- 4540 - HOW TO WEIGH A NEUTRON STAR? - 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.
---------------------------------- 4540
- HOW TO
WEIGH A NEUTRON STAR?
-
- 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.
-
- The dense cores of these exotic Neutron
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.
-
- Astronomers used radio signals from a
fast-spinning neutron star to measure its 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.
-
- 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.
-
- NASA’s “Neutron star Interior Composition
ExploreR” (NICER) mission is trying to solve the mysteries of this extreme
matter. NICER is an X-ray telescope on
the International Space Station. It detects X-rays coming from hot spots on the
surface of neutron stars where temperatures can reach millions of degrees.
-
- 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.
-
- 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. 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. 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). We have been observing it for almost 30 years
with Murriyang, CSIRO’s Parkes radio telescope in New South Wales.
-
- This pulsar's X-rays coming from a nearby
galaxy made it hard to accurately model the hot spots on the neutron star’s
surface. Fortunately, we 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.
-
- To measure the neutron star’s mass, they
rely on an effect described by Einstein’s theory of general relativity, called
the “Shapiro delay”. 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.
-
- 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.
-
- 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.
-
- They calculated that the mass of this
pulsar as 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.
-
- 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.
-
- 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”.
-
- 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”.
-
-
August 21, 2024 HOW
TO WEIGH A NEUTRON
STAR? 4540
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--------------------- --- Tuesday, August 20,
2024
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