Neutron Star
A neutron star is the collapsed core left by a supernova, packing more mass than the Sun into a sphere about 20 kilometers across.
Definition
A neutron star is the crushed leftover core of a giant star that exploded as a supernova: only about $20$ kilometers wide, the size of a city, yet heavier than the Sun. Gravity has squeezed its protons and electrons together into neutrons, packing about $1.4$ solar masses into that tiny sphere, and because it kept the original star's spin in a much smaller body, a young neutron star can rotate hundreds of times per second. It is supported by neutron degeneracy pressure and the repulsive nuclear force, with central densities exceeding nuclear density, around $10^{17}$ to $10^{18}\ \text{kg/m}^3$. Masses range from about $1.2$ to a maximum near $2$ to $2.5$ solar masses, above which collapse to a black hole is unavoidable, and the conserved angular momentum and magnetic flux produce millisecond rotation and fields of $10^8$ to $10^{15}\ \text{T}$.
Example
A sugar-cube-sized piece of neutron star would weigh about a billion tons on Earth, more than a mountain. The pulsar in the Crab Nebula spins about $30$ times per second, sweeping a beam of radiation past Earth on each turn like a lighthouse; the first pulsar was discovered in 1967 by Jocelyn Bell Burnell. If the Sun's core collapsed from a radius of $10^5\ \text{km}$ to $10\ \text{km}$ while conserving angular momentum, its rotation rate would rise by a factor of $(10^5/10)^2 = 10^8$, turning a once-a-month spin into hundreds of turns per second.
Key Insight
A neutron star is like a giant atomic nucleus held together by gravity instead of the nuclear force, with surface gravity about $100$ billion times Earth's. Pulsars are so regular that they serve as precise clocks. The 2017 detection of gravitational waves from two merging neutron stars (GW170817), followed by light from the same event, confirmed that such mergers forge heavy elements like gold and platinum, so neutron stars are element factories, not just remnants.