A sugar cube of neutron star weighs about as much as every human alive.
When a giant star collapses, it can crush more than the Sun's mass into a ball about the width of a city. The matter is so dense that one cubic centimetre weighs hundreds of millions of tonnes, roughly the weight of all eight billion of us.
Stars many times heavier than the Sun end their lives in a supernova. The outer layers are blasted into space, but the core collapses under its own gravity. If the core is not quite heavy enough to become a black hole, the collapse stops at something almost as extreme: a neutron star.
Gravity in the collapsing core is strong enough to force electrons into protons, turning them into neutrons. What is left is a sphere roughly 20 kilometres across, about the size of a city, holding one to two times the mass of the Sun. Its density is comparable to that of an atomic nucleus: a few hundred million tonnes per cubic centimetre. A sugar-cube-sized piece would weigh about as much as the entire human population.
Neutron stars also spin astonishingly fast, because the collapsing core keeps its angular momentum, just as a skater spins faster by pulling in their arms. Some rotate hundreds of times a second. When a neutron star's powerful magnetic field sweeps a beam of radio waves across Earth, we see it as a pulsar, ticking with a regularity that rivals atomic clocks. The first was found in 1967 by Jocelyn Bell Burnell, then a PhD student at Cambridge.
In 2017, observatories detected gravitational waves from two neutron stars spiralling into each other, followed by a flash of light across the spectrum. The collision confirmed that such mergers forge heavy elements, including much of the universe's gold and platinum.
Source: Wikipedia — Neutron star · Text summarised from Wikipedia (CC BY-SA 4.0)
