Sagittarius A*

The centre of our Galaxy

Sagittarius A* sits at the heart of the Milky Way, hidden behind thick clouds of interstellar dust. Visible-light telescopes cannot see it directly; infrared, radio, and X-ray observations peer through the dust to reveal the dense star cluster and the black hole at its centre.

Its name comes from its position in the constellation Sagittarius and the “A*” label used by radio astronomers for a particularly energetic source. The star at the end is pronounced “A-star”.

How we know it is there

Astronomers tracked stars whipping around an invisible point at the Galactic Centre for decades. One star, S2, completes a highly elliptical orbit in about 16 years, reaching speeds of several percent of the speed of light. Its orbit can only be explained by a compact mass of about four million Suns.

Infrared interferometry from the GRAVITY instrument later detected the orbital motion of gas close to the event horizon, providing one of the strongest tests yet of the black-hole interpretation.

The first image

In 2022, the Event Horizon Telescope collaboration released the first image of Sagittarius A*. It shows a bright ring of glowing gas surrounding a dark central shadow, the silhouette of the event horizon. The image required a network of radio telescopes across the globe acting as one Earth-sized instrument.

The ring is not a photograph of the black hole itself. Black holes emit no light; what we see is the hot, magnetised plasma around it, bent and brightened by gravity.

A quiet giant

Compared with many supermassive black holes, Sagittarius A* is quiet. It consumes very little gas, so its surroundings are faint. Occasional flares in infrared and X-rays show that small amounts of material are still falling inward or being ejected.

The black hole’s mass is tiny compared with the mass of the Galaxy, but its gravity helps shape the orbits of the dense stellar cluster at the centre. Understanding this region helps astronomers connect black-hole growth to the evolution of galaxies.

Keep exploring

See where Sagittarius A* sits in the Milky Way, compare our Galaxy with Andromeda, or browse the Black Holes directory.