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The Sun's death

How the Sun will swell into a red giant, then fade into a white dwarf

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. The Sun today

The Sun is 4.6 billion years old, about halfway through its life. Every second its core turns some 600 million tonnes of hydrogen into helium, and the energy this frees keeps it shining.

Deep in the core, at about 15 million °C, hydrogen nuclei crash together and fuse into helium. A little mass goes missing each time and becomes energy, as Einstein's E = mc² says: about four million tonnes of it every second. That energy takes many thousands of years to work its way out to the surface we see.

No one can wait billions of years to watch a star grow old, but no one needs to. The sky is full of stars of about the Sun's mass, caught at every stage of life from newborn to dead. Computer models of how stars work, checked against all of them, tell the story that follows.

The sizes, temperatures and dates here come from one such model of the Sun, by Klaus-Peter Schröder and Robert Connon Smith, which follows it until it has shed the last of its outer layers.

2. Brighter and brighter

As helium piles up in its core, the Sun slowly brightens, by about 10% every billion years. In about a billion years that will be enough to boil Earth's oceans away.

Helium, the ash of the Sun's fire, builds up in the core. That squeezes the core tighter and heats it, so hydrogen fuses faster and the Sun shines brighter. When it was born it shone only about 70% as brightly as it does today.

The green ring is the habitable zone, where a planet like Earth could keep liquid water. As the Sun brightens, the ring moves outwards. In about a billion years its inner edge passes Earth: the air fills with water vapour, which traps more heat, and the oceans start to boil away. High in the air, sunlight splits the water apart, and its hydrogen escapes into space.

By the end of this stage, about 5.4 billion years from now, the Sun will be nearly twice as bright as today, and the habitable zone will have moved out to the orbit of Mars.

3. The core runs out

In about 5.4 billion years the core's hydrogen is used up. The core shrinks and heats up, hydrogen burns on in a shell around it, and the Sun's outer layers begin to swell.

With no fuel left in the middle, the core of helium cannot hold itself up and starts to shrink. Being squeezed heats it, and the heat sets fire to hydrogen in a thin shell around it, which burns faster than the core ever did.

The extra energy pushes the Sun's outer layers outwards. They cool as they spread, so the Sun turns from yellow-white towards orange. The dashed circle shows its size today.

Every star comes to this point once the hydrogen in its core runs out, and the heavier the star, the sooner. A star of twice the Sun's mass gets here in about a billion years; the smallest red dwarfs will shine on for trillions of years, far longer than the universe has existed so far.

4. A red giant

Over about two billion years the Sun swells slowly, then faster and faster: in its last few million years it grows from dozens of times its size today to hundreds. Its surface cools towards 2,330 °C, and it swallows Mercury.

At its biggest, the Sun will be about 356.2 million km across, 256 times as wide as today. Its surface will reach beyond the orbit Earth has now. Yet it will have lost a third of its mass, spread so thin that most of the giant is thinner than the air we breathe.

Its gravity at the surface will be about a hundred thousand times weaker than today. Boiling gas rises there in a few giant cells, each almost as wide as the distance from Earth to the Sun. In 2017 the VLT telescope in Chile saw cells like these on the red giant π¹ Gruis.

Held so weakly, the surface easily blows away. A strong wind carries off a third of the Sun's mass before it reaches its largest. With less mass pulling on them, the planets' orbits widen.

5. Earth's fate

Venus is swallowed next. Earth's orbit should widen as the Sun loses mass, but the tides Earth raises on the giant drag it back. Most likely it is swallowed too, half a million years before the Sun reaches its largest, 356.2 million km across.

Long before that, Earth will be a scorched, molten world: once the Sun is a thousand times brighter than now, the heat reaching Earth would be enough to melt rock.

A planet close to a giant star raises a bulge on it, the way the Moon raises tides in Earth's oceans. The bulge's pull slows the planet down, and it spirals inwards. In Schröder and Smith's model this beats the widening of Earth's orbit, and Earth is swallowed a million years after Venus.

Other models, with different amounts of mass loss, let Earth escape on a wider orbit. It is a close race, and the answer depends on how giants lose their mass, which is still being measured.

6. The helium flash

At the tip, the core reaches about 100 million °C and its helium starts to fuse, all at once, in a flash. Then the Sun shrinks to about 11 times its size today and burns helium for over a hundred million years.

Helium needs about 100 million °C to fuse into carbon and oxygen. By now the Sun's helium core is packed so tightly that heating does not make it expand and cool off, as a gas would. So once helium starts to fuse, it runs away: for a few seconds the core makes about as much energy as a whole galaxy.

None of it gets out. The energy goes into loosening the core, and nothing shows at the surface. Then the core burns its helium calmly, and the Sun settles down, far smaller than as a red giant, though still about 50 times brighter than today.

Stars at this stage are common. On charts of stars' colours and brightnesses they gather in a clump, or in old star clusters line up in a band that astronomers call the horizontal branch.

7. A giant again

When the core's helium runs out too, the Sun swells into a giant once more, now round a core of carbon and oxygen. Every hundred thousand years or so its burning shells flare up, and it puffs off its outer layers.

Now helium burns in a shell round the core of carbon and oxygen, and hydrogen in another shell outside it. The two take turns: the hydrogen shell piles up helium until the helium shell flares, a thermal pulse, and then things settle again.

This time the Sun grows to about 180 times its size today, smaller than as a red giant, so Mars and the planets beyond are spared. It sheds almost a fifth of the mass it has left, in a slow wind of gas and dust.

Dying giants like this are wrapped in shells of gas and dust. Round the red giant R Sculptoris, the ALMA telescope has seen a shell thrown off in one of these pulses. Much of the dust in our galaxy was made in winds like these.

8. A glowing cloud

The last of the outer layers drifts away, laying bare the core. At more than 100,000 °C, it lights up the gas it has thrown off, and for about 10,000 years the gas glows as a planetary nebula.

Planetary nebulae have nothing to do with planets: in early telescopes they looked round, like planets. Thousands are known in our galaxy, each one the last breath of a star like the Sun.

The bare core shines mostly in ultraviolet light, which tears electrons off the atoms in the gas. As the atoms catch electrons again, each element glows in its own colours: oxygen teal-green near the hot core, hydrogen and nitrogen red further out. The colours here follow that pattern, as real nebulae do.

For a long time astronomers thought a star as light as the Sun could not make a visible nebula: its core would heat up too slowly, and the gas would drift away before it was lit. Newer models, published in 2018, show that it heats up fast enough. The Sun's nebula will be a faint one, but bright enough to be seen from other galaxies.

The gas flies outwards at tens of kilometres a second. After about 10,000 years it has spread too thin to glow, and drifts off into the galaxy, where it may one day become part of new stars and planets.

9. A white dwarf

What is left is the Sun's core: a white dwarf with 54% of the Sun's mass today, squeezed into a ball about 18,000 km across, not much bigger than Earth. It makes no new energy at all.

A white dwarf is so dense that a teaspoonful of it would weigh about as much as a car. Nothing holds it up but its electrons: squeezed as close as nature allows, they resist being pressed any closer.

There is a limit to what electrons can hold. No white dwarf can be heavier than about 1.4 Suns: beyond that even they give way, and the star collapses further.

Sirius B, the companion of the brightest star in the night sky, is a white dwarf like this, about as heavy as the Sun and smaller than Earth.

10. Slowly fading

The planets that survive circle the white dwarf in orbits nearly twice as wide as today's. Over billions of years it cools and fades. No white dwarf in the universe has had time to go dark yet.

With only 54% of its mass left, the Sun holds its planets less tightly, and they have slowly moved outwards: Mars to about 419.1 million km from it, Jupiter to about 1.4 billion km.

A white dwarf shines only with the heat it has left. It cools quickly at first, then ever more slowly. The oldest white dwarfs known have been cooling for about ten billion years and still glow faintly. To cool into a cold, dark black dwarf would take far longer than the universe has existed so far.

Many white dwarfs have rocky dust falling onto them, the wreckage of their asteroids and planets. Some still have planets: in 2020 a planet the size of Jupiter was found circling the white dwarf WD 1856+534.

Objects in this story

Real photos