A city that weighs a sun
What Is Left After the Explosion?
The crushed core survives the blast — a ball the width of a city holding more material than the Sun.
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Step 1 of 5
The explosion throws away the outside and keeps the middle.
A supernova blows off the star's outer layers, but the collapsed core is not blown away. It stays where it was, crushed far past anything ordinary matter can survive.
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Step 2 of 5
Squeezed that hard, atoms stop being atoms.
An atom is mostly empty space. Under this much pressure that space is closed up entirely, and what is left is packed solid — which is where the name comes from.
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Step 3 of 5
The density is genuinely hard to picture.
A single teaspoon of it would weigh about as much as a mountain. The whole object is roughly the width of a city and outweighs the entire Sun.
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Step 4 of 5
Losing width makes it spin fantastically fast.
The old core was already turning. Shrink a spinning thing and it speeds up, the way a skater pulls their arms in — so some of these spin hundreds of times a second.
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Step 5 of 5
Squeeze harder still and you get the other ending.
A neutron star is the last stable stop. If the leftover core is heavy enough that even this cannot hold it up, nothing can, and it keeps collapsing into a black hole.
The first one found was mistaken for a signal from another civilisation, and nicknamed LGM-1 for Little Green Men.
The short version
A supernova leaves its crushed core behind as a neutron star: city-sized, heavier than the Sun, and spinning hundreds of times a second.
Try it yourself
Tuck your arms in while spinning on an office chair. That speed-up is the same rule, on a much gentler scale.