Five extreme facts about neutron stars (2023)

When a massive star dies, ejecting most of its innards across the universe in a supernova explosion, its iron heart, the stellar core, collapses to form the densest observable form of matter in the universe: a neutron star.

A neutron star is basically a giant nucleus, says Mark Alford, a professor at the University of Washington.

"Imagine a little lead ball surrounded by cotton candy," says Alford. "This is an atom. All the mass is in a little lead ball in the middle, and around it is a big fluffy cloud of electrons, like cotton candy."

In neutron stars, all the atoms have collapsed. All the electron clouds have been absorbed and everything becomes one entity, with electrons racing side by side with protons and neutrons in the gas or fluid.

Neutron stars are quite small as far as stellar objects are concerned. While scientists are still working to determine their exact diameter, they estimate they are about 12 to 17 miles across, about the length of Manhattan. Still, they are about 1.5 times the mass of our sun.

If a neutron star were any bit denser, it would collapse into a black hole and disappear, says Alford. "This is the penultimate stop on the line."

These extreme objects offer intriguing test cases that could help physicists understand fundamental forces, general relativity, and the early universe. Here are some fascinating facts for you to familiarize yourself with:

1. Already in the first few seconds after the transformation of a star into a neutron star begins, the energy released by neutrinos is equal to the total amount of light emitted by all stars in the observable universe.

Ordinary matter contains approximately the same number of protons and neutrons. But most of the protons in a neutron star become neutrons: Neutron stars are about 95 percent neutrons. When protons turn into neutrons, they release ubiquitous particles called neutrinos.

Neutron stars are created in supernova explosions, which are giant neutrino factories. A supernova emits 10 times more neutrinos than particles, protons, neutrons and electrons in the Sun.

2. It has been speculated that if life existed in neutron stars, it would be two-dimensional.

Neutron stars have some of the strongest gravitational and magnetic fields in the universe. Gravity is strong enough to flatten almost anything on the surface. The magnetic fields of neutron stars can be trillions to trillions of times greater than the magnetic field on Earth's surface.

"Everything about neutron stars is extreme," says James Lattimer, a professor at Stony Brook University. "It gets to the point where it's almost ridiculous."

Because they are so dense, neutron stars provide an excellent testing ground for the strong interaction, allowing scientists to study how quarks and gluons interact under these conditions. Many theories predict that the core of a neutron star squeezes neutrons and protons, releasing their constituent quarks. Scientists have created a hotter version of this "quark material" released at the Relativistic Heavy Ion Collider and the Large Hadron Collider.

The intense gravity of neutron stars requires scientists to use general relativity to describe the physical properties of neutron stars. In fact, the measurements of neutron stars provide us with some of the most accurate evidence of general relativity that we currently have.

Despite their incredible density and extreme gravity, neutron stars still manage to retain a surprising amount of internal structure, encompassing crusts, oceans and atmospheres. "They're a strange mix of some of the star's mass with other properties of the planet," says Chuck Horowitz, a professor at Indiana University.

But whereas here on Earth, we're used to having an atmosphere that extends hundreds of miles into the sky, because a neutron star's gravity is so extreme that its atmosphere can extend for less than a foot.

3. The fastest rotating neutron star rotates about 700 times per second.

Scientists believe that most neutron stars are now or once were pulsars, stars that shoot out beams of radio waves as they spin rapidly. If the pulsar is pointed towards our planet, we see these rays sweep across the Earth like the light from a lighthouse.

Scientists first observed neutron stars in 1967, when graduate student Jocelyn Bell noticed repeated radio pulses coming from a pulsar outside our solar system. (The 1974 Nobel Prize in Physics went to his promoter, Anthony Hewish, for this discovery.)

Pulsars can rotate tens to hundreds of times per second. If it were at the equator of the fastest known pulsar, it would rotate at about 1/10 the speed of light.

The 1993 Nobel Prize in Physics went to scientists who measured the rate at which a pair of neutron stars spiraling around each other rotated together as a result of the emission of gravitational radiation, a phenomenon predicted by Albert Einstein's theory of general relativity.

Scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced in 2016 that they had directly detected gravitational waves for the first time. In the future, it may be possible to use pulsars as giant, scaled-up versions of the LIGO experiment, trying to detect tiny changes in the distance between pulsars and Earth as a gravitational wave passes.

4. The wrong kind of neutron star can wreak havoc on Earth.

Neutron stars can be dangerous because of their strong fields. If a neutron star entered our solar system, it could wreak havoc, disrupting planetary orbits, and if it got close enough, even trigger tidal waves that would tear the planet apart.

But the closest known neutron star is about 500 light years away. And given that Proxima Centauri, the closest star to Earth just over 4 light years away, has no connection to our planet, it's unlikely we'll feel these catastrophic effects any time soon.

Probably even more dangerous would be the radiation from the neutron star's magnetic field. Magnetars are neutron stars whose magnetic fields are a thousand times stronger than the extremely strong fields of "normal" pulsars. Sudden changes in these fields can cause flares similar to solar flares, but much more powerful.

On December 27, 2004, scientists observed a giant gamma-ray burst from Magnetar SGR 1806-20, about 50,000 light years away. In 0.2 seconds, the flare radiated as much energy as the Sun produces in 300,000 years. The flare saturated many spacecraft detectors and caused detectable disturbances in Earth's ionosphere.

Fortunately, we are not aware of any nearby magnetars powerful enough to cause any damage.

5. Despite the extremes of neutron stars, scientists still have ways to study them.

There's a lot we don't know about neutron stars, including how many there are, says Horowitz. “We know about 2,000 neutron stars in our galaxy, but we expect there will be billions more. So most neutron stars, even in our own galaxy, are completely unknown."

Many radio, X-ray and optical telescopes are used to study the properties of neutron stars. NASA's upcoming Neutron Star Interior Composition ExploreR (NICER) mission, due to join the International Space Station's wing in 2017, is a mission dedicated to understanding these extreme objects. NICER will look at the X-rays of rotating neutron stars to try to determine their mass and radius more precisely.

We could also study neutron stars by detecting gravitational waves. LIGO scientists hope to detect gravitational waves produced by the merger of two neutron stars. Studying these gravitational waves could give scientists clues about the properties of the extremely dense matter that makes up neutron stars.

Studying neutron stars can help us discover the origin of heavy chemical elements, including gold and platinum, in our universe. There is a possibility that when neutron stars collide, not all of it will be sucked into a more massive neutron star or black hole, but some will be ejected and form these heavy nuclei.

"If you want to use a 24th or 25th century lab," says Roger Romani, a professor at Stanford University, "this study of neutron stars is a way of looking at conditions that we can't create in laboratories on Earth."