Nib asked
What happens when a star dies?
A star's mass decides its death. Most stars, including the Sun, never explode: they swell into red giants and fade into white dwarfs. Only stars over about eight Suns collapse and blow up in a supernova.
8 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1a balance
A star is not on fire. Deep in its core, fusion welds hydrogen nuclei into helium and releases heat. That heat pushes outward, while the star's own gravity pulls inward. For billions of years the two forces cancel, and the star holds its size.
A star is a balance between fusion pushing out and gravity pulling in.
a common mix-upStars do not burn like a fire; they fuse atoms.
2mass decides
How a star dies is set by its mass, measured in Suns. Stars under about eight times the Sun's mass, which is nearly all of them, die quietly. Only the heavy ones, over about eight Suns, end in an explosion.
Mass decides a star's death; most stars are light and never explode.
Does the Sun end in a supernova?
Show the answer
No. It is a light star, so it ends as a red giant and then a white dwarf.
3red giant
When its core runs out of hydrogen, a Sun-like star's outer layers swell and cool. It becomes a red giant, perhaps a hundred times wider than the Sun is now, wide enough to swallow Mercury and Venus, and possibly Earth. This is about five billion years away for our Sun.
A light star swells into a red giant when its core runs out of hydrogen.
4white dwarf
The star's outer layers drift away and glow as a planetary nebula, a name that comes from old telescopes, not from planets. What remains is the hot core: a white dwarf, about as heavy as half the Sun but only as wide as the Earth. It no longer fuses anything and simply cools for billions of years.
A light star sheds a glowing shell and leaves a white dwarf the size of Earth.
How big is a white dwarf?
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About as wide as Earth, with roughly half the Sun's mass.
5fuel to iron
A heavy star's core is hot and dense enough to keep fusing: hydrogen into helium, helium into carbon and oxygen, and on through heavier atoms, each stage faster than the last. The star ends up layered like an onion. Iron sits at the center, and iron is the dead end: fusing it takes in energy instead of releasing any, so the core has no way to hold itself up.
Heavy stars fuse up to iron, and iron fusion absorbs energy.
Why does fusion stop at iron?
Show the answer
Fusing iron absorbs energy instead of releasing it, so it cannot hold the star up.
6supernova
Without fusion's push, nothing resists gravity. The iron core, about as big as the Earth, falls in on itself in less than a second, to a ball only a few tens of kilometers wide. The material bounces off it and a shock wave rips the rest of the star outward. For weeks, the explosion can outshine a whole galaxy.
The iron core collapses in under a second and the star explodes as a supernova.
7what is left
What survives depends on the core's mass. A neutron star packs more than the Sun's mass into a ball about 20 kilometers across. If the original star was heavy enough, roughly more than 20 or 25 Suns, even that cannot hold, and the core collapses all the way into a black hole.
A supernova leaves a neutron star, or a black hole if the star was heaviest.
a common mix-upNot every dying star becomes a black hole; most become white dwarfs.
8the whole idea
A star's mass decides how it dies. A light star like the Sun swells into a red giant, sheds a glowing shell and leaves a white dwarf. A star over about eight Suns fuses all the way to iron, collapses, explodes as a supernova, and leaves a neutron star or, if it was heaviest, a black hole.
Light stars fade into white dwarfs; heavy stars explode and leave neutron stars or black holes.
Every star is a balance between heat and gravity, and when the fuel runs out, its mass alone writes the ending.