How stars die
One number decides how a star ends: the mass it started with. Follow the Sun to its white dwarf, see why an iron core collapses in under a second, and learn how we check any of it.
THE FIRST CRACK
Running out doesn't mean empty
Fusion only happens where it is hot and dense enough, which is the core: a small fraction of the star. When the core's hydrogen has all become helium, fusion there stops. Pressure drops, gravity squeezes the core, and squeezing heats it, just as it did before the star was born. Soon the hydrogen in a thin shell around the core is hot enough to fuse, and it burns faster than the core ever did.
The result looks upside down. The core shrinks and gets hotter while the outer layers swell enormously and cool. That swollen, cooled star is a red giant: a tiny furnace inside a huge, thin, red envelope.
The rest of the Sun's life
- Now · roughly halfway
The Sun is 4.6 billion years old with about 5 billion years of core hydrogen left. It is brightening very slowly as it goes.
- In about 5 billion years · red giant
Core hydrogen is gone, shell burning takes over and the Sun swells to more than a hundred times its present width. Its surface cools to red. Mercury and Venus end up inside it.
- Helium catches
The squeezed core passes roughly 100 million °C and helium starts fusing into carbon and oxygen. This buys about a hundred million years, not billions. Helium is a much poorer fuel than hydrogen.
- The envelope drifts off
The Sun does not have the mass to squeeze its core hot enough for the next step. The outer layers float away as a glowing shell called a planetary nebula, a misleading name from early telescopes that showed a small round disc.
- A white dwarf is left
The bare carbon and oxygen core stays behind: about the size of the Earth, white hot, with no fusion in it at all. From here it only cools, for billions of years.
Check yourself
Negar reads that the Sun will swell up when it 'runs out of fuel', and also that most of the Sun's hydrogen will still be there when it happens. How do both go together?
- Fusion only runs in the hot core, and it is the core's hydrogen that runs out
- The Sun blows the rest of its hydrogen into space before it swells
- The leftover hydrogen is a form that cannot fuse
- Red giants run on helium from the very beginning
Show the answer
Fusion only runs in the hot core, and it is the core's hydrogen that runs out
Right. Only the innermost part of the Sun is hot and dense enough to fuse anything. When that small core turns to helium the engine stops there, even though most of the star is still hydrogen sitting in the wrong place: too cool.
Two endings, decided by mass
Up to about 8 Suns
The star becomes a red giant, drifts its outer layers away and leaves a white dwarf: an Earth-sized ball of carbon and oxygen that will never fuse again. There is a ceiling on it. No white dwarf can be heavier than about 1.4 Suns. Our Sun ends here, leaving a white dwarf of roughly half its present mass.
More than about 8 Suns
Gravity is strong enough to squeeze the core hotter at every stage, so the star fuses carbon, then oxygen, then heavier things, in shells like an onion, up to iron. Then the core collapses in less than a second and the star tears itself apart as a supernova. A neutron star or a black hole is left behind.
Check yourself
A white dwarf goes on shining for billions of years even though no fusion at all happens inside it.
Show the answer
True
True. Nothing is burning in there. It glows only because it is still extremely hot, like an iron bar pulled out of a fire. What makes it different from the bar is that it is tiny and dense, so it loses that heat very slowly: the oldest white dwarfs have been cooling since before the Sun existed and are still warm.
THE LAST STEP
Iron: where fusion stops paying
Every fusion step so far released energy. That stops at iron. Iron nuclei are the most tightly bound of all, so fusing iron into anything heavier takes energy in rather than giving it out. A massive star therefore builds an iron core it cannot burn. The core grows past about 1.4 Suns, pressure loses, and a ball roughly the size of the Earth collapses to about 20 km across in under a second.
The inner core slams to a halt when it can be squeezed no further. It rebounds, and a shock wave plus a flood of particles called neutrinos blows the rest of the star outward. For a few weeks the explosion can outshine every other star in its galaxy put together.
the Sun today diameter 1,400,000 km
a white dwarf diameter about 12,000 km (an Earth)
a neutron star diameter about 20 km (a city)
how much smaller across is the white dwarf?
1,400,000 / 12,000about 120 times smaller across, with a comparable mass inside itSqueeze a Sun's worth of mass into an Earth-sized ball and a sugar cube of it weighs more than a tonne. Squeeze a bit more than a Sun into 20 km and that same sugar cube weighs hundreds of millions of tonnes. Lesson 07 goes there.
Check yourself
What does each star leave behind at the end?
- The Sun
- A star of 25 Suns
- A star of 3 Suns
- A star of 12 Suns
- A red dwarf, far in the future
- A blue-white star of 20 Suns
Show the answer
A white dwarf, quietly: The Sun, A star of 3 Suns, A red dwarf, far in the future
A supernova, then a neutron star or black hole: A star of 25 Suns, A star of 12 Suns, A blue-white star of 20 Suns
Check yourself
- Star A starts life with 3 times the Sun's mass. It swells, lets its outer layers drift off, and leaves a hot ball the size of the Earth.
- Star B starts life with 20 times the Sun's mass. It explodes, and a ball 20 km across is left spinning in the middle of the wreckage.
What decides which of these two endings a star gets?
- How close it is to other stars
- The mass it started with
- How much hydrogen is left in its outer layers
- How old the galaxy was when it formed
Show the answer
The mass it started with
Yes. Below roughly 8 solar masses the star ends quietly as a white dwarf. Above that, gravity can squeeze the core hot enough to fuse its way up to iron, and an iron core cannot hold itself up.
Three things to hold on to
- The core is the whole story. A star dies when its core can no longer make heat fast enough to hold itself up, not when the star is out of hydrogen.
- Mass decides. Under about 8 Suns: red giant, then a white dwarf. Over it: supernova, then a neutron star or a black hole.
- The lines are fuzzy, and honestly so. Roughly 8 solar masses for a supernova, and somewhere around 20 to 25 for a black hole rather than a neutron star, are today's best numbers. How much mass a star sheds on the way changes the answer, and that part is still being worked out.
Check yourself
Sara asks what happens to the Earth when the Sun becomes a red giant in about 5 billion years. Which answer is the most honest?
- It collapses straight into a black hole and pulls the planets in
- The Sun explodes as a supernova, and the blast destroys every planet in the solar system, the Earth included
- Nothing much happens to the Earth; the Sun only changes colour
- Mercury and Venus end up inside the Sun, the Earth is scorched bare, and its own fate is still argued
Show the answer
Mercury and Venus end up inside the Sun, the Earth is scorched bare, and its own fate is still argued
Right. The Sun is far too light to explode or to make a black hole. It swells, and the inner planets are then inside it. Earth's own fate is a real open question, because a red giant sheds mass and the planets drift outward as it does.
Lesson recap
- A star dies when its core stops making heat, not when the star runs out of hydrogen. Only the core's supply counts.
- The Sun has about 5 billion years left. It will swell into a red giant, shed its outer layers and leave an Earth-sized white dwarf.
- A white dwarf does no fusion at all; it is a hot ember cooling for billions of years, and it cannot be heavier than about 1.4 Suns.
- Above roughly 8 solar masses a star builds an iron core, which collapses in under a second and blows the star apart as a supernova.
- What is left in the middle is a neutron star or a black hole, and again it is mass that decides.