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.
