Neutron Stars

A neutron star is the collapsed core left behind when a massive star dies in a supernova. It packs roughly one and a half times the mass of the Sun into a sphere only about 20 kilometres across — so dense that a sugar-cube-sized piece would weigh as much as a mountain. Neutron stars sit at the boundary between ordinary stellar physics and the extreme regime where gravity, nuclear forces, and quantum mechanics all matter at once.

How a neutron star forms

When a star much heavier than the Sun exhausts its nuclear fuel, its core can no longer support itself and collapses in a fraction of a second. The infalling matter crushes protons and electrons together into neutrons, and the collapse halts only when the neutrons themselves resist being squeezed further. The outer layers rebound in a supernova explosion, leaving the compact neutron core behind. If the original star is heavy enough, even neutron pressure fails and the core collapses further into a black hole instead.

What holds it up: degeneracy pressure

A neutron star does not burn fuel, so what stops it from collapsing? The answer is a purely quantum effect. The Pauli exclusion principle forbids two neutrons from occupying the same quantum state, which forces them to move even at the lowest energies. This creates a powerful outward neutron degeneracy pressure that supports the star against its own immense gravity — the same principle that holds up a white dwarf, but pushed to a far more extreme density.

Cosmic laboratories

Exactly how matter behaves at the densities inside a neutron star — described by its equation of state — is still not fully known, because no laboratory on Earth can reach such conditions. The core may contain exotic states of matter such as free quarks or hyperons. Astronomers probe this by measuring neutron-star masses and radii, and by watching them merge: the 2017 detection of gravitational waves from a neutron-star collision, seen alongside light, opened a powerful new way to study dense matter and even the origin of heavy elements like gold.

Pulsars and magnetars

Many neutron stars spin rapidly and beam radiation from their magnetic poles; as the beam sweeps past Earth we see regular pulses, and the object is called a pulsar. The most extreme, with the strongest magnetic fields, are magnetars. These spinning, magnetised remnants are among the most precise natural clocks in the universe.

A common misconception

A neutron star is not made of "neutronium" packed like ordinary matter; it is a single, gravitationally bound object more like a colossal atomic nucleus than a normal star, with a solid crust over a fluid interior. And although often called "dead" stars, neutron stars remain physically active for billions of years, cooling, spinning, and occasionally flaring.