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A star torn apart

How a black hole shreds a star that strays too close, and lights up a galaxy's heart

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 8 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 comes close

At the heart of a galaxy, a star like the Sun falls towards a black hole of a million Suns. The hole's horizon is 5.9 million km across, and the star is heading inside its tidal radius, 69.6 million km from it.

Almost every big galaxy has a giant black hole at its centre. The Milky Way's, Sagittarius A*, has four million times the Sun's mass; this one has a million, typical of those caught tearing stars apart. Stars swarm round such holes on orbits of their own, and now and then two pass so close that one is flung onto a path that dives straight at the hole.

The hole itself is small for its mass. Its event horizon, the surface from inside which not even light gets out, is about four times as wide as the Sun. Its gravity bends the light of the stars behind it, which is why the sky looks warped round its black shadow.

The dashed ring is the tidal radius. Outside it, the star's own gravity holds it together. Inside it, nothing can.

2. Stretched

The hole pulls harder on the star's near side than on its far side. Near the tidal radius that difference, the tide, grows as strong as the star's own gravity, and the star is pulled long.

Tides are not special to black holes. The Moon raises them in Earth's oceans because it pulls harder on the side of Earth that faces it. Here the pull changes so steeply with distance that, across the width of a star, the difference outweighs the star's own gravity.

Astronomers call this spaghettification, because whatever falls in is drawn out long and thin. For a black hole of a million Suns, a star like the Sun reaches that point about 70 million km away: half Earth's distance from the Sun, and still far outside the horizon.

It happens fast. Moving at tens of thousands of kilometres a second, the star takes only about 18 minutes to go from the tidal radius to its closest approach.

3. Torn apart

At its closest, 34.8 million km from the hole and moving at 87,349 km/s, almost a third of the speed of light, the star comes apart. Each bit of it now flies on an orbit of its own.

Nothing explodes. The star simply stops being one thing: its gas keeps the speed it had, but each part is at a slightly different distance from the hole, so each follows a slightly different orbit. Squeezed flat as it swings past, then drawn out, the star becomes a long, thin stream.

The side that was nearer the hole moves a little too slowly to escape, and is now bound to it. The far side moves a little too fast, and will never come back. That small difference decides everything that follows.

Computer simulations show the whole star unravelling within a few hours.

4. A river of gas

Within days the stream is longer than Earth's distance from the Sun. Its bound half, nearer the hole, is slowing and will fall back; the other half is flying out for good.

The stream stays thin because its own gravity holds it together from the sides, even as it stretches to thousands of times the star's length. Its gas cools as it spreads, so the real stream is dim. Telescopes cannot see it, but simulations follow it closely.

How the gas divides is set by the energy each part carried away: the nearer the hole it lay in the star, the more tightly it is bound. The most tightly bound gas is on an orbit that takes it about 46 times Earth's distance from the Sun before it turns back.

The dashed ring is Earth's orbit, drawn to the same scale.

5. Half flies away

The unbound half leaves at up to 6,200 km/s, never to return. The bound half slows as it climbs away from the hole, stops, and begins to fall back.

The escaping gas carries half the star's mass off into the galaxy. At thousands of kilometres a second it will drift between the stars for millions of years: a thin arc of gas that was once a sun.

The bound gas is on long, thin orbits round the hole, like comets round the Sun. The gas that lay nearest the hole has the shortest orbit and comes back first; the rest follows over months and years, less and less of it at a time.

The dashed ring is Neptune's orbit, to scale. The stream soon reaches far beyond the size of our whole planetary system.

6. Coming back

After about 41 days the first gas returns and swings round the hole. Relativity turns its orbit a little each time round, so the returning gas runs into the gas still arriving.

In Newton's gravity an orbit closes on itself. Close to a black hole it does not: Einstein's theory makes each orbit swing a little further round, as Mercury's does round the Sun, only here by about 23 degrees each time instead of a hair's breadth.

So gas that has just swung round the hole flies back out along a slightly different line and crashes into the stream still falling in, some 16 times Earth's distance from the Sun away. The collision heats the gas and robs it of speed, and bit by bit it settles into a hot, swirling disk round the hole.

Just how the returning gas becomes a disk is still being worked out, with some of the largest simulations in astronomy.

7. The flare

The hot gas lights up. At its peak it shines like 9.4 billion Suns, as bright as all the stars of a galaxy together, and it glows blue-white at about 20,000 °C.

Telescopes catch these flares as a sudden brightening at the very centre of a galaxy that had been quiet. AT2019qiz, found in 2019 in a galaxy 215 million light-years away, is the nearest yet; the flare's brightness and temperature here are its measured ones, though its timing is the simplest model's.

Its visible light comes from a cloud of hot gas round the hole, tens of times wider than Earth's orbit, thrown out by the flare and lit from inside. The disk itself, further in, shines mostly in ultraviolet light and X-rays.

Near the hole, its gravity bends the disk's light, so the far side of the disk shows above and below the black shadow.

8. Fading

Less and less gas comes back each day, so the flare fades, as the chart shows. Within a few years it is all but gone, and the black hole goes quiet again.

The returning gas thins out in a way that follows from how the stream's energy was shared: the rate falls as time to the power of minus five thirds. The light of many flares fades just like that, one of the clearest signs that they really are stars being torn apart.

About half of the star falls back. Part of it is swallowed and much is blown away again by the heat. The hole gains a fraction of a Sun: barely a change for something a million times heavier.

In a galaxy like ours this happens about once every 10,000 to 100,000 years. With so many galaxies to watch, survey telescopes now find dozens a year, and the Vera C. Rubin Observatory in Chile is expected to find thousands.

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