Neutron stars are extraordinarily compact stellar remnants left behind when certain massive stars explode as supernovae. Despite containing around a Sun’s worth of mass or more, a typical neutron star is only about 20 kilometers (12 miles) across.
Packing so much matter into something roughly the size of a city produces some of the most extreme conditions found anywhere in the universe. Here are some of the most fascinating facts about neutron stars and what makes these tiny stellar remnants so extraordinary.
Neutron Stars Are Born in Supernova Explosions
A neutron star begins with a star much more massive than the Sun. After millions of years of nuclear fusion, the star eventually reaches a point where its core can no longer support itself against the inward pull of gravity. The core then collapses in a fraction of a second, squeezing matter so intensely that electrons and protons are forced together to form neutrons. The star’s outer layers are blasted into space in a supernova explosion, while the compressed core can remain behind as a neutron star. Not every supernova produces one, however. If the collapsing core is too massive to remain stable as a neutron star, it can continue collapsing and form a black hole instead.
Their Gravity Is Almost Unimaginably Strong
Squeezing more mass than the Sun into an object only about the size of a city produces almost unimaginable density. Concentrating so much mass into such a small space also creates an extraordinarily powerful gravitational field, with gravity at the surface around 100 billion times stronger than on Earth.
That immense gravity affects anything that approaches the star. An object falling toward the surface would accelerate to a significant fraction of the speed of light, but the effects do not stop with matter. The neutron star also strongly distorts the space and time around it.
Even light has to struggle against this gravity. Light escaping from the surface loses energy and is shifted toward longer wavelengths, while light traveling from the far side can be bent around the star and into our view. This means an observer can actually see more than half of a neutron star’s surface at once.
Matter as We Know It Breaks Down Inside a Neutron Star
The enormous pressure does more than simply pack matter tightly together. Near the surface, a neutron star has a solid crust made largely from atomic nuclei and electrons, but deeper inside, increasing pressure changes the nature of the matter itself.
Farther beneath the crust, the pressure becomes so intense that ordinary atomic nuclei can no longer remain intact in the familiar way. More and more neutrons are squeezed out of them, until much of the star’s interior consists of an incredibly dense fluid dominated by neutrons. This is the origin of the name neutron star.
The deeper we go, however, the less certain the picture becomes. Physicists understand the outer layers reasonably well, but the conditions near the center are too extreme to reproduce in laboratories. The core may contain unfamiliar forms of ultra-dense matter, and discovering what actually exists there remains one of the great mysteries of modern astronomy.
Neutron Stars Can Spin Hundreds of Times Per Second
The same collapse that makes a neutron star so compact can also make it spin extraordinarily fast. As the dying star’s core shrinks, its rotation speeds up because its angular momentum is concentrated into a much smaller object — similar to a spinning ice skater rotating faster after pulling in their arms.
Some neutron stars therefore rotate many times every second. The fastest known, PSR J1748−2446ad, completes 716 rotations per second, equivalent to more than 43,000 revolutions per minute.
Neutron stars do not necessarily keep the same rotation rate throughout their lives. They generally slow down as they lose rotational energy, while those in binary systems can sometimes be spun up again by taking material and angular momentum from a companion star.
Some Neutron Stars Have Incredibly Powerful Magnetic Fields
Neutron stars can also possess enormously strong magnetic fields. As the stellar core collapses, its magnetic field can become enormously amplified, helping produce fields that are billions or even trillions of times stronger than Earth’s.
The most extreme examples are known as magnetars — neutron stars with exceptionally powerful magnetic fields that can reach around 100 billion tesla. Their fields are so intense that stresses in the star’s crust can trigger enormous outbursts of X-rays and gamma rays. The most powerful of these flares can release in a fraction of a second more energy than the Sun produces in tens of thousands of years, making magnetars some of the most extreme objects in the universe.
Pulsars Are Cosmic Lighthouses
Some rapidly spinning neutron stars are observed as pulsars, named because they appear to send out remarkably regular pulses of radiation. The star itself is not flashing on and off. Instead, its powerful magnetic field channels radiation into beams that emerge from regions around its magnetic poles.
Those magnetic poles can be tilted away from the neutron star’s axis of rotation, so as the star spins, the beams sweep around with it — rather like the beam from a lighthouse. If one of them happens to sweep across Earth, our telescopes detect a pulse every time the neutron star turns and the beam passes us again.
For the fastest pulsars, that can happen hundreds of times every second, with astonishing regularity. Astronomers can measure tiny changes in the timing of these pulses, turning the neutron star’s own rotation into an extraordinarily precise tool for studying its motion, its surroundings and even the effects of gravity.
The First Confirmed Exoplanets Were Found Around a Neutron Star
Neutron stars might seem like unlikely places to find planets, yet the first confirmed planets beyond our Solar System were discovered orbiting one. In 1992, astronomers found two planets around the pulsar PSR B1257+12, with a third confirmed later. They were detected through tiny changes in the timing of the pulsar’s otherwise remarkably regular pulses.
Their existence was astonishing. The neutron star had been created in a supernova explosion, so it is unclear whether any original planets could have survived such a violent event. The worlds we see today may instead have formed afterward from material surrounding the neutron star.
And they would be strange worlds indeed. Instead of a huge, steadily shining Sun, their central object would be a neutron star only about the size of a city. The planets would receive little visible light, yet could be exposed to intense radiation and high-energy particles from the pulsar. Dark, cold and hostile, they orbit the tiny collapsed remains of a star that once exploded as a supernova.
Neutron Stars Slowly Cool Over Time
A newly formed neutron star is extraordinarily hot, with a surface temperature that can initially reach around a million degrees or more. But with no sustained nuclear fusion to replace the energy it loses, the remnant gradually cools.
Much of the early cooling happens through neutrinos — elusive particles that can escape from deep inside the star and carry enormous amounts of energy with them. Later, heat escaping from the surface becomes increasingly important, and the neutron star grows progressively colder and fainter.
Some neutron stars we observe are only hundreds or thousands of years old, while others have survived for billions of years. As they age they generally cool and spin more slowly, although interaction with a companion star can sometimes spin an old neutron star up again.
Cooling does not make the neutron star disappear. Unless it gains enough additional mass to collapse into a black hole, it can remain as a dense stellar remnant for an immensely long time — a slowly fading relic of the star that created it.