What Is a Neutron Star? A Sun Squeezed Into a City
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When a star much heavier than the Sun runs out of fuel, its core collapses in less than a second and the outer layers blow off in a supernova. What is left behind, if the core was not heavy enough to become a black hole, is a neutron star: an object with about one and a half times the mass of the Sun crushed into a sphere the size of a city, spinning up to hundreds of times a second, with a magnetic field a trillion times stronger than Earth's. A teaspoon of its material would weigh about a billion tonnes.
How the core collapses
A massive star spends its life fusing lighter elements into heavier ones until its core is iron, which cannot be fused for energy. With nothing holding it up, the core, already the size of the Earth and heavier than the Sun, collapses under its own gravity at a quarter of the speed of light. The pressure becomes so great that electrons are forced into protons, converting them to neutrons and releasing a flood of neutrinos, and the collapse halts only when the neutrons are packed as tightly as they are inside an atomic nucleus.
The rebound from that sudden halt, together with the neutrinos, drives the explosion that destroys the rest of the star. The core has shrunk from thousands of kilometres across to about twenty, and its spin has increased enormously in the process, in the way a skater speeds up by pulling in her arms. The magnetic field is compressed along with the matter and becomes far stronger than anything that can be made on Earth.
Matter at its limit
Ordinary matter is mostly empty space; the nucleus of an atom is a tiny speck inside a cloud of electrons. In a neutron star that space has been squeezed out. The outer crust is a lattice of iron nuclei; deeper down the nuclei are crushed together and drip neutrons; the interior is a fluid of neutrons, with a few protons and electrons, denser than an atomic nucleus. What lies at the very centre is not known and is one of the open questions in physics: possibly exotic particles, possibly a soup of free quarks.
The density means the gravity at the surface is around a hundred billion times that of Earth. A marshmallow dropped from a height would hit the surface with the energy of a nuclear bomb, and light leaving the star is bent so strongly that an observer could see part of the far side. Neutron stars are the densest objects that are still objects; squeeze one further, past about two or three solar masses, and it becomes a black hole.
Pulsars: the lighthouses
In 1967 a Cambridge graduate student, Jocelyn Bell Burnell, noticed a radio signal pulsing every 1.337 seconds with a regularity no natural object was thought capable of; the first working name for it was LGM-1, for little green men. It was a neutron star. The magnetic poles of a neutron star fire beams of radio waves into space, and because the magnetic axis is usually tilted from the spin axis, the beams sweep round like a lighthouse, flashing past Earth once per rotation. Over three thousand pulsars are now known.
Some spin astonishingly fast. The millisecond pulsars, spun up by matter falling onto them from a companion star, rotate several hundred times a second, so that a point on their equator moves at a sizeable fraction of the speed of light, and their pulses arrive with a regularity that rivals atomic clocks. Astronomers use arrays of them as a galaxy-sized detector for gravitational waves, watching for the tiny irregularities that passing ripples in space-time would cause.
Magnetars and collisions
A minority of neutron stars, the magnetars, have magnetic fields a thousand times stronger still, so strong that they would wipe a credit card from a distance of halfway to the Moon and distort the very atoms in their crust. When the crust cracks under the magnetic strain, the star releases a burst of gamma rays; in 2004 a magnetar on the far side of the galaxy briefly outshone every other source in the sky and measurably compressed Earth's upper atmosphere.
Neutron stars also collide. In August 2017 gravitational-wave detectors caught two of them spiralling together 130 million light years away, and telescopes around the world watched the aftermath: a burst of gamma rays and a glowing cloud of debris whose spectrum showed freshly made heavy elements. The event settled a long argument by showing that neutron star mergers forge much of the universe's gold, platinum and uranium. The gold in a wedding ring was most likely made in one. A neutron star at a glance:
- •Mass: about 1.4 times the Sun, up to a limit near 2 to 3 solar masses
- •Diameter: roughly 20 kilometres
- •Density: comparable to an atomic nucleus, about a billion tonnes per teaspoon
- •Spin: from once every few seconds to over 700 times a second
- •Magnetic field: a trillion times Earth's, a quadrillion times for magnetars
Weighing a star you cannot see
A neutron star is too small and far away for any telescope to show its surface, so its size and mass have to be worked out indirectly. Mass is easiest when the star has a partner. Many pulsars orbit another star, and as the pulsar moves, its pulses arrive a little early or late depending on which part of the orbit it is in. Timing those shifts precisely reveals the shape and size of the orbit, and from that, using the laws of gravity, the masses of both objects. Some pulsars are heavy enough to rule out many proposed theories of the matter inside.
The most famous example was discovered in 1974 by Russell Hulse and Joseph Taylor: two neutron stars orbiting each other, one of them a pulsar. Over the following years its orbit was seen to shrink at exactly the rate Einstein's theory predicted if the pair were losing energy as gravitational waves, the first strong evidence that such waves exist, which earned the pair a Nobel Prize in 1993. Measuring the radius is harder. An X-ray instrument on the International Space Station, NICER, does it by tracking hot spots on a spinning neutron star and seeing how strongly gravity bends their light.
Predicted thirty years before it was seen
The neutron itself was discovered by James Chadwick in 1932, and within two years astronomers had imagined a star made of them. In 1934 Walter Baade and Fritz Zwicky proposed that the brilliant stellar explosions they called super-novae marked the moment an ordinary star collapsed into a tiny, extremely dense ball of neutrons, an idea remarkably close to the modern picture. In 1939 J. Robert Oppenheimer and George Volkoff calculated that such a star could only be so heavy before gravity overwhelmed it, an early estimate of the limit beyond which a black hole forms instead.
Then the subject went quiet for three decades, because a cooling object twenty kilometres across seemed impossible to observe at such distances. Pulsars changed that. In 1968, the year after the first pulsar was found, astronomers detected one flashing about thirty times a second at the heart of the Crab Nebula, the debris of a supernova recorded in 1054, and another inside the Vela supernova remnant. Nothing but a neutron star could spin that fast without flying apart, and finding them inside the wreckage of exploded stars confirmed what Baade and Zwicky had guessed: supernovae make neutron stars.
Using pulsars to listen for gravitational waves
Some pulsars spin with extraordinary regularity, sending pulses that arrive on Earth with a precision rivalling atomic clocks. Astronomers have turned groups of these stars into a detector the size of the galaxy, called a pulsar timing array. If a long gravitational wave passes through, it should very slightly stretch and squeeze the space between Earth and the pulsars, shifting the arrival times of their pulses in a recognisable pattern.
After more than fifteen years of careful timing, several international collaborations announced in 2023 evidence for a background of very low-frequency gravitational waves, most likely produced by pairs of supermassive black holes orbiting each other across the universe. It was a striking use of dead stars, the leftovers of supernovae, as some of the most precise instruments in science.
The takeaway
A neutron star is the collapsed core of a massive star, more than the Sun's mass packed into twenty kilometres of matter as dense as an atomic nucleus. Its beams make the pulsars that flash across the sky, its extreme cousins the magnetars unleash the brightest bursts in the galaxy, and when two of them collide they make the heaviest elements there are.