What is Sagittarius A*?

Supermassive black hole at the Milky Way’s centre, 4.297 million solar masses.

Can you see it?

No small telescope will show you this one; it is here to be understood, not spotted.

Worth knowing about Sagittarius A*

Anything light from there tells us is news 27,000 years old.

Why it stands out: It rules the orbits of the stars in the galaxy’s innermost few light-years, which makes it a natural laboratory for general relativity.
How much mass it has: 4.297 million solar masses.; How we know: Astronomers tracked individual stars near the galactic centre for over 30 years, watching them whip around an invisible point (2020 Nobel Prize in Physics). The Event Horizon Telescope photographed the glowing ring around its shadow: the observations were made in 2017, and the picture was published in 2022.
Its name and its story: Named after Sagittarius A, the brightest radio source in Sagittarius, as the compact source inside it; Robert Brown added the asterisk in 1982, borrowing the notation for an excited atom.

Sagittarius A* — A model, seen from the Earth, with north up.

The numbers

Distance from us	27,000 light-years

We use 27,000 light-years, from the stars racing round it (GRAVITY Collaboration 2022, 8.277 kiloparsecs). Their 2019 result was 26,670; the 26,000 often quoted is older.

How anyone knows this

Show Sagittarius A* in the model 📖 How the Milky Way fits together 🌙 Plan an evening outside