Matter at the Extreme
When a massive star collapses, its core can become a neutron star: a city-sized object containing more mass than the Sun.
Neutron stars are the compact remnants of some massive stars. After a supernova, the star's core collapses so intensely that protons and electrons are forced together, producing matter dominated by neutrons.
A collapse measured in seconds
The collapse happens extraordinarily quickly. As the core contracts, conservation of angular momentum can make the remnant rotate many times each second. Its magnetic field can also become enormously stronger as the stellar material is compressed.
Pulsars: cosmic clocks
Some neutron stars emit beams of electromagnetic radiation from near their magnetic poles. If the beams sweep across Earth as the star rotates, telescopes detect regular pulses. These pulsars can be exceptionally stable, making them useful as natural clocks and probes of interstellar space.
When neutron stars collide
Binary neutron-star systems can lose orbital energy through gravitational radiation and eventually merge. Such events produce gravitational waves and can also generate a powerful burst of electromagnetic radiation. The violent environment can create heavy elements, including some elements found on Earth.
A laboratory we cannot build
The pressure and density inside a neutron star are far beyond anything reproducible in an ordinary laboratory. Astronomers therefore use observations of masses, radii, spin and gravitational waves to constrain models of ultra-dense matter.
Neutron stars occupy a remarkable boundary in physics. They are small enough to behave like compact objects, massive enough to distort spacetime strongly, and dense enough to test theories of matter under conditions found nowhere else nearby.