Neutron stars and pulsars

What is a neutron star?

When a star more than about eight times the Sun's mass runs out of fuel, its core collapses in a supernova. If the leftover core is between roughly 1.4 and 2 or so solar masses, it becomes a neutron star.

During the collapse, gravity crushes electrons into protons, turning them into neutrons, particles with no electric charge. The result is essentially a giant atomic nucleus held together by gravity instead of nuclear forces.

If the core is even heavier, nothing can stop the collapse and it becomes a black hole. Neutron stars are the last stop before that point.

Small, heavy and extreme

A typical neutron star is only about 12 miles (20 km) across, roughly the size of a city, yet it holds more mass than the whole Sun. A sugar cube of its material would weigh about as much as all humans on Earth combined, and a teaspoon perhaps a billion tons.

Gravity at the surface is so strong that an object dropped from about three feet would hit the ground at millions of miles per hour. The surface may be extremely smooth, with "mountains" smaller than an inch tall.

Because a collapsing core keeps its spin as it shrinks, like a skater pulling in their arms, newborn neutron stars rotate very fast. Their magnetic fields also get squeezed to trillions of times Earth's.

Pulsars: cosmic lighthouses

In 1967, Jocelyn Bell Burnell, then a graduate student at Cambridge, noticed a strange, regular "bit of scruff" in her radio telescope data: pulses arriving every 1.34 seconds. The signal was so steady that the team jokingly called it LGM-1, for "little green men".

More sources soon turned up, and astronomers realized they were spinning neutron stars. Beams of radio waves pour out near the magnetic poles, and when the poles are tilted from the spin axis, the beams sweep across space. If one crosses Earth, we see a pulse with every turn.

Some pulsars spin a few times per second; "millisecond pulsars", spun up by stealing gas from a companion, turn hundreds of times per second. Their ticks are so regular that they rival atomic clocks, and astronomers use groups of them to search for slow ripples in spacetime.

Magnetars

A magnetar is a neutron star with a magnetic field roughly a thousand times stronger than an ordinary neutron star's, the strongest known anywhere. Such a field could wipe a credit card from about halfway to the Moon and distort atoms near the star.

Stress in the crust can snap and release sudden bursts of X-rays and gamma rays. In December 2004, a giant flare from a magnetar called SGR 1806−20, about 50,000 light-years away, was strong enough to measurably affect Earth's upper atmosphere.

Magnetars are also linked to some fast radio bursts, brief and very bright radio flashes. In 2020 one was traced to a magnetar inside our own galaxy.

GW170817: when two neutron stars collided

On August 17, 2017, the LIGO and Virgo detectors recorded gravitational waves, tiny ripples in space itself, from two neutron stars spiraling together about 130 million light-years away. About 1.7 seconds later, NASA's Fermi satellite caught a short burst of gamma rays from the same patch of sky.

Within hours, telescopes found a new point of light in the galaxy NGC 4993. It was a kilonova, a glow powered by radioactive material flung out in the crash, and it was watched in light from gamma rays to radio.

The spectra showed that such mergers create heavy elements, likely including gold and platinum. It was the first time a single cosmic event was studied with both gravitational waves and light, a field now called multi-messenger astronomy.

Can I see a neutron star?

Neutron stars are far too small and faint to see with your eyes or a backyard telescope as individual objects. You can, however, look at their homes.

The Crab Nebula (M1) in Taurus contains a pulsar that spins about 30 times per second; in a small telescope you see the glowing cloud, not the pulsar. Large professional telescopes can record its flashing.

Many radio astronomy clubs and university outreach programs let the public listen to pulsar signals converted into sound, which is a memorable way to hear a dead star ticking.