How dense is a neutron star?

A typical neutron star packs about 1.4 solar masses (2.8 × 10³⁰ kg) into a sphere roughly 20 km across. Its density is around 4 × 10¹⁷ kg/m³ — comparable to the density of an atomic nucleus. A teaspoon of neutron-star material would weigh about a billion tonnes on Earth. At this density, the pressure forces electrons and protons to combine into neutrons, hence the name. The NASA Chandra neutron star resource gives further context.

What is a pulsar and who discovered them?

Pulsars are rapidly rotating neutron stars that emit beams of radio waves from their magnetic poles. Because the magnetic axis is tilted from the rotation axis, the beam sweeps across the sky like a lighthouse — and when it points toward Earth we detect a regular pulse. The first pulsar was discovered in 1967 by Jocelyn Bell Burnell and Antony Hewish at Cambridge; the signal was so regular it was briefly nicknamed LGM-1 (Little Green Men). Bell Burnell's BBC interview recounts the discovery. The ATNF Pulsar Catalogue lists over 3,300 known pulsars.

A magnetar is a neutron star with an exceptionally powerful magnetic field — up to 10¹⁵ gauss, roughly a trillion times Earth's field. They are the most powerful magnets known to exist. When a magnetar's crust cracks under magnetic stress, it releases enormous bursts of gamma rays and X-rays. On 27 December 2004, a magnetar burst from SGR 1806-20 (about 50,000 light-years away) was briefly bright enough to partially ionise Earth's upper atmosphere despite the distance. NASA's Imagine the Universe covers magnetar physics.

Can two neutron stars merge?

Yes — and it is one of the most violent events in the universe. When two neutron stars orbit each other, they lose energy to gravitational waves and spiral inward. The merger produces a kilonova — an explosion rich in heavy elements including gold and platinum. On 17 August 2017, the LIGO-Virgo network detected gravitational waves from a neutron star merger (GW170817) and 70 telescopes observed the resulting kilonova, confirming that neutron star mergers are the primary source of heavy elements heavier than iron. This was the first multi-messenger astronomical event. LIGO's official announcement details the discovery.

Neutron Stars & Pulsars — Guide

From supernova to neutron star

When a star more than about 8 solar masses exhausts its nuclear fuel, its iron core collapses in less than a second. The outer layers rebound off the stiffened core and are blasted away in a supernova explosion. If the remaining core mass is between roughly 1.4 and 3 solar masses, neutron degeneracy pressure halts the collapse and a neutron star is born. Above ~3 solar masses, not even neutron pressure can resist gravity, and a black hole forms instead.

Newly formed neutron stars spin extremely fast due to conservation of angular momentum — the same physics that makes a spinning ice skater speed up when they pull in their arms. A star 500,000 km across collapses to 20 km, spinning up from once per month to potentially hundreds of revolutions per second. Over millions of years, magnetic braking gradually slows the spin.

Pulsars: cosmic clocks

About 3,300 pulsars are known. The fastest, millisecond pulsars (MSPs), spin up to 716 times per second — thought to be old pulsars that have been spun up again by accreting matter from a binary companion. MSPs are so stable in their timing that arrays of them (pulsar timing arrays, or PTAs) are used as gravitational wave detectors, sensitive to the very low-frequency gravitational wave background. In 2023, the NANOGrav collaboration announced strong evidence for a gravitational wave background using a 15-year dataset from 67 millisecond pulsars.

Kilonovae and heavy elements

Neutron star mergers create kilonovae — electromagnetic transients powered by the radioactive decay of freshly synthesised heavy elements. The 2017 event GW170817 produced an estimated 5 Earth masses of gold and 500 Earth masses of strontium in just a few seconds. These events are now understood to be the primary cosmic source of gold, platinum, uranium, and other elements heavier than iron. For the full story of how stars make elements, see the stellar life cycle guide.

The ESA XMM-Newton and NASA Chandra X-ray observatories study neutron stars routinely, revealing their structure through X-ray timing and spectroscopy.

Neutron stars & pulsars

From supernova to neutron star

Pulsars: cosmic clocks

Kilonovae and heavy elements

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