At the centre of almost every large galaxy sits a black hole far heavier than any star could make: from a hundred thousand to many billions of times the mass of the Sun. These supermassive black holes power the brightest objects in the universe, the quasars, and appear to grow together with the galaxies around them.
Weighing our own
The Milky Way’s supermassive black hole is called Sagittarius A*, about 27 000 light-years away in the direction of the constellation Sagittarius. It cannot be seen directly through the dust of the galaxy’s disc in visible light, but in infrared light astronomers can watch individual stars orbiting it. One star, S2, completes an orbit in about 16 years, swinging close to the black hole at enormous speed.
Two teams, led by Reinhard Genzel in Germany and Andrea Ghez in the United States, tracked those orbits for decades. They showed that the stars circle an invisible object of about 4.3 million solar masses packed into a tiny volume: it can only be a black hole. The work earned them a share of the 2020 Nobel Prize in Physics.
Growing with their galaxies
In 2000 two teams found a surprisingly tight link: the heavier a galaxy’s central bulge of stars, and the faster those stars move, the heavier its black hole. This M–σ relation suggests that black holes and galaxies grow together, probably because the energy released by a feeding black hole regulates how fast its galaxy forms stars.
Too big, too early
The puzzle is speed. JWST and Chandra have found supermassive black holes when the universe was only a few hundred million years old. GN-z11, seen about 400 million years after the Big Bang, already has a black hole of about 1.6 million solar masses, and UHZ1 one of about 40 million. Growing that fast from the remnants of ordinary stars is hard. One proposed answer is that some started big, as “direct collapse” black holes of around 100 000 solar masses formed straight from giant gas clouds.
Pairs
When galaxies merge, their black holes should eventually orbit each other and merge too. In 2023 the NANOGrav collaboration, timing the steady pulses of 67 pulsars, reported evidence for a background of very slow gravitational waves, consistent with countless pairs of supermassive black holes spiralling together across the universe.