[Music throughout] Matter at the heart
of a neutron star, the crushed remnant of a massive sun, is on
the brink of becoming a black hole. For decades, scientists have
wondered about the properties of that matter – the densest in the universe we can
measure – and what form it takes. Now they have new
insights, thanks to NASA’s NICER X-ray telescope on the
International Space Station. A neutron star
forms when a massive star’s core runs out of fuel.
With nothing left to fight gravity, the star collapses.
Here, protons and electrons crush together to form neutrons,
as well as lightweight particles called neutrinos that escape the star.
The core continues to collapse until the matter at its center
has twice the density of an atom’s nucleus, but on a
city-sized scale. When the core can’t compress further, it rebounds.
The expanding core crashes into the star’s collapsing
inner layers, creating a shock wave that rips outward through the star.
The result is a powerful supernova explosion with a
newborn neutron star at its center. Scientists have
many questions about neutron star physics, including: How
squeezable is the matter in their cores? In more squeezable models,
the internal pressure and density break neutrons in the center into a sea of
even tinier particles, or combinations of those particles,
resulting in a squishy core and a smaller star for a given mass.
In some less squeezable models, the neutrons hold up against those
forces, resulting in a larger star.
Scientists used NICER’s precise mass and size measurements
of two pulsars, a kind of rapidly rotating neutron star,
to narrow down how compressible these objects are.
A pulsar is so dense that its strong gravity warps nearby
space-time, allowing us to see light emitted from its far side.
This distortion makes it look bigger than it actually is.
The more massive the pulsar, the greater the warping and the larger it appears.
Scientists measure this distortion by tracking the
brightness of X-ray-emitting hot spots on the pulsar’s surface as it
spins. They can then precisely determine the pulsar’s mass and
radius and obtain important clues about conditions in the core.
NICER used this method to analyze J0740,
the heaviest known pulsar with about 2.1 times the
Sun’s mass. Two research groups using different approaches
both estimate it’s about 16 miles across.
NICER’s measurements of J0740 and pulsar J0030
disfavor squeezable models, where cores contain only
quarks or other exotic matter. And J0740's size
and mass together challenge less squeezable theories where cores
contain only neutrons. Physicists will have to develop new
models, perhaps containing both neutrons and quarks, to
explain NICER’s observations.
The cores of neutron stars represent matter’s final, stable
form short of becoming a black hole. Scientists can’t recreate
those conditions in Earth laboratories, so NICER will continue to
measure pulsars to probe deeper and deeper into the
hearts of these mysterious objects.
[NASA]