Sagittarius A*

The supermassive black hole at the heart of our galaxy weighs about four million Suns. Astronomers released the first picture of it on 12 May 2022, using a network of radio telescopes.

The picture shows the glowing gas around the black hole. The black hole itself cannot be seen.

Context

Sagittarius A* (pronounced A-star) sits in the direction of the constellation Sagittarius, about 26,000 to 27,000 light-years from Earth. Its existence had been suspected for decades because of a strong, compact radio source at the galactic centre. The strongest proof came from tracking stars that orbit it. One of them, S2, swings around it in about 16 years at very high speed.

Those orbits let astronomers calculate the mass using the same laws of motion that govern planets. The answer is about 4 million times the mass of the Sun. The teams led by Reinhard Genzel and Andrea Ghez shared the 2020 Nobel Prize in Physics for this work, together with Roger Penrose for his theory of black holes.

The 2022 image came from the Event Horizon Telescope, a network of radio dishes spread across the globe that acts like one Earth-sized telescope. Making the picture was hard because the gas near Sgr A* changes within minutes, so the team had to combine many observations and use extensive computer modelling.

The bright ring shows hot gas bent by the black hole's gravity around a dark central region. It matched what Einstein's general relativity predicts.