- 3866 - PULSARS - beacons of rotating stars. Pulsars are rapidly rotating neutron stars that blast out pulses of radiation at regular intervals ranging from seconds to milliseconds. These 'cosmic lighthouses' can spin as fast as 700 rotations per second.
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3866 - PULSARS
- beacons of rotating star?
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- Pulsars
have strong magnetic fields that funnel particles along their magnetic poles
accelerating them to relativistic speeds, which produces two powerful beams of
light, one from each pole.
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- Because the
poles of the magnetic field aren't aligned with the axis of spin of the pulsar,
the beams of particles and the light they produce are swept around as the
pulsar rotates.
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- The
periodicity of pulsars is caused by these beams of light crossing the line of
sight here on Earth, with the pulsar appearing to 'switch off' at points when
the light is facing away from us. The time between these pulses is the 'period'
of the pulsar.
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- Pulsars
can be thought of as 'cosmic lighthouses . Because they spin so rapidly this
takes the appearance of 'flickering.'
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- This also
means that the name 'pulsar' may actually be misleading as these neutron stars
don't actually pulse by periodically shrinking and swelling in size, as
astronomers once thought. Their pulsing is merely a factor of their orientation
in relation to our view of them, their light output is mostly consistent.
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- There are
two types of pulsars, ones with periods of a few milliseconds with this
periodicity changing very slowly over time, called millisecond pulsars, and
other pulsars which are just called 'ordinary pulsars.'
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- Like all
neutron stars, pulsars are born when stars with masses between four and eight
times that of the sun run out of fuel for nuclear fusion. When the fusion of
lighter elements into heavier elements stops, the production of energy that
supports the massive star against the inward pressure of its own tremendous
gravity also ceases. The balance of forces the star has enjoyed all of its life
ends and it begins to collapse.
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- As the
collapse proceeds the outer layers of the star are blown away in a supernova
explosion with only the iron core of the massive star containing masses
equivalent to that of the sun up to about 1.5 times that of our star remaining.
This crushes down into a width no greater than 12 to 17 miles.
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- This
creates neutron star matter composed of 95% neutrons, because the collapse has
forced electrons and protons together.
The material that comprises neutron stars is so dense that a mere
teaspoon of it would weigh
4 billion tons.
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- This
superdense material is prevented from cramming further together as the mass of
the stellar core cannot overcome the quantum properties of its neutrons. If the
star was massive enough to overwhelm this effect the neutron star would
continue to collapse until it transforms into a black hole.
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- A star with
the mass of the sun will never become a neutron star, instead, our star will
end its life having run out of hydrogen to fuse into helium as a smoldering
stellar remnant called a “white dwarf” star.
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- The process
of core collapse may be what causes young neutron stars to spin rapidly as
pulsars. Think of this as akin to an ice skater drawing in their arms. As they
do so, the ice skater spins more rapidly. This is analogous to the rapidly
shrinking diameter of a collapsing stellar core.
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- One theory
as to why millisecond pulsars rotate even more rapidly is that they are born
from massive stars in a binary system. After the neutron star creation process
has concluded the newborn neutron star strips material from its close binary
companion. This transfers angular momentum from the doner star to the 'feeding'
neutron star which increases its rotation or 'spins it In short, all pulsars
are neutron stars but, not all neutron stars are pulsars.
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- The
majority of neutron stars are pulsars, but that's because they are far more
obvious than neutron stars. With large beacons of radiation blasting out from
magnetic poles, astronomers can observe these cosmic lighthouses far easier
than small, dim, neutron stars.
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- However,
some pulsars may not be observable from Earth, because their radiated beams of
light don't ever orientate themselves toward us.
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- Other
neutron stars that currently don't appear to be pulsars and look like
non-rotating neutron stars, may have once been pulsars but the process that
causes them to blast out beams of radiation may have 'turned off' or the emissions
may be too weak to be observed. The average lifetime of a pulsar is around 10
million years and as they age their rotation slows.
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- Following
the discovery of neutrons in 1932 by English physicist James Chadwick, the
concept of a neutron star was first predicted in 1934 by Lev Landau in the
Soviet Union and separately by Walter Baade and Fritz Zwicky in the U.S. during
the same year.
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- Five years
later in 1939, Robert Oppenheimer and George Volkoff would develop a
theoretical model for neutron stars, but it would take a further three decades
for the first neutron star, in the form of a pulsar to be discovered.
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- In 1967
radio astronomers in Cambridge were involved in the search for quasars and had
developed an instrument capable of detecting rapid and random changes in the
intensity of radio waves that saved such signals for later analysis.
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- Examining
this data a then 24-year-old graduate student at New hall, conducting research
at Cambridge's Cavendish Laboratory, Jocelyn Bell under the supervision of
Antony Hewish, discovered mysterious highly regular signals very different from
the random signals expected from quasars affected by solar winds.
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- The signals
were so regular that when they were first discovered they were attributed to
human activity. When it was deduced that these highly regular signals must
originate from the universe, the suggestion was made that they could be the
result of intelligent extraterrestrial life.
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- This led to
the source of these pulses earning the name 'LGM1' or 'Little Green Man
1.' Bell and Hewish went on to discover
the actual source of the emission was a pulsar with a period of 1.3373 seconds.
This pulsar now has the slightly less sensational name of “PSR B1919+21”.
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- Over 3,000
pulsars have been discovered and while these were initially found in radio
waves, we have since discovered these cosmic lighthouses in X-ray, gamma-ray,
and even visible light.
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- Like all
neutron stars, pulsars have quite tightly constrained masses and sizes, most
neutron stars have a mass of around 1.5 times that of the sun. One respect in
which pulsars vary a lot is their rate of rotation. Some spin at a rate of
hundreds of times per second.
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- In 2022
astronomers discovered the heaviest neutron star to date with a mass of 2.35
times that of the sun, which is also the fastest-rotating pulsar ever found in
the Milky Way. The pulsar designated “PSR J0952−0607” is also known as the
Black Widow pulsar as it is believed to have reached record-breaking rotational
speed and mass by consuming a companion binary star.
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- The Black
Widow pulsar rotates at 707 Hertz , or 707 times per second. This is topped by
the pulsar “PSR J1748–2446ad” which rotates at 716 HZ or 716 times per second.
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- The light
emitted by a pulsar carries information about these objects and what is
happening inside them. That means pulsars give scientists information about the
physics of neutron stars, which are the densest material in the universe.
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- Under such
incredible pressure, matter behaves in ways not seen before in any other
environment in the universe. The strange state of matter inside neutron stars
is what scientists call 'nuclear pasta': Sometimes, the atoms arrange
themselves in flat sheets.
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- Some pulsars
also prove extremely useful because of the precision of their pulses. There are
many known pulsars that blink with such precise regularity; they are considered
the most accurate natural clocks in the universe.
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- The first
planet outside Earth's solar system ever found was orbiting a pulsar. Because pulsars are moving through space
while also blinking a regular number of times per second, scientists can use
many pulsars to calculate cosmic distances.
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- The changing
position of the pulsar means the light it emits takes more or less time to
reach Earth. Thanks to the exquisite timing of the pulses, scientists have made
some of the most accurate distance measurements of cosmic objects.
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- Pulsars
have been used to test aspects of Albert Einstein's theory of general
relativity, such as the universal force of gravity. The regular timing of pulsars also may be
disrupted by gravitational waves, the ripples in space-time predicted by
Einstein and directly detected for the first time in February 2016. There are
multiple experiments currently searching for gravitational waves via this
pulsar method.
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- All pulsars
slow down gradually as they age. The radiation emitted by a pulsar is jointly
powered by its magnetic field and its spin. As a result, a pulsar that slows
down also loses power, and gradually stops emitting radiation, or at least, it
stops emitting enough radiation for telescopes to detect.
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-
Observations thus far suggest that pulsars drop below the detection
threshold with gamma rays before radio waves. When pulsars reach this stage of
life, they enter what's known as the pulsar graveyard. Pulsars that have stopped emitting may be
considered ordinary neutron stars.
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- When a
pulsar forms from the wreckage of a supernova, it spins fast and radiates a lot
of energy. The well-studied Crab Pulsar is an example of such a young pulsar.
This phase may last for a few hundred thousand years, after which the pulsar
begins to slow down and only emit radio waves.
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- These
'middle-age' pulsars likely make up most of the population of pulsars
identified as emitting only radio waves. These pulsars live for tens of
millions of years before eventually slowing down so much that they 'die' and
enter the pulsar graveyard.
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- But if the
pulsar sits near a stellar companion, it may be 'recycled,' meaning it siphons
material and energy from its neighbor, increasing its spin to hundreds of times
per second creating a millisecond pulsar, and giving the once-dead pulsar new
life.
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- This change
can occur at any time in a pulsar's life, meaning a 'dying' pulsar's rotation
rate can increase over hundreds to millions of years. The pulsar begins to emit
X-rays, and the pair of objects is known as a 'low-mass X-ray binary”.
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- These cannibalistic
pulsars have been called 'black widow' pulsars or 'redback' pulsars in
reference to two species of spiders that are known to kill their
companions. Millisecond pulsars are the
oldest known pulsars, some are billions of years old and will continue to spin
at those high rates for billions of years.
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- Pulsars
aren't the only extreme example of neutron stars in the universe. “Magnetars”
are a type of neutron star that are thought to have the most powerful magnetic
fields in the universe.
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February 8, 2023 PULSARS -
beacons of rotating stars 3866
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