*, yet the two rings look similar in size on the sky, because Sgr A* is about 2,000 times closer. The EHT's 2017 observations used eight observatories, and more than 300 researchers from 80 institutes built the image, comparing it with a large library of simulated black holes.

What we still don't know

Whether it drives a faint jet, as its magnetic field hints, and why it is so quiet compared with the quasars of the early universe.

Missions

1. 2017–

   Event Horizon Telescope

   International collaborationActive

   Earth-sized radio interferometer at 1.3 mm; imaged M87* (2019) and Sgr A* (2022).

   Mission page (opens in a new tab)

Related entries

• The Milky WayOur barred spiral home galaxy
• M87*The first black hole ever imaged
• Black holeEvent horizon · gravity light cannot escape

Sources

1. ESO: eso2006: S2's orbit precesses (opens in a new tab)
2. GRAVITY Collaboration 2022, A&A (opens in a new tab)
3. ESO: eso2208: first image of Sgr A* (opens in a new tab)
4. ESO: eso2406: Sgr A* magnetic fields (opens in a new tab)
5. Goss, Brown & Lo 2003: the discovery of Sgr A* (arXiv) (opens in a new tab)
6. EHT Collaboration 2022, ApJL 930:L12 (arXiv) (opens in a new tab)
7. ESO: eso1825: S2 and general relativity (opens in a new tab)

Last reviewed 10 Oct 2026 against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.