What a neutron star is
When a massive star runs out of fuel, its core can collapse in a fraction of a second, and the outer layers are blown away in a supernova. If the collapsed core is not massive enough to become a black hole, it settles into a neutron star. This is a ball of matter with a mass of roughly one and a half times the Sun’s, squeezed into a sphere only about 20 kilometers (around 12 miles) across.
That makes neutron stars among the densest objects known that are not black holes. A teaspoon of neutron star material would weigh billions of tons on Earth. The gravity at the surface is enormously stronger than anything we experience, and the escape speed is a sizable fraction of the speed of light.
Why the name “neutron”
Under the crushing pressure of the collapse, electrons and protons are forced together and combine into neutrons. What is left is mostly neutrons, packed together at a density comparable to that inside an atomic nucleus. It is almost as if a whole star had become one giant nucleus, although the true interior is not fully understood.
What stops the collapse from continuing is a quantum effect. Neutrons, like electrons, cannot all occupy the same state, and the resulting “degeneracy pressure” pushes back against gravity. This is a direct consequence of quantum physics. Beyond a certain mass, thought to be somewhere around two to three times the Sun’s, even this pressure is not enough, and the object would collapse further. The exact limit depends on the poorly known behavior of matter at such densities, which is one reason neutron stars are studied so intensely.
The structure, as far as we know
Models suggest a thin outer crust of dense nuclei, possibly with a solid lattice, around a deeper interior of neutron-rich matter. The core is the least certain part. Some theories propose exotic forms of matter there, while others do not. Observations of neutron-star masses and sizes help narrow down the options, but the question is not settled.
Fast spinners and strong magnets
Two properties of neutron stars follow from how they form.
Rapid spin. Stars rotate, and when a core shrinks from many thousands of kilometers to a few kilometers across, it spins faster, much as a skater spins faster when pulling in their arms. Newborn neutron stars can rotate many times per second.
Powerful magnetic fields. The star’s magnetic field gets compressed along with its matter. Neutron stars have magnetic fields that are vastly stronger than any produced in laboratories. Compare this with ordinary magnetic fields, such as the one that makes a compass point north: those are tiny by comparison.
Pulsars: the blinking stars
In 1967, a graduate student at Cambridge named Jocelyn Bell, working with her supervisor Antony Hewish, noticed a strange signal in radio telescope data: a series of regular pulses arriving about 1.3 seconds apart. The regularity was so striking that the team jokingly considered whether it might be a signal from an intelligent source, though they quickly turned toward natural explanations. Soon more of these sources were found, and the objects became known as pulsars.
The explanation is now well established. A pulsar is a rotating neutron star whose magnetic field directs beams of radio waves, and sometimes other radiation, out from its magnetic poles. The magnetic poles are usually not aligned with the spin axis. So as the star rotates, the beams sweep around the sky like the beam of a lighthouse. If one of them passes across Earth, we see a flash once per rotation.
The star is not truly blinking. It shines steadily, but the beam points at us only briefly each turn. Many neutron stars have beams that never cross our direction, so they are invisible to us as pulsars even though they exist.
How fast do they pulse?
Pulse periods range widely:
- Many pulsars rotate about once every second or so.
- The pulsar in the Crab Nebula, left over from a supernova that people on Earth recorded in the year 1054, spins about 30 times each second.
- So-called millisecond pulsars spin hundreds of times per second. They are thought to have been spun up by drawing in material from a companion star.
Pulsars gradually slow as they lose energy, though over very long timescales, and their timing can be extremely regular. The best millisecond pulsars are steady enough that astronomers compare them to precise clocks. Because of this stability, pulsar timing can reveal tiny disturbances, including the passing of gravitational waves.
Other kinds of neutron stars
Magnetars are neutron stars with exceptionally strong magnetic fields, even by neutron-star standards. They occasionally produce powerful bursts of X-rays and gamma rays, and a few have been linked to brief radio flashes.
Binary systems include neutron stars orbiting a companion. In some, gas flows from the companion onto the neutron star and heats up, creating X-ray emission.
Mergers happen when two neutron stars spiral toward each other and collide. In 2017, gravitational waves and light from such a collision were detected together for the first time. Observations suggested that these collisions produce heavy elements, and they are considered a likely source of a good share of elements such as gold in the periodic table, although the exact contribution is still being worked out.
Why they matter
Neutron stars let scientists test physics in conditions that cannot be recreated on Earth. They test the theory of gravity in strong fields, the behavior of matter at extreme densities, and the effects of enormous magnetic fields. Much of this work depends on observations made with space telescopes in X-rays and gamma rays, which are blocked by Earth’s atmosphere, along with radio telescopes on the ground.
Pulsars are also simply a good example of how a puzzling signal can lead to a new class of objects. What first looked like an odd glitch in a chart turned out to be a stellar remnant spinning in the dark, carrying information about how stars die.