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[Music throughout] Matter at the heart

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of a neutron star, the crushed remnant of a massive sun, is on

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the brink of becoming a black hole. For decades, scientists have

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wondered about the properties of that matter – the densest in the universe we can

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measure – and what form it takes. Now they have new

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insights, thanks to NASA’s NICER X-ray telescope on the

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International Space Station. A neutron star

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forms when a massive star’s core runs out of fuel.

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With nothing left to fight gravity, the star collapses.

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Here, protons and electrons crush together to form neutrons,

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as well as lightweight particles called neutrinos that escape the star.

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The core continues to collapse until the matter at its center

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has twice the density of an atom’s nucleus, but on a

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city-sized scale. When the core can’t compress further, it rebounds.

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The expanding core crashes into the star’s collapsing

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inner layers, creating a shock wave that rips outward through the star.

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The result is a powerful supernova explosion with a

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newborn neutron star at its center. Scientists have

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many questions about neutron star physics, including: How

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squeezable is the matter in their cores? In more squeezable models,

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the internal pressure and density break neutrons in the center into a sea of

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even tinier particles, or combinations of those particles,

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resulting in a squishy core and a smaller star for a given mass.

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In some less squeezable models, the neutrons hold up against those

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forces, resulting in a larger star.

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Scientists used NICER’s precise mass and size measurements

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of two pulsars, a kind of rapidly rotating neutron star,

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to narrow down how compressible these objects are.

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A pulsar is so dense that its strong gravity warps nearby

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space-time, allowing us to see light emitted from its far side.

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This distortion makes it look bigger than it actually is.

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The more massive the pulsar, the greater the warping and the larger it appears.

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Scientists measure this distortion by tracking the

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brightness of X-ray-emitting hot spots on the pulsar’s surface as it

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spins. They can then precisely determine the pulsar’s mass and

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radius and obtain important clues about conditions in the core.

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NICER used this method to analyze J0740,

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the heaviest known pulsar with about 2.1 times the

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Sun’s mass. Two research groups using different approaches

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both estimate it’s about 16 miles across.

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NICER’s measurements of J0740 and pulsar J0030

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disfavor squeezable models, where cores contain only

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quarks or other exotic matter. And J0740's size

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and mass together challenge less squeezable theories where cores

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contain only neutrons. Physicists will have to develop new

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models, perhaps containing both neutrons and quarks, to

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explain NICER’s observations.

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The cores of neutron stars represent matter’s final, stable

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form short of becoming a black hole. Scientists can’t recreate

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those conditions in Earth laboratories, so NICER will continue to

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measure pulsars to probe deeper and deeper into the

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hearts of these mysterious objects.

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[NASA]