Neutron stars are the extremely dense remnants left behind after a massive star explodes in a supernova. When such a star exhausts its nuclear fuel, its core collapses under gravity. If the remaining core is between about 1.4 and 3 times the mass of the Sun, it becomes a neutron star rather than a black hole.
Despite containing up to twice the mass of the Sun, a neutron star is only about 20 kilometers (12 miles) in diameter—roughly the size of a city. This means the matter inside is so dense that a single teaspoon of neutron star material would weigh billions of tons on Earth.
The collapse causes protons and electrons to merge into neutrons, creating a star composed almost entirely of neutrons. These stars have:
- Immense gravitational pull due to their compactness,
- Rapid rotation (many spin dozens to hundreds of times per second),
- Powerful magnetic fields, often trillions of times stronger than Earth’s.
Some neutron stars emit beams of radiation and are observed as pulsars, which appear to pulse as the star spins.
Neutron stars are fascinating because they provide insights into:
- The behavior of matter at nuclear densities,
- Gravitational waves, especially when two neutron stars merge,
- The life cycle of massive stars and the extreme conditions of the universe.