What Is a Neutron Star?

A neutron star is what remains after a massive star runs out of fuel and explodes in a supernova. When the outer layers of the star blast away, the core gets crushed together by gravity into an incredibly dense ball. Neutron stars are only about 12 miles (20 kilometers) across, roughly the size of a city, but they contain more mass than our entire Sun. A single teaspoon of neutron star material would weigh about a billion tons on Earth. They are among the densest objects in the entire universe.

How Neutron Stars Form

Stars much larger than our Sun, at least eight times more massive, can become neutron stars at the end of their lives. When these giant stars use up all their nuclear fuel, they can no longer hold themselves up against gravity. The core collapses in less than a second, and the outer layers bounce off and explode outward in a supernova. What is left behind is a super-compressed ball made mostly of particles called neutrons. This is why scientists call them neutron stars.

Extreme Density and Gravity

Neutron stars have some of the strongest gravity in the universe. Their surface gravity is about two billion times stronger than Earth’s gravity. If you could stand on a neutron star, you would be crushed instantly because everything weighs so much more there. Light itself bends as it passes near a neutron star because gravity pulls on it. The material inside a neutron star is packed so tightly that atoms are squeezed together until only neutrons remain.

What Are Pulsars?

A pulsar is a special type of neutron star that spins very quickly and sends out beams of radio waves and light from its magnetic poles. As the pulsar spins, these beams sweep through space like a lighthouse beam sweeps across the ocean. When one of those beams points toward Earth, our telescopes detect a pulse of energy. Some pulsars spin hundreds of times every second, which is hard to imagine for an object so dense. The fastest known pulsar spins 716 times per second.

The Discovery of Pulsars

Pulsars were first discovered in 1967 by Jocelyn Bell Burnell, a graduate student at Cambridge University in England. She noticed a strange repeating signal in her radio telescope data that pulsed every 1.3 seconds. At first, the signal was so regular that some scientists jokingly wondered if it could be a message from aliens. They even nicknamed it “LGM-1,” which stood for “Little Green Men.” Soon more pulsars were found, and scientists realized these signals came from rapidly spinning neutron stars.

Magnetars: The Most Magnetic Objects

Some neutron stars have magnetic fields that are incredibly powerful, and these are called magnetars. A magnetar’s magnetic field can be a thousand trillion times stronger than Earth’s magnetic field. These extreme magnetic fields can cause starquakes on the surface of the magnetar, releasing huge bursts of energy. Magnetars are rare, with only about 30 known in our galaxy. Even from thousands of light-years away, a magnetar’s energy burst can be detected by telescopes on Earth.

Why Scientists Study Neutron Stars

Neutron stars help scientists learn about physics in extreme conditions that cannot be recreated in any laboratory on Earth. By studying how pulsars spin and send out signals, astronomers can test ideas about gravity and the nature of matter. Pulsars are such accurate timekeepers that they rival the best atomic clocks. Scientists have even used pulsars to prove that gravitational waves exist, which earned the Nobel Prize in Physics in 1993. Neutron stars also help us understand what happens to matter when it is squeezed beyond anything we can imagine.

Fun Facts About Neutron Stars

Neutron stars spin so fast when they first form because of the same principle that makes an ice skater spin faster when they pull in their arms. Some neutron stars have been found racing through space at over 600 miles per second after being kicked by their supernova explosions. If two neutron stars crash into each other, they can create heavy elements like gold and platinum. Scientists detected such a collision for the first time in 2017 using gravitational wave detectors. There are estimated to be about one billion neutron stars in our Milky Way galaxy alone.