Sagittarius A*
Supermassive black hole
At the centre of our galaxy lies Sagittarius A* (say “A-star”), a supermassive black hole. Stars race around it in tight orbits, and by tracking them astronomers proved it’s there.
- 26,700 light-years awayEven light – the fastest thing there is – needs 26,700 years to get here.
- ~4 million × SunMass
- Centre of the Milky WayLocation
Take a closer look
Heavy but compact
Sagittarius A* has the mass of about 4 million Suns, yet its event horizon is only around 25 million km across. That would fit well inside the orbit of Mercury. If you shrank the Milky Way to the size of a football field, the black hole would be smaller than a grain of sand.
A picky eater
Compared with many giant black holes, Sagittarius A* is surprisingly quiet, because very little gas falls into it. Even so, it flickers with small flares in X-rays and infrared light, often about once a day. Astronomers watch these flares to learn how black holes feed.
Testing Einstein
When the star S2 swings closest to the black hole, its light is stretched by gravity, exactly as Einstein’s theory predicts. Its orbit also slowly turns like a spirograph pattern, another of Einstein’s predictions. Scientists confirmed both effects in 2018 and 2020.
How we found out
- 1974
Bruce Balick and Robert Brown discover a compact radio source at the galactic centre.
- 1990s–2010s
Teams led by Reinhard Genzel and Andrea Ghez track stars orbiting an invisible heavy object.
- 2020
Genzel and Ghez share the Nobel Prize in Physics (with Roger Penrose).
- 2022
The Event Horizon Telescope releases the first image of Sagittarius A*.
Will Sagittarius A* swallow the Milky Way?
No. It is enormous, but tiny compared to the galaxy. Stars far from it orbit safely, just as planets orbit the Sun without falling in.
See it yourself
Point towards the “teapot” of Sagittarius, where the Milky Way is brightest. The centre is hidden behind dust.
From the encyclopedia
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