Neutron stars, the remnants of massive stars that have exhausted their fuel and collapsed in on themselves, are some of the most extreme and fascinating objects in the universe. A single teaspoon of neutron star material would weigh about four billion tonnes on Earth, which is roughly the mass of a mountain. This is because the star has crushed the equivalent of the Sun into a sphere the width of a city. The first neutron star ever detected, PSR B1919+21, was picked up by Jocelyn Bell Burnell in Cambridge in 1967 as a steady radio pulse arriving every 1.3 seconds from a source no larger than a small city. The star did this by taking something with more mass than the Sun and squeezing it into a ball about 20 kilometres across. The density inside a neutron star is around 10^17 kilograms per cubic metre, which is so high that a teaspoon of its material would weigh around four billion tonnes on Earth. This is the mass of about 1,000 Great Pyramids of Giza, the mass of every car and truck built worldwide in a decade, or the mass of a small mountain range, all sitting in a spoon. What stops a neutron star from continuing to shrink into a black hole is a quantum effect called neutron degeneracy pressure. Neutrons, like electrons, refuse to occupy the same quantum state as their neighbours. Push them close enough and they push back with a force that has nothing to do with heat or motion. This pressure holds the star up against a gravitational field so intense that the surface gravity is billions of times Earth’s. A marshmallow dropped from a metre above the surface would hit the ground with the energy of an atomic bomb. Above a critical mass — somewhere around 2.2 to 2.5 solar masses — even neutron degeneracy fails. Anything heavier than that ceiling becomes a black hole instead. The fastest neutron stars known rotate hundreds of times per second, their equators moving at roughly a quarter of the speed of light. The magnetic fields are similarly extreme. A class of neutron stars called magnetars carries fields of around 10^11 tesla — a trillion times stronger than a fridge magnet, and strong enough to disrupt the chemistry of atoms from a thousand kilometres away. In 2004, a magnetar released a starquake burst that briefly ionised the upper atmosphere of our planet and saturated satellites designed to watch gamma-ray bursts across the entire visible universe. It was among the brightest events ever recorded from beyond the solar system. Nobody knows what sits at the very centre of a neutron star. The pressure is so extreme that neutrons themselves may stop being the fundamental unit — they might dissolve into a soup of free quarks, or form exotic particles, or crystallise into structures with no analogue in ordinary matter. Researchers at Michigan State University have been running simulations of how neutrinos travel through this material, using the way spin and density correlate in the neutron fluid as a probe of what lies deeper in. Neutrinos are one of the few things that can escape the interior carrying information about it. Neutron stars matter for reasons that go beyond exotic physics. In August 2017, the LIGO and Virgo gravitational wave detectors picked up a signal called GW170817 — two neutron stars in the galaxy NGC 4993 spiralling into each other and merging in a burst that lit up telescopes across every wavelength. The collision produced, among other things, substantial amounts of gold and platinum, flung outward at a fraction of the speed of light. Similar mergers, occurring across billions of years before the Sun formed, are where most of Earth’s heavy elements came from. The gold in a wedding band was, quite literally, made in the collision of two of these city-sized corpses. The surface of a neutron star is not remotely like the surface of a planet. It is a crust of iron nuclei arranged in a crystal lattice, compressed to millions of tonnes per cubic centimetre, sitting on top of the neutron fluid below. Mountains on this crust exist, but they cannot be tall. The surface gravity flattens them considerably. A neutron star is smoother than any billiard ball humans have ever polished. Occasionally the crust cracks. When it does, the star releases the energy of a stellar flare in fractions of a second, and its rotation rate abruptly changes in an event pulsar astronomers call a glitch. Because neutron stars are so extreme, they double as natural laboratories for physics that cannot be tested any other way. A recent paper covered by Science Daily suggested that neutron stars might be key to understanding dark matter — the invisible material that makes up most of the mass of galaxies but has never been directly detected. If dark matter particles interact even weakly with ordinary matter, they should accumulate inside neutron stars over billions of years, subtly changing how the stars cool, spin, and vibrate. Precision measurements of those properties are one of the few practical ways to hunt for particles that pass through Earth without noticing it is there. If a fragment of neutron star material somehow reached Earth intact, it would not stay in the spoon. Without the crushing gravity holding it together, it would explode outward with roughly the energy of a nuclear weapon, unbinding back into ordinary matter in microseconds. The stuff only exists because a whole star’s worth of gravity is pressing down on it from every direction at once. PSR B1919+21 is still out there, still pulsing every 1.3 seconds the way it was when Jocelyn Bell Burnell first noticed it on a paper chart in 1967. It has been doing it for a very long time, and it will keep doing it long after every human who reads this sentence is gone. A teaspoon of it would still weigh a mountain.
