Neutron Stars
A neutron star is the small, ultra-dense leftover core of a massive star that has exploded in a supernova. Neutron stars are tiny by stellar standards (only about 20 km across, the size of a small city), but each one packs in more mass than our entire Sun. The matter inside is so squeezed that ordinary atoms cannot survive: protons and electrons combine into neutrons, and the whole star becomes essentially one giant atomic nucleus. Neutron stars are second only to black holes as the densest objects in the universe.
- Typical sizeapprox. 20 km acrossSmaller than London
- Typical mass1.4 to 2.5 SunsWithin a 20-km ball
- Densityapprox. 400 million tonnes per cm³A teaspoon weighs as much as Mount Everest
- Surface gravityapprox. 2 billion times Earth'sAnything that falls would be flattened
- SpinMany times per secondFastest known: 716 rotations per second
- Famous exampleCrab PulsarIn the Crab Nebula, leftover of a supernova seen in 1054
How does a neutron star form?
When a star at least 8 times the mass of the Sun runs out of fuel, its core collapses in a fraction of a second. The outer layers blast outwards as a supernova, but the inner core keeps falling inward. Gravity squeezes the leftover core so hard that electrons are forced into protons, turning them into neutrons. The result is a ball of pure neutrons, only about 20 km across but containing the mass of a star.
Pretty much everything you have heard about neutron stars is some consequence of how absurdly dense they are. A neutron star is around 100 trillion times denser than water. If you could (somehow) bring a sugar-cube's worth of neutron star material to Earth, it would weigh about 400 million tonnes, the weight of a small mountain. The surface gravity is around two billion times that of Earth, strong enough to bend the path of light passing close to the star.
Pulsars: cosmic lighthouses
When a star collapses into a neutron star, two things happen at once: the star spins much faster (like an ice skater pulling her arms in), and its magnetic field gets squashed thousands of times stronger. The combination produces a pulsar: a rapidly spinning neutron star with a strong magnetic field that flings out beams of radio waves from its magnetic poles.
If one of those beams happens to point at Earth, we see a quick pulse of radio waves every time the beam sweeps past us, like the beam of a lighthouse. The pulses are incredibly regular: some pulsars are so steady that their rotation period rivals atomic clocks for accuracy. The first pulsar, called CP 1919, was discovered by the British astronomer Jocelyn Bell Burnell in 1967. She was a young PhD student and the regular pulses were so strange she initially nicknamed the signal "LGM-1" (for Little Green Men) just in case it was an alien message.
The Crab Pulsar
One of the most famous pulsars in the sky is the Crab Pulsar, in the centre of the Crab Nebula in the constellation Taurus. The Crab Nebula is the expanding wreckage of a supernova that was seen and recorded by Chinese, Japanese and Korean astronomers in the year 1054 AD. The supernova was so bright that for several weeks it was visible during the day.
At the heart of the modern Crab Nebula sits the dead core of the original star: a neutron star spinning around 30 times every second. The Crab Pulsar pumps powerful energy into the surrounding nebula, which is why the Crab is one of the brightest objects in the sky at almost every wavelength of light, from radio waves to gamma rays.
Magnetars: the universe's strongest magnets
Some neutron stars have magnetic fields hundreds or even thousands of times stronger than ordinary pulsars. Those are called magnetars, and they have the strongest magnetic fields in the known universe, around a quadrillion times stronger than the magnetic field of Earth.
The magnetic field of a magnetar is so powerful that if you got within 1,000 km, the field alone would warp the atoms in your body and rip them apart. From a safe distance, you would just see the magnetar do occasional violent eruptions: short bursts of X-rays and gamma rays as the crust of the star cracks under magnetic stress. Magnetar eruptions detected from across the galaxy can briefly outshine almost every other source in the sky.
Neutron star mergers
Sometimes two neutron stars form in a close binary system, and over millions of years their orbits gradually shrink as they radiate away energy. Eventually they spiral into each other and merge in one of the most violent events in the universe. A neutron star merger releases more energy in a few seconds than the Sun will release in its entire life.
In August 2017, astronomers detected the first neutron star merger from both gravitational waves (ripples in space itself) and light. The event, called GW170817, happened 130 million light years away. Among other things, it confirmed a long-suspected idea: that most of the heavy elements in the universe (gold, platinum, uranium) are actually made when neutron stars collide. The collision threw off a cloud of gas containing many times the mass of the Earth in gold and silver. So most of the gold on Earth (in jewellery, electronics and in your blood) was probably forged in a neutron star merger billions of years before the Solar System even formed.
Deeper dive: what is matter actually made of inside a neutron star?
The inside of a neutron star is the most exotic state of matter that scientists know about. Astronomers and physicists are still working out exactly what is going on, because it is impossible to recreate that level of pressure in any laboratory on Earth.
The outermost layer is a thin atmosphere of hot atomic gas, perhaps only a centimetre thick, sitting on a solid crust of ordinary atomic nuclei (mostly iron) crushed by gravity into a rigid lattice billions of times stronger than steel. Tiny cracks in the crust are thought to cause "starquakes" that briefly speed up a pulsar's rotation.
Beneath the crust, the density rises until atomic nuclei dissolve into a soup of neutrons, with a few protons and electrons mixed in. This soup behaves as a superfluid: a strange friction-free liquid that can flow forever without losing energy. Even deeper, no one quite knows what is going on. One leading idea is that the very core may be made of quark matter, where neutrons themselves dissolve into their building blocks: free-floating up and down quarks. Future observations of neutron star mergers, especially with gravitational waves, may be able to settle the question for the first time.
For the deaths that lead to neutron stars, see supergiants and life cycle of a star. For black holes (the next step up), visit what is a black hole.