Intermediate · 1:59

A massive star explodes

How a star many times heavier than the Sun collapses and blows itself apart

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 supergiant

Near the end of its life, a star of 18 Suns has swollen into a red supergiant 1.1 billion km across, like Betelgeuse. In place of the Sun, it would reach past the orbit of Mars.

Massive stars live fast. This one began with about 18 times the Sun's mass and burns its fuel so fiercely that it shines about 90,000 times as brightly as the Sun. Its whole life lasts only about ten million years, a thousandth of the Sun's.

Its outer layers are so swollen and thin that just a few huge cells of boiling gas cover its whole face, each many times wider than the Sun.

Betelgeuse, in Orion, is such a star, about 550 light-years away. It will explode some time in the next 100,000 years: a moment for a star, but most likely not in our lifetimes.

2. Layers like an onion

Inside, it is built in layers. Round a core of iron lie shells of silicon, oxygen and neon, carbon and helium, each the ash of the fuel burning outside it, all wrapped in a vast envelope of hydrogen.

A star shines by fusing light elements into heavier ones. When the hydrogen in its centre runs out, the core shrinks and heats up until it can burn the helium ash, making carbon and oxygen. Each time a fuel runs out the core contracts, grows hotter and starts on the ash of the last: carbon, then neon, oxygen and silicon.

Each fuel carries on burning in a thin shell just outside the core, so the star ends up layered like an onion. The layers are drawn roughly to scale: nearly all of the star's width is hydrogen, and everything else is packed into a core only about as big as the Sun.

Zooming in shows how deep they go: the iron at the centre is about a hundred thousand times narrower than the star.

3. Faster and faster

Each new fuel runs out faster than the last: hydrogen lasts about 8 million years, helium a million, carbon a thousand, neon and oxygen about a year each, and silicon only about ten days.

Heavier fuels give less energy each time they fuse, and the core must be hotter to fuse them at all. At such temperatures it also pours energy away as neutrinos, which fly straight out of the star, so it has to burn ever faster just to hold itself up.

By the time it burns silicon, at about 3 billion degrees, the core goes through its fuel in days. From outside no one could tell: the star's surface changes far too slowly to show the core racing towards its end.

These times are those of a star of about 20 Suns in Woosley, Heger and Weaver's models; a heavier star goes through them faster still.

4. An iron core

Silicon burns to iron, and iron is where fusion stops: making anything heavier takes energy instead of giving it. The iron core, a little smaller than Earth, grows to 1.4 times the Sun's mass.

Iron's nucleus is among the most tightly bound there is. Fuse it into anything heavier, or break it into anything lighter, and energy has to be put in. So iron piles up in the middle as dead ash, with nothing to burn it.

The core is held up only by its electrons, squeezed so tightly that they push back: the same pressure that holds up a white dwarf. That pressure can bear only about 1.4 times the Sun's mass, and the silicon burning round the core keeps adding iron to it.

The dashed circle is Earth, to scale. Into a ball smaller than our planet, the core packs more mass than the Sun.

5. Collapse

When the core passes the limit, it gives way. In about a quarter of a second it falls in on itself at up to 70,000 km/s, a quarter of the speed of light, from the size of a planet to that of a city.

As the core is squeezed, electrons are forced into protons, turning them into neutrons and giving off neutrinos. With fewer electrons left to push back, the collapse only speeds up. The rest of the star has no time to notice: to it, nothing has happened yet.

The core's own gravity does the work, and it sets free a staggering amount of energy: a few hundred times more than the Sun will give out in its whole ten-billion-year life, in less than a second.

In less time than a blink, the core shrinks from nearly the size of Earth, the dashed circle, to a ball a few dozen kilometres across: the neutron star to be.

6. Bounce

At the density of an atom's nucleus, the falling core stiffens and bounces, sending a shock wave out. A neutron star is born, and 10⁵⁸ neutrinos burst out of it, carrying off 99% of the energy.

Matter this dense is packed as tightly as matter can be: a teaspoonful would weigh about a billion tonnes. When the inner core reaches it, it cannot be squeezed any further, and gas still falling onto it rebounds like a ball off a wall.

Neutrinos barely touch anything: trillions from the Sun pass through your body every second. Even they are slowed by this crush, but within about ten seconds they have all escaped, carrying off almost all the energy of the collapse.

They are the only thing ever caught coming straight from a collapsing core: in 1987, detectors in Japan, the United States and Russia caught about two dozen from SN 1987A, hours before its light arrived.

7. The shock stalls, then breaks free

Within a few hundredths of a second the shock runs out of push and stalls about 150 km out. Neutrinos heating the gas behind it set it boiling, and after a few tenths of a second the shock breaks free.

The shock spends its energy breaking up the iron nuclei that fall onto it, and grinds to a halt. For decades, computer models of supernovae failed right here: in simple models the star does not explode at all.

The answer lies in the neutrinos. A few per cent of their energy is caught by the gas behind the shock, which boils and sloshes in huge bubbles and pushes the shock outwards until it breaks free. Only three-dimensional simulations on some of the largest supercomputers capture this, and in them stars now explode.

Once free, the shock sweeps out through the star's layers at thousands of kilometres a second.

8. Breakout

About a day later the shock reaches the star's surface and bursts through it in a flash of ultraviolet light lasting about half an hour. The whole star is blown apart.

Only one such flash has been caught as it began: in 2008 the Swift satellite happened to be watching a galaxy when the shock of supernova SN 2008D burst out, glowing in X-rays for about five minutes.

For a star this big the shock takes about a day to get out. SN 1987A's star was smaller, a blue supergiant, and its light began about three hours after the neutrinos.

The star's outer layers, more than ten Suns' worth of gas, are flung out at around 10,000 km/s.

9. A supernova

The debris glows as it spreads, shining like 350 million Suns for about three months. Then it fades, lit by radioactive cobalt made in the blast, which halves every 77 days.

For months this single star shines like hundreds of millions of Suns, bright enough to be seen across much of the universe.

The long, steady glow comes from the hydrogen it threw off. Heated by the blast, the gas cools as it spreads, and its light escapes from the outside in. When that is over, the light comes from nickel-56 made in the explosion, which decays into cobalt and then into iron, heating the debris.

SN 1987A, 168,000 light-years away in the Large Magellanic Cloud, was the nearest supernova seen since the telescope was invented, bright enough to see with the naked eye.

10. Star stuff

Centuries later the debris is a shell light-years across, rich in oxygen, silicon, calcium and iron made in the star. At its heart spins the neutron star.

The remnant keeps growing as it ploughs into the gas round it, and glows in X-rays where its shock heats that gas to millions of degrees. Cassiopeia A, 11,000 light-years away, is one about 340 years old and 10 light-years across; the colours here follow the elements Chandra has mapped in it.

In time the debris mixes into the galaxy's gas, from which new stars and planets form. Much of the oxygen we breathe, and some of the iron in our blood, was made in stars like this one and scattered by explosions like this.

The neutron star left behind may shine as a pulsar, like the one in the Crab Nebula, the remains of a supernova seen in 1054. A heavier star's core can collapse even further, into a black hole.

Objects in this story

Real photos