Neutron stars

A neutron star is the collapsed core of a massive star that exploded as a supernova: more mass than the Sun, typically about 1.4 solar masses, crushed into a sphere about 20 kilometers across. Its matter is so dense that a teaspoon would weigh about a billion tons on Earth. Pulsars and magnetars are neutron stars, and they are the densest objects known short of black holes.

Neutron stars are one of three possible endings for a star, alongside white dwarfs and black holes. For the full life cycle, see our stars guide.

How do neutron stars form?

Neutron stars form when stars of roughly 8 to 20 times the Sun’s mass run out of fuel, according to NASA. The core, now iron that cannot fuse further, collapses in a fraction of a second. Electrons are forced into protons, turning them into neutrons, and the collapse stops when the neutrons are packed as tightly as in an atomic nucleus. The rebound and a flood of neutrinos blow the rest of the star apart as a core-collapse supernova. Heavier stars tend to leave black holes instead.

Walter Baade and Fritz Zwicky predicted this in 1934, in the same papers that coined the word “super-novae”: “with all reserve”, they wrote, a supernova may represent “the transition of an ordinary star into a neutron star” (Baade and Zwicky 1934). The idea stayed speculative for more than three decades.

Pulsars: how neutron stars were found

The first neutron stars were found by accident. In 1967 Jocelyn Bell, a graduate student at Cambridge, noticed a radio source pulsing every 1.337 seconds with clock-like regularity. The discovery paper appeared in Nature in February 1968 (the discovery of pulsars), and within a year pulsars were interpreted as spinning neutron stars. Antony Hewish shared the 1974 Nobel Prize in Physics for “his decisive role in the discovery of pulsars”; Bell’s exclusion became one of the best-known credit controversies in science.

A pulsar is a neutron star whose magnetic field channels beams of radiation from its poles. As the star spins, the beams sweep across Earth like a lighthouse. The catalog maintained by CSIRO in Australia lists 4,503 pulsars as of its current version. The fastest known, PSR J1748-2446ad in the star cluster Terzan 5, spins 716 times a second (Hessels et al. 2006).

Pulsars became precision tools. The Hulse-Taylor binary pulsar gave the first evidence of gravitational waves (the Hulse-Taylor binary pulsar), and arrays of millisecond pulsars now detect a background of very long gravitational waves.

How big and how heavy is a neutron star?

Measuring a neutron star’s size is hard, because it is a 20-kilometer object thousands of light-years away. NASA’s NICER X-ray instrument on the International Space Station did it by modeling hot spots on spinning pulsars:

Neutron star	Mass (Suns)	Radius	How measured
PSR J0030+0451	About 1.34 to 1.44	About 12.7 to 13 km	NICER X-ray hot spots (two teams, 2019)
PSR J0740+6620	2.08	About 12.4 to 13.7 km	Radio timing (mass) and NICER (radius), 2021
PSR J0952-0607	2.35 (plus or minus 0.17)	Not measured	Optical study of its companion, 2022

Sources: Riley et al. and Miller et al. 2019 and 2021; Fonseca et al. 2021; Romani et al. 2022 (all The Astrophysical Journal Letters).

The surprise is that PSR J0740+6620, almost 50% heavier than PSR J0030+0451, is essentially the same size. That tells physicists how “squeezable” neutron star matter is, a property of nuclear physics that no laboratory on Earth can reach.

Is there a maximum mass?

Yes, but nobody knows exactly where it is. Above some limit, neutron pressure cannot hold the star up and it collapses into a black hole. Analyses of the GW170817 merger suggested a ceiling around 2.2 solar masses, while the 2.35-solar-mass PSR J0952-0607 implies the limit is at least about 2.2. A safe statement, as of October 2026: the maximum lies somewhere a little above 2.2 solar masses.

Neutron star vs black hole

	Neutron star	Black hole
What holds it up	Pressure of packed neutrons	Nothing: complete collapse
Surface	Yes, a solid crust	No, an event horizon
Typical mass	About 1.4 Suns; up to about 2.3 measured	From about 3 Suns up (stellar)
Size	About 20 to 25 km across	About 6 km per 1 Sun of mass (horizon diameter)
Light	Can emit radio, X-rays, gamma rays from its surface and beams	No light from inside; glow comes from surrounding gas
Can it spin and pulse?	Yes, as a pulsar	No pulses

The horizon size follows from the Schwarzschild radius, about 3 kilometers per solar mass. A neutron star is only a few times larger than a black hole of the same mass, which is why it sits just one step from collapse. More in our black holes guide.

Magnetars: the strongest magnets in the universe

A magnetar is a neutron star with a magnetic field of about 100 trillion to a quadrillion (10¹⁵) gauss, according to NASA, up to a thousand times stronger than an ordinary neutron star’s. Magnetars release sudden flares of X-rays and gamma rays as their fields rearrange. In October 2026 JWST found that the most distant fast radio burst yet came from a young host galaxy, supporting the idea that some bursts come from young magnetars (the most distant fast radio burst).

When neutron stars collide

Pairs of neutron stars slowly spiral together by emitting gravitational waves. In 2017 LIGO and Virgo caught the final collision of two of them, GW170817, and telescopes saw the explosion that followed, a kilonova that forged heavy elements such as gold (GW170817). Our explainer on neutron star mergers tells that story. Lighter stars end very differently, as Earth-sized white dwarfs.