Friday, March 10, 2023

3010 - PULSARS - are rapidly spinning stars?

 

-   3910  -    PULSARS  -  are rapidly spinning stars?    Pulsars are 'cosmic lighthouses' can spin as fast as 700 rotations per second.    They are rapidly rotating neutron stars that blast out pulses of radiation at regular intervals ranging from seconds to milliseconds.

         


------------   3010  -  PULSARS  -  are rapidly spinning stars?

-   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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-   Thus, in effect, pulsars can be thought of as 'cosmic lighthouses.' Even though the lighthouse may be sending out beams of light all night but stricken sailors only see it when it is facing toward them.

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- Because they spin so rapidly this takes the appearance of 'flickering.'   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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-  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 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 around 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 quantum 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.”

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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 up.'

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-    All pulsars are neutron stars but, not all neutron stars are pulsars.  The majority of neutron stars we have discovered thus far 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.  We can be fairly certain that pulsars are rapidly spinning neutron stars because young pulsars have been detected within the remnants of supernovas, exactly where neutron stars are expected to be found.

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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.  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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-  The source of these pulses earning the tongue-in-cheek name 'LGM1' or 'Little Green Man 1.' Of course 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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-   Today 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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-   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 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 (HZ),  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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-   In 1974 astronomers discovered another extreme type of pulsar, one existing in a binary system with a neutron star.  Named a “Hulse–Taylor binary”, this type of pulsar system has become extremely important to astronomers looking to study the limits of Albert Einstein's theory of general relativity and for the study of gravitational waves, tiny ripples in the very fabric of spacetime that propagate from the universe's most powerful and violent events and objects.

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-    The light emitted by a pulsar carries information about these objects and what is happening inside them.  Pulsars give scientists information about the physics of neutron stars, which are the densest material in the universe, with the exception of whatever happens to matter inside a black hole.

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-    The strange state of matter inside neutron stars is what scientists call 'nuclear pasta': Sometimes, the atoms arrange themselves in flat sheets, like lasagna, or spirals.

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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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-   It was with this method that scientists began to identify the presence of alien planets orbiting these dense objects.   The first planet outside Earth's solar system ever found was orbiting a pulsar.

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-    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. 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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-   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 are slowing down gradually as they spin; but those used for precision measurements are slowing down at an incredibly slow rate, so scientists can still use them as stable time-keeping devices.

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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 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.  That is pulsars that have stopped emitting may be considered ordinary neutron stars by astronomers.

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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, thus 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 year

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                   March 10, 2023         PULSARS  -  are rapidly spinning stars?           3910                                                                                                                          

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--------------------- ---  Friday, March 10, 2023  ---------------------------

 

 

 

 

         

 

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