How it works

	The centre of the Milky Way
Where to look	Towards the constellation Sagittarius
Distance	About 27,000 light years
What is there	A black hole called Sagittarius A* (said “A star”)
Its mass	About four million times the Sun’s
How big it looks	About as big as a doughnut would look on the Moon

Why infrared. Dust grains between the stars scatter and soak up ordinary light. Over 27,000 light years of the dusty disc of the galaxy there is enough of it to hide the centre altogether. Longer waves get past the grains more easily, so infrared, radio waves and X-rays all reach us. Everything known about the centre comes from those.

Weighing something you cannot see. The speed of an orbit depends on the mass at the middle of it. Measure how big a star’s orbit is and how long it takes, and the mass follows. Two teams did this for the stars at the centre for more than two decades, and the work won a share of the Nobel Prize in Physics in 2020.

S2’s close pass. In May 2018 S2 came within 20 billion kilometres of the black hole. That is about 130 times the distance from the Earth to the Sun (worked out here), so it would be well outside the orbit of Neptune if the black hole sat where the Sun is. The team at the European Southern Observatory watched its light as it went by. The light was stretched to slightly longer waves by the black hole’s gravity, by the amount Einstein’s theory says it should be and not the amount Newton’s would give. It was the first time that had been checked so close to a black hole of this size.

Why the picture took so long. The black hole in the galaxy M87 was pictured in 2019, three years before our own, though it is far further away. Ours is more than a thousand times smaller. Gas takes days or weeks to circle M87’s black hole but only minutes to circle ours, so the view kept changing while the telescopes were looking. The picture in this lesson is an average of many, from several nights in April 2017.

A quiet one. Some galaxies have central black holes that are swallowing a great deal of gas, and the glow of that gas can outshine all their stars. Ours is not doing that at present. Those galaxies are the subject of the lesson on active galaxies.