Active galaxies and quasars

Most galaxies shine with the light of their stars and nothing else. About one in ten is different. Right at its centre is a tiny point that gives out more light than all the rest of the galaxy. These are called active galaxies.

The light does not come from stars. Nearly every large galaxy has a giant black hole at its centre. A black hole gives out no light of its own, but in an active galaxy gas is falling towards it. The gas does not drop straight in. It swirls round in a flat disc, like water round a plughole, and as it is squeezed and rubbed it grows so hot that it glows fiercely.

The brightest active galaxies are called quasars. A quasar can give out thousands of times as much light as our whole galaxy. They are so far away that from the Earth one looks like a faint star, a single point of light.

Some active galaxies also fire out two thin jets of fast particles, one each way, reaching far beyond the galaxy itself.

Our own galaxy has a giant black hole too, but it is quiet at present. The Milky Way is not an active galaxy.

What makes a galaxy active. In an ordinary galaxy, the light is the sum of its stars. Around 10 per cent of known galaxies give out far more than that, from a small core whose brightness changes with time. The centre of an active galaxy is more than 100 times brighter than all its stars together, and much of the extra comes out as infrared, radio waves, ultraviolet and X-rays. Spirals, ellipticals and irregulars can all be active.

The engine. Most large galaxies, perhaps all, have a black hole at the centre with a mass of hundreds of thousands to billions of Suns. In an active galaxy that black hole is being fed.

Part	What it is
Black hole	Hundreds of thousands to billions of times the mass of the Sun. It gives out no light itself
Accretion disc	Gas and dust spiralling inwards. The black hole’s gravity squeezes and heats it until it glows, from infrared through to X-rays
Torus	A clumpy ring of cooler dust round the disc, shaped like a doughnut, which hides the centre from some directions
Jets	In about one active galaxy in ten, a small part of the gas is driven out in two narrow beams at right angles to the disc, at nearly the speed of light

How anyone knew it was a black hole. The first clue was how quickly the brightness changes. Quick changes point to a small source, because something very large cannot brighten and fade all at once. Great power, quick changes and a small volume together pointed to gas falling onto a black hole, one of the only things that can give out so much energy from so little room.

In 2019 the engine of one active galaxy was photographed. M87 is a massive elliptical galaxy in the Virgo cluster. A network of eight radio telescopes working as one showed the shadow of its black hole, which is about 55 million light years away and has the mass of 6.5 billion Suns.

One kind of object, seen from different sides. Astronomers named several kinds of active galaxy before they understood them. Much of the difference is now thought to be the angle we happen to look from: straight down the jet, into the bright centre, or side on with the dust ring in the way.

Kind	What is seen
Seyfert galaxy	The commonest and the least powerful. Looks like a normal galaxy in ordinary light but is bright in infrared, and sometimes in X-rays. First picked out by Carl Seyfert in 1943
Radio galaxy	A galaxy with strong jets that reach far out into the space between galaxies and glow in radio waves. The jets of the elliptical galaxy NGC 4261 span 88,000 light years
Quasar	The most luminous kind. Can give out thousands of times the energy of a galaxy like the Milky Way
Blazar	One whose jet points almost straight at the Earth, so it looks brighter and flickers more than the rest

Quasars. The first quasar was recognised in 1963 by Maarten Schmidt. It is called 3C 273, and it looked like a faint star. Its distance turned out to be 2.5 billion light years, far too great for any star to be seen. It is more than ten times brighter than the brightest giant elliptical galaxies. If it were a few tens of light years away it would look as bright as the Sun.

The name is short for quasi-stellar object, which only means something that looks like a star. In 2024 the Hubble telescope blocked out the glare of 3C 273 and looked eight times closer to the black hole than before. It found blobs and threads within 16,000 light years of the centre, some of which may be small galaxies falling in. Its jet is 300,000 light years long. By our own sum that is three times the width of the Milky Way’s disc, taken as 100,000 light years.

Why quasars are far away. The nearest quasar, Markarian 231, is about 600 million light years off. More and more are found the further out astronomers look, and over a million are now known. The farthest is about 13 billion light years away.

Looking far away is looking back in time, so this says that quasars were commoner in the past. They were most abundant about 3 billion years after the big bang, when collisions between galaxies were more frequent. A collision can send gas pouring to the centre and feed the black hole. When the supply runs out the quasar fades. The bright phase may last only about 10 million years, and may come back more than once.

That makes a quasar a stage in a galaxy’s life, not a separate kind of thing. The Milky Way is quiet now, though its centre probably had a burst of activity within the past few million years.

What the 3D view shows. The view opens on the map of real galaxies, each drawn as a point. About one in ten of them is active, but the map does not mark which, and no discs or jets are drawn. By the distance of 3C 273 the map has thinned to a scattering of the brightest galaxies, and 3C 273 is not singled out. The model of the Milky Way has a brighter bulge in the middle, which is the glow of old stars. It has no bright point at the very centre and no jet, which is right for a galaxy that is not active.