Sagittarius A*
Supermassive black hole
Facts
- Distance
- about 27,000 light-years
- Mass
- 4.3 million times the Sun
- Event horizon
- about 25 million km across
- Ring in the EHT image
- 52 millionths of an arcsecond
- Position (ICRF3, 2015)
- 17h 45m 40.034s · −29° 00′ 28.22″
Sagittarius A*, or Sgr A*, is the supermassive black hole at the centre of the Milky Way, about 27,000 light-years from us in the direction of the constellation Sagittarius. It has a mass of 4.3 million times the Sun, measured from the orbits of the stars that circle it.
Its position is known to within about half a thousandth of an arcsecond, from VLBA radio measurements. On the sky it moves by 6.4 thousandths of an arcsecond a year, but this is almost entirely a reflection: it is the Sun that is circling the centre of the Galaxy, while Sgr A* stays put at the centre.
In 2022 the Event Horizon Telescope, a network of eight radio telescopes spread around the world, released its first image, from observations made in 2017: a bright ring of hot gas around the black hole's shadow.
Here the accretion disc is drawn as in simulations and films, thin and bright: the real one is more tenuous and shines mostly in radio waves. Its tilt is real, measured from the flashes of light of gas in orbit, and so is the gravitational lensing of general relativity, which bends the image of the disc above and below the shadow and brightens the side coming towards us.
Sources
- GRAVITY Collaboration 2022, A&A 657, L12 (mass and distance from the orbits of the S stars, § 6)
- Event Horizon Telescope Collaboration 2022, ApJL 930, L12 (first image of Sgr A*)
- Gordon, de Witt and Jacobs 2023, AJ 165, 49 (ICRF3 position from the VLBA, table 1)
- Reid and Brunthaler 2020, ApJ 892, 39 (proper motion of Sgr A*, table 2)
- GRAVITY Collaboration 2023, A&A 677, L10 (orbits of the near-infrared flares, table 2)