Stellar Explosions & Extreme Stars
Neutron Stars
A neutron star is the collapsed core of a massive star, packing more than the Sun's mass into a ball about the size of a city.
They are the densest stars known, and they appear as pulsars, magnetars and merger sources.
- Observe
Some neutron stars pulse in radio, X-ray or gamma-ray light; others are seen in mergers.
- Measure
Pulse timing, X-ray measurements of size, and gravitational-wave signals from mergers.
- Model
Nuclear-physics models predict how mass and radius are related.
- Explain
A collapsed star core is a giant atomic nucleus with extreme gravity and magnetism.
What is it?
When a massive star's core collapses in a supernova, what remains can be a neutron star: an object with roughly 1.2 to 2 times the Sun's mass squeezed into a sphere only about 20 kilometers across. A teaspoon of neutron-star material would weigh billions of tonnes.
Gravity is held off by neutron degeneracy pressure, a quantum effect that stops neutrons from being packed any tighter. Exactly what matter is like in the deepest interior is unknown, and measuring neutron-star sizes and masses is one of the main ways scientists test it.
How does it happen?
- Stage 1Massive star exhausts its fuel
- Stage 2Core collapses under gravity
- Stage 3Neutron degeneracy halts the collapseSupernova blows off outer layers
- Stage 4Neutron star remainsSpinning, magnetized, cooling
What do we actually observe?
- Pulses
- A rotating neutron star with beams pointed at Earth appears as a pulsar.
- Magnetism
- Ordinary neutron stars carry magnetic fields trillions of times stronger than Earth's; magnetars are stronger still.
- Mergers
- Two neutron stars in orbit lose energy and eventually merge, producing gravitational waves and, in the observed case, a kilonova.
- Size
- X-ray timing of hot spots, as done by NASA's NICER instrument, is used to estimate radii.
How do scientists detect it?
- Radio telescopes. Find pulsars through their regular pulses.
- X-ray observatories. NICER, Chandra and XMM-Newton study heat and pulses from the surface.
- Gravitational-wave detectors. LIGO and Virgo detect signals from neutron-star mergers.
Why does it matter?
- They are natural laboratories for matter at densities beyond anything on Earth.
- They help test general relativity in strong gravity.
- Their mergers are candidate sites for making heavy elements.
The SpaceWeatherTracker connection
Earth's magnetic field is about half a gauss. A typical neutron star's is around a trillion times stronger, a reminder of how far magnetism can be pushed beyond anything in space weather.
This is an explainer. SpaceWeatherTracker does not show live data for this phenomenon; see the “Space Phenomena Now” section on the hub for what is currently monitored.
Related phenomena
Primary family: Stellar Explosions & Extreme Stars
Neutron Stars: common questions
What is a neutron star?
The extremely dense, compact remnant left when a massive star's core collapses in a supernova, made mostly of neutrons.
How dense is a neutron star?
Its matter is roughly as dense as an atomic nucleus. A teaspoon would weigh billions of tonnes on Earth.
What is the difference between a neutron star and a black hole?
A neutron star has a solid surface and is supported by neutron pressure. A black hole has collapsed so far that not even light escapes. The heaviest neutron stars sit near the limit where collapse into a black hole would occur.