Sagittarius A* was imaged by the Event Horizon Telescope in 2017
The very centre of our galaxy is home to a slumbering monster: a supermassive black hole. Objects like this are thought to lie at the hearts of most galaxies – they originated from the collapse of giant stars in the early days of the universe, then grew prodigiously by feeding on gas, dust and stars as galaxies coalesced, and by merging together as smaller galaxies collided to form larger ones. Today, most material stays well out of reach of the black hole, but radiation from a gentle stream of infalling gas turns it into a source of radio waves known as Sagittarius A*. Astronomers have confirmed the black hole’s mass by mapping the orbits of stars near the galactic centre, revealing that they orbit an invisible object with the mass of about 4.1 million Suns and a diameter substantially smaller than the orbit of Mercury.