First look
The idea, in plain words
On a dark night, look towards the constellation Sagittarius and you are looking at the middle of our galaxy. This is a real photograph of that part of the sky. The glow is the light of millions of far-off stars. The black lanes across it are clouds of dust lying in the way, and they hide the centre completely.
Dust blocks ordinary light, but it lets a good deal of infrared through. This is the same direction photographed in infrared from a mountain in Chile. The dark lanes have mostly gone and the crowded heart of the galaxy shows through, about 27,000 light years away.
Go right in to the very middle and the stars are packed together as nowhere near us. This picture is only about three light years from side to side. Round the Sun, a space that size holds no other star at all. Here there is a whole crowd of them.
Astronomers watched these stars year after year and found that they move. One of them, called S2, goes round an empty-looking point once every 16 years. At its closest, in May 2018, it was moving at more than 25 million kilometres an hour, which is nearly three hundredths of the speed of light.
Only something very heavy could swing a star round that fast. From S2’s orbit, the thing at the centre weighs about four million times as much as the Sun, and yet it gives off almost no light. It is a black hole. In 2022, eight radio observatories working together as one telescope the size of the Earth showed it: a ring of glowing gas round a dark middle.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Milky Way
- The galaxy we live in. From inside it shows as a pale band of light across a dark sky.
- Supermassive black hole
- A black hole of hundreds of thousands to billions of times the mass of the Sun, found at the centre of a galaxy.
- Infrared
- Light with wavelengths a little longer than red, which we feel as heat.
- Light year
- The distance light travels in one year, about 9.46 million million km.
- Event horizon
- The edge of a black hole: the boundary from inside which nothing can get out.
- Interferometer
- An instrument that splits a beam of light in two, sends the halves along different paths and compares them when they meet again.
- Redshift
- A stretching of light to longer, redder wavelengths.
Where this comes from
- ESO, Astronomers reveal first image of the black hole at the heart of our galaxy (2022)
- ESO, First successful test of Einstein’s general relativity near a supermassive black hole (2018)
- ESO, The centre of the Milky Way (NACO image)
- ESO, VISTA gigapixel mosaic of the central parts of the Milky Way
- ESO, A 340-million pixel starscape from Paranal
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.