A NEW DEATH LEADS TO A NEW BIRTH
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 into a Supernova. The star shoots most of its innards into space, seeding the galaxy with heavier elements. However, what this cataclysmic eruption leaves behind might be even more outstanding: a ball of matter so dense that atomic electrons collapse from their quantum orbits, into the depths of their atomic nuclei. The death of that star is the birth of a Neutron Star: one of the densest known objects in the universe.
WHAT IS A NEUTRON STAR?
This star is the laboratory for the strange physics of super-condensed matter. However, what is a Neutron Star? Think of it as a compact ball inside of which protons and electrons have fused into neutrons and form a frictionless liquid, called a superfluid, surrounded by a Crust. This material is incredibly dense; the same as the mass of a giant cargo ship of steel squeezed into a human hair, or the mass of Mount Everest compressed into the size of a sugar cube. Deeper in the Crust the neutron superfluid forms different phases that physicists call ‘Nuclear Pasta’ for it is crammed from lasagna to spaghetti-like shapes.
The massive precursors to the Neutron Stars often spin. When they collapse, stars that are typically millions of kilometers wide, compress into Neutron Stars that are about 25 kilometers across. Nevertheless, the original star's angular momentum is preserved. In other words, because of the same reason a figure skater’s spin accelerates when they bring in their arms, the Neutron Star spins more hurriedly than its parent. The fastest Neutron Star on record rotates over 700 times in one second. This means that a point on its surface travels through space at more than a fifth the speed of light!
Neutron Stars also have a magnetic field that is stronger than any known object in the universe. This magnetic field also forms vortexes that radiate beams from the magnetic poles. Since the poles are not 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 method of detection of one of these bright signals by the astrophysicist Jocelyn Bell in 1967 was in fact the method by which we indirectly discovered Neutron Stars. An aging Neutron Star’s furious rotation slows down over billions of years, as it radiates away its energy in the form of electromagnetic and gravitational waves.
Yet, not all Neutron Stars disappear so peacefully and quietly. For example, we have observed some binary systems where a Neutron Star co-orbits another Neutron Star. A Neutron Star can feed on a lighter companion, devouring its loosely bound atmosphere before eventually collapsing tempestuously into a black hole. While many stars exist as binary systems, only a small percentage of them 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.
THE MOST STUDIED EVENT IN THE HISTORY OF ASTRONOMY
Einstein's theory of general relativity predicted this phenomenon over 100 years ago, but it was not directly verified until 2017 when gravitational-wave observatories Laser Interferometer Gravitational-wave Observatory (LIGO) and Variability of solar IRradiance and Gravity Oscillations (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 has 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 have not completely disclosed their secrets yet. LIGO and VIRGO are being upgraded to detect more collisions. That will help us learn what else the spectacular demise of these incredibly dense, pulsating, spinning magnets tell us about the universe.
Bibliography:
Naeye, Robert “Neutron Stars” NASA.gov. 28 Aug, 2008. Web. 30 Mar, 2022. <https://www.nasa.gov/mission_pages/GLAST/science/neutron_stars.html>
Marttson, Barbara “Neutron Stars” Imagine the Universe! Nasa.gov. 23 Sept, 2021. Web. 27 Mar, 2022. <https://imagine.gsfc.nasa.gov/science/objects/neutron_stars1.html>
Briggs, Andy “What are neutron stars? ” Earth Sky. 12 Feb, 2020. Web. 30 Mar, 2022. <https://earthsky.org/astronomy-essentials/definition-what-is-a-neutron-star/>
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