A neutron star is what remains when a massive star’s core collapses in a supernova. It packs more mass than the Sun into a sphere only about 20 kilometers (12 miles) across, roughly the size of a city.
How extreme are they?
- Density: a teaspoon of neutron-star material would weigh on the order of a billion tons on Earth.
- Gravity: hundreds of billions of times stronger than Earth’s surface gravity.
- Spin: newborn neutron stars can rotate many times per second.
- Magnetic fields: typically a trillion times stronger than Earth’s, and far more in magnetars.
The core is thought to be made mostly of neutrons, though what happens at the very center is still an open question in physics.
Pulsars
A pulsar is a spinning neutron star that sweeps beams of radio waves (and sometimes X-rays or gamma rays) across space like a lighthouse. Every time a beam crosses Earth, we detect a pulse. The first pulsar was discovered in 1967 by Jocelyn Bell Burnell, then a graduate student, working with Antony Hewish. The regular signal was briefly nicknamed “LGM-1” for “little green men”.
Pulsars, magnetars and other kinds
- Radio pulsars: the classic type, spinning down slowly over millions of years.
- Millisecond pulsars: old neutron stars “spun up” by taking material from a companion star, rotating hundreds of times per second. They are among the most precise natural clocks known and are used to search for gravitational waves, see how gravitational waves are detected.
- Magnetars: neutron stars with extremely strong magnetic fields that release powerful bursts of X-rays and gamma rays.
- X-ray binaries: neutron stars pulling gas from a companion; the hot gas shines in X-rays.
- Neutron stars that do not pulse toward us: their beams simply miss Earth, or they are isolated and cooling.
Collisions
When two neutron stars merge, they produce gravitational waves and a “kilonova” explosion. The 2017 event GW170817 was seen both in gravitational waves and across the electromagnetic spectrum, and confirmed that such mergers create heavy elements such as gold and platinum.
Neutron star or black hole?
Above roughly two to three solar masses, a collapsed core cannot resist gravity and becomes a black hole instead, see how black holes form. Lighter dead stars become white dwarfs, see white dwarfs.
