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NEUTRON STARS -
Neutron stars are some of the most extreme objects in the universe.
Formed from the collapsed cores of supergiant stars, they weigh more than our
Sun and yet are compressed into a sphere the size of a city.
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- NEUTRON STARS
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- The dense cores of these exotic stars
contain matter squashed into unique states that we can’t possibly replicate and
study on Earth. That’s why NASA is on a mission to study neutron stars and
learn about the physics that governs the matter inside them.
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- Astronomers have used radio signals from a
fast-spinning neutron star to measure their mass. This enabled scientists
working with NASA data to measure the star’s radius, which in turn gave us the
most precise information yet about the strange matter inside.
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- Matter in the core of neutron stars is even
denser than the nucleus of an atom. As the densest stable form of matter in the
universe, it is squashed to its limit and on the brink of collapse into a black
hole. Understanding how matter behaves under these conditions is a key test of
our theories of fundamental physics.
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- Scientists model the timing and energies of
these X-rays to map the hot spots and determine the mass and size of the
neutron stars. Knowing how the sizes of
neutron stars relate to their masses will reveal the “equation of state” of the
matter in their cores. This tells scientists how soft or hard – how
“squeezeable” – the neutron star is, and therefore what it is made of.
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- A “softer” equation of state would suggest
that neutrons in the core are breaking apart into an exotic soup of smaller
particles. A “harder” equation of state might mean neutrons resist, leading to
larger neutron stars.
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- The equation of state also dictates how and
when neutron stars get ripped apart when they collide. One of NICER’s primary targets is a neutron
star called PSR J0437-4715, which is the nearest and brightest millisecond
pulsar.
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- A “pulsar” is a neutron star that emits
beams of radio waves that we observe as a pulse every time the neutron star
rotates. This particular pulsar rotates
173 times per second (as fast as a blender).
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- The team working with NICER data faced a
challenge for this pulsar. X-rays coming from a nearby galaxy made it hard to
accurately model the hot spots on the neutron star’s surface. They were able to use radio waves to find an
independent measurement of the pulsar’s mass. Without this crucial information,
the team would not have recovered the correct mass.
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- Massive and dense objects such as pulsars,
and in this case its companion star, a white dwarf, warp space and time. The
pulsar and this companion orbit one another once every 5.74 days. When pulses
from the pulsar travel to us across the compressed spacetime surrounding the
white dwarf, they are delayed by microseconds.
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- Such microsecond delays are easy to measure
with Murriyang from pulsars like “PSR J0437-4715”. This pulsar, and other
millisecond pulsars like it, are observed regularly by the Parkes Pulsar Timing
Array project, which uses these pulsars to detect gravitational waves.
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- Because PSR J0437-4715 is relatively close
to us, its orbit appears to wobble slightly from our point of view as Earth
moves around the Sun. This wobble gives us more details about the geometry of
the orbit. We use this together with the Shapiro delay to find the masses of
the white-dwarf companion and the pulsar.
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- We calculated that the mass of this pulsar
is typical of a neutron star, at 1.42 times the mass of our Sun. That’s
important because the size of this pulsar should also be the size of a typical
neutron star.
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- Scientists working with the NICER data were
then able to determine the geometry of the X-ray hot spots and calculate that
the neutron star’s radius is 11.4 kilometers. These results give the most
precise anchor point yet found for the neutron star equation of state at
intermediate densities.
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- Our new picture already rules out the
softest and hardest neutron star equations of state. Scientists will continue
to decode exactly what this means for the presence of exotic matter in the
inner cores of neutron stars. Theories suggest this matter may include “quarks”
that have escaped their normal homes inside larger particles, or rare particles
known as “hyperons”.
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- This new data adds to an emerging model of
neutron star interiors that has also been informed by observations of
gravitational waves from colliding neutron stars and an associated explosion
called a kilonova.
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July 26, 2024 Neutron Stars 4532
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--------------------- --- Monday, July 29, 2024
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