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  • 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 nitrogenall 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 callnuclear 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.

About once every century,

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中性子星のライフサイクル - デイビッド・ルニー (The life cycle of a neutron star - David Lunney)

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    April Lu に公開 2021 年 01 月 14 日
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