About once every century, a massive star somewhere in our galaxy  runs out of fuel. This happens after millions of years  of heat and pressure have fused the star’s hydrogen into heavier elements like helium, carbon, and nitrogen— all the way to iron. No longer able to produce sufficient  energy to maintain its structure, it collapses under its own gravitational  pressure and explodes in a supernova. The star shoots most of its  innards into space, seeding the galaxy with heavy elements. But what this cataclysmic eruption leaves behind might be even more remarkable: a ball of matter so dense that  atomic electrons collapse from their quantum orbits  into the depths of atomic nuclei. The death of that star  is the birth of a neutron star: one of the densest known objects  in the universe, and a laboratory for the strange physics  of supercondensed matter. But what is a neutron star? Think of a compact ball inside of which  protons and electrons fuse into neutrons and form a frictionless liquid  called a superfluid— surrounded by a crust. This material is incredibly dense – the equivalent of the mass of a  fully-loaded container ship squeezed into a human hair, or the mass of Mount Everest  in a space of a sugar cube. Deeper in the crust, the neutron  superfluid forms different phases that physicists call “nuclear pasta,” as it’s squeezed from lasagna  to spaghetti-like shapes. The massive precursors to  neutron stars often spin. When they collapse, stars that are typically millions of  kilometers wide compress down to neutron stars that are only about 25 kilometers across. But the original star’s angular  momentum is preserved. So for the same reason that a figure  skater’s spin accelerates when they bring in their arms, the neutron star spins much more  rapidly than its parent. The fastest neutron star on record  rotates over 700 times every second, which means that a point on its surface  whirls through space at more than a fifth  of the speed of light. Neutron stars also have the strongest  magnetic field of any known object. This magnetic concentration forms vortexes that radiate beams  from the magnetic poles. Since the poles aren’t always aligned  with the rotational axis of the star, the beams spin like lighthouse beacons, which appear to blink  when viewed from Earth. We call those pulsars. The detection of one of these  tantalizing flashing signals by astrophysicist Jocelyn Bell in 1967 was in fact the way we indirectly discovered neutron stars in the first place. An aging neutron star’s furious rotation slows over a period of billions of years as it radiates away its energy in the form of electromagnetic and gravity waves. But not all neutron stars  disappear so quietly. For example, we’ve observed binary systems where a neutron star  co-orbits another star. A neutron star can feed on  a lighter companion, gorging on its more  loosely bound atmosphere before eventually collapsing cataclysmically into a black hole. While many stars exist as binary systems, only a small percentage of those end up  as neutron-star binaries, where two neutron stars circle each other  in a waltz doomed to end as a merger. When they finally collide, they send  gravity waves through space-time like ripples from a stone  thrown into a calm lake. Einstein’s theory of General Relativity predicted this phenomenon over 100 years ago, but it wasn't directly verified until 2017, when gravitational-wave observatories  LIGO and VIRGO observed a neutron star collision. Other telescopes picked up a burst of  gamma rays and a flash of light, and, later, x-rays and radio signals,  all from the same impact. That became the most studied event  in the history of astronomy. It yielded a treasure trove of data that’s helped pin down the speed  of gravity, bolster important theories  in astrophysics, and provide evidence for the origin  of heavy elements like gold and platinum. Neutron stars haven’t given up  all their secrets yet. LIGO and VIRGO are being upgraded  to detect more collisions. That’ll help us learn what else the spectacular demise of these dense,  pulsating, spinning magnets can tell us about the universe.  