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ASTROPHYSICS

Why are neutron stars so dense?

It all comes down to how they’re built. Matter as we know it is a bit like candy floss – it’s very fluffy and sparse. On Earth, the nuclei of atoms are separated by vast gulfs of space filled with electrons. Imagine an atom was large enough that we could hold its nucleus in our hand: if the nucleus was a penny, the electron cloud in the atom around it would be far bigger than a tennis court. Most importantly, protons and neutrons are a thousand times heavier than electrons, so even though the electrons fill most of the space in atoms, it’s actually the nuclei that give them most of their weight. This means we can make matter much denser just by squeezing nuclei together.

Neutron stars, being the imploded cores of massive stars, have effectively crushed all their nuclei together into a giant pudding of neutrons, with the occasional