Observatory · the engine
Active galactic nuclei
At the centre of almost every big galaxy sits a black hole of millions to billions of Suns. Most of them, like ours, are quiet. But when gas falls in, it piles up in a disc that spins faster and hotter the closer it gets, and glows brighter than all the galaxy's hundreds of billions of stars together, from a region not much bigger than the solar system. Some also fire two jets of particles out of their poles at nearly the speed of light, thousands of light years long. Astronomers found these engines under a dozen names, quasars, blazars, Seyferts, radio galaxies, before realising they are mostly the same thing, seen from different sides.
One engine, many faces
Fig. I. The engine at the heart of a galaxy, seen from the chosen side.
- Seen as
- Jet towards us
- Blobs seem to move at
The disc is brightest on the side turning towards us, the beaming of the siren and the lens. Around it lies a thick, dusty ring. Look from the side and the ring hides the centre: you see the jets and the galaxy, a radio galaxy or a type 2 Seyfert. Tilt it towards you and you look into the ring at the blazing disc and the fast-moving gas clouds just outside it, whose spectrum lines are smeared out by thousands of kilometres a second: a quasar or a type 1 Seyfert. Look straight down a jet and its light, beamed forward at 99% of the speed of light, outshines everything else and flickers within days: a blazar.
The read-outs show two tricks of a jet aimed nearly at us. Its light is beamed, thousands of times brighter than the jet going the other way, which is often too faint to see at all. And blobs in it seem to cross the sky faster than light. Nothing breaks the speed limit: the blob almost keeps up with its own light, so light it sent years apart arrives only months apart, and the motion looks squeezed into a fraction of the time.
One kilogram, four ways
Fig. II. One kilogram, four ways of turning it into energy.
All of these release energy by turning a little mass into it, E = mc². Burning coal turns about three parts in ten billion of its mass into heat. Fusion in the Sun does far better: 0.7%. But matter spiralling into a black hole has to lose most of the energy of its fall before it may cross the edge, and it radiates it away as it rubs and heats up in the disc: 5.7% for a black hole that does not spin, up to about 32% for one that spins as fast as nature allows, the gas orbiting so close in before the plunge. Nothing else in the universe gets that much out of matter, which is how the brightest things in it are black holes.
Who worked it out
Where it already matters
A star that was not a star. 3C 273 looks like a faint star of magnitude 13, visible in a good amateur telescope. In 1963 the Dutch astronomer Maarten Schmidt, at Palomar, recognised its strange spectrum lines as hydrogen, shifted 16% to the red: it was 2.4 billion light years away, so it had to be more than a hundred times brighter than the whole Milky Way. The name quasar, quasi-stellar, stuck.
The first pictures. In 2019 the Event Horizon Telescope, radio dishes all over the Earth linked into one the size of the planet, showed the shadow of the black hole in M87, 6.5 billion Suns, 55 million light years away. Its jet, 5,000 light years long, was first noticed as a "curious straight ray" in 1918, and the Hubble telescope later timed blobs in it that seem to move at six times the speed of light. In 2022 it was our own black hole's turn, Sagittarius A*, four million Suns, quiet, whose weight had been found by timing stars orbiting it, the Nobel prize of 2020.
Shaping galaxies. The heaviest black holes sit in the heaviest galaxies, in a tight ratio, even though a black hole weighs only a few tenths of a percent of the stars in its galaxy's bulge. The energy an active nucleus pours out heats and blows away the gas its galaxy would make stars from, so the engine seems to regulate its own galaxy's growth.