Sgr A* is the supermassive black hole at the centre of the Milky Way, weighed by the stars that orbit it and imaged in silhouette by the Event Horizon Telescope.
At a glance
| type | supermassive black hole |
|---|---|
| distance from the Sun | 26,700 light-years (8.18 kpc) |
| its light left it | 26,700 years ago |
Read off the piece’s own catalogue, the same values it draws with.
About Sgr A*
Sagittarius A* was first found as a compact, faint radio source at the centre of the Galaxy, in the direction of Sagittarius. Thick dust in the plane of the Milky Way hides it completely in visible light, so everything known about it comes from radio, infrared and X-ray observations. Its own motion across the sky is almost entirely a reflection of the Sun’s orbit around the Galaxy: it sits nearly still at the centre.
Two teams, at the European Southern Observatory and at the Keck Observatory, followed a cluster of stars close to it, the S-stars, for many years using infrared adaptive optics. The stars trace ellipses around an unseen point, and Kepler’s laws give its mass: a great many Suns packed into a region smaller than the orbit of Mercury. At its closest approach the star S2 showed the redshift and the slowly turning orbit that general relativity predicts. The work was recognised with a Nobel Prize.
The Event Horizon Telescope linked radio dishes across Earth into a single Earth-sized instrument and imaged a bright ring of hot gas around a dark shadow. The ring’s size matches what relativity predicts for the mass found from the stars. Measured: the orbits, the mass and the ring. Inferred: that the mass is a black hole, because no other known object could be so heavy and so small.
What you see in the piece
In visible light the centre is lost behind dust, so switch the band row to infrared or radio to find it. The WONDERS (W) frame Sgr A* close up, its shadow, and the S-stars on their orbits.
Measured or modelled
Its spin, the orientation of that spin and the structure of the gas flowing onto it are inferred from models fitted to the data. It is very dim for its mass, feeding slowly, though it flickers and flares in infrared and X-rays; signs that it was far more active in the past are still being interpreted.
Questions
How far away is Sgr A*?
Sgr A* is 26,700 light-years (8.18 kpc) from the Sun, so the light reaching us now left the Galactic centre 26,700 years ago.
How do we know Sgr A* is a black hole?
Stars orbiting it show that a huge mass sits in a tiny region, and the Event Horizon Telescope imaged a shadow of the size relativity predicts for that mass. No known object other than a black hole fits both.