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Quasar

A black hole feeding so fast it outshines its whole galaxy

Each is simulated from real physics and measurements; anything no eye could see, like magnetic fields and radio waves, is drawn and labelled as drawn.

Below are the 10 chapters of this simulation in Cosmic Scale: each one's caption, then the fuller explanation the app shows under “Read more”. Colours, arrows and lines mentioned in the text belong to the 3D scene in the app.

Open Cosmic Scale then the Phenomena tab

1. A star that wasn't one

In 1963 Maarten Schmidt found that a faint point of light called 3C 273 was no star: its light had been stretched by the expanding universe on a journey of about 2 billion years.

3C 273 was first known as a source of radio waves, the 273rd in the third Cambridge catalogue. In 1962 the Moon passed in front of it, and timing when its radio waves blinked out pinned down where it was. There sat an ordinary-looking star of the 13th magnitude, with a faint streak beside it.

Its spectrum made no sense until Schmidt saw that it was the familiar pattern of hydrogen, shifted almost 16% towards the red. Only the expansion of the universe could do that, so its light had been travelling for about 2 billion years. Anything that bright from so far had to be staggeringly powerful. Such objects were named quasi-stellar radio sources: quasars.

2. Brighter than a hundred Milky Ways

3C 273 shines like 4 trillion Suns, about a hundred times the whole Milky Way, and it flickers within days: all that light comes from a region only light-days across.

Around 3C 273 is a giant elliptical galaxy, but in ordinary pictures it vanishes in the glare of its centre. Only with a coronagraph, which blocks the central light as the Moon blocks the Sun in an eclipse, could Hubble see it clearly in 2024.

If 3C 273 were 30 light-years away, it would shine in our sky as brightly as the Sun. Yet its light changes within days, and nothing can brighten all at once faster than light can cross it: the source is only light-days across, about the size of our Solar System.

The galaxy is drawn from a simple model of a giant elliptical; how much brighter its centre is follows the measurements.

3. A black hole feeding

At the centre is a black hole of about 260 million Suns. Gas spiralling in is squeezed and heated in a disk until it shines brighter than all the galaxy's stars together.

Almost every large galaxy has a supermassive black hole at its heart, and most are quiet. A quasar is one that is being fed: gas from its galaxy, often stirred up by a collision with another galaxy, falls towards it faster than it can be swallowed.

The gas cannot fall straight in. Like water round a drain, it swirls into a flat disk, rubs against itself, loses energy and spirals slowly inwards, heating up as it goes. The black hole's shadow and the light bent round it are drawn as general relativity has them; the disk's streaks are drawn.

4. The fiercest light

Near the black hole the disk glows mostly in ultraviolet. Falling in turns about a tenth of the gas's mass into light: fourteen times more than nuclear fusion does in the stars.

Matter falling towards a black hole speeds up and heats up. By the time gas in the disk reaches the innermost stable orbit, a few times the size of the horizon, about a tenth of its mass has been turned into heat and light, and more if the black hole spins fast.

The Sun, fusing hydrogen into helium, turns only 0.7% of the mass it burns into energy. That is why feeding black holes are the most powerful steady lights in the universe. The side of the disk turning towards us looks brighter and bluer: gas moving at a large fraction of the speed of light beams its light forwards.

Ultraviolet is invisible; the disk is drawn in the colours of its heat.

5. Clouds on fast orbits

Farther out, clouds of gas circle at about 3,000 km/s, some 145 light-days from the centre. In 2018 an instrument called GRAVITY saw them turning, and that weighed the black hole.

Quasar light shows broad lines: each element's colours are smeared out, because the gas making them moves at thousands of kilometres a second, some towards us and some away. These clouds are lit by the disk inside them.

In 2018 GRAVITY, which joins the four 8-metre telescopes of ESO's Very Large Telescope into one, measured where the clouds' light came from across 3C 273 and saw them rotating: coming towards us on one side and going away on the other. How fast they circle at that distance gives the black hole's mass, about 260 million Suns (GRAVITY Collaboration 2018).

The clouds' colours are drawn: blue coming towards us, red going away. Their turning is sped up; one orbit really takes centuries.

6. A jet across the galaxy

Some of the gas never falls in. Flung out along the axis at almost the speed of light, it forms a jet 200,000 light-years long, twice as long as the Milky Way is wide.

Near the black hole, magnetic fields wound up by the spinning disk and hole fling a little of the gas out along the axis at nearly the speed of light. About one quasar in ten has jets strong enough to shine brightly in radio waves.

3C 273's jet shows even in ordinary Hubble pictures, a streak about 200,000 light-years long. Knots in it seem to race outwards faster than light: an illusion, because they move almost straight towards us at nearly light's speed, so their light arrives bunched up. Its twin, pointed away from us, is too faint to see.

The jet is drawn in pale blue, as it looks in Hubble's pictures.

7. The brightest of all

The brightest quasar known, J0529-4351, shines like 500 trillion Suns. Its black hole, of 17 billion Suns, swallows more than a Sun's worth of gas every day.

J0529-4351 is so bright that for decades sky surveys listed it as a star. In 2024 a team led by Christian Wolf showed with ESO's Very Large Telescope that it is a quasar whose light set out more than 12 billion years ago.

Its black hole is about 17 billion times the Sun's mass and still growing. The disk feeding it is about 7 light-years across, the largest known, and it shines like more than 500 trillion Suns (Wolf et al. 2024).

The disk is drawn to its measured size. The black hole's shadow at its middle is far too small to see at this scale.

8. The age of quasars

Quasars were most common about 10 billion years ago, when galaxies were young and full of gas. Most have since run out of food, and bright ones are rare today.

Because light takes time to reach us, the farthest quasars are also the oldest. Counting them at each distance shows that bright quasars were about a hundred times more common 10 to 11 billion years ago than they are today.

Galaxies then were rich in gas and often colliding, which drove gas into their centres. As the gas was used up in stars or blown away, most black holes went quiet. Many of today's giant galaxies carry the sleeping black holes of former quasars.

The chart gives the shape the surveys found, roughly. The quasar's fading here is drawn.

9. Our own black hole

The Milky Way's black hole, Sagittarius A*, weighs 4.3 million Suns and barely eats. But two giant bubbles of gamma rays above and below the galaxy suggest it flared up a few million years ago.

Sagittarius A* is about sixty times lighter than 3C 273's black hole and almost starved: it shines only about as much as a few hundred Suns, mostly as radio waves and X-rays.

In 2010 the Fermi space telescope found two huge lobes of gamma rays, each reaching about 25,000 light-years above and below the centre of the galaxy. Their size and shape suggest an outburst from the centre a few million years ago, perhaps when Sagittarius A* last had a big meal.

The bubbles are invisible to the eye; here they are drawn in violet. The Milky Way is drawn from a simple model.

10. Beacons that never move

Quasars are so far away that they never seem to move. Astronomers use thousands of them as the fixed points against which every position in the sky is measured.

Stars drift across the sky over the years as they move through the galaxy. Quasars, billions of light-years away, stay put, so they make the best landmarks there are.

The International Celestial Reference Frame, used to steer spacecraft and to track the Earth's turning, is built on the positions of 4,536 distant radio sources, most of them quasars, measured by radio telescopes around the world (ICRF3, 2018). The Gaia spacecraft ties its map of nearly two billion stars to about 1.6 million quasars.

The rings mark quasars as a map would; the points in them are drawn.

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