Thursday, November 8, 2018

Magnetars and pulsars, and neutron stars



-  2156  -  Magnetars and pulsars belong to a class of objects called neutron stars, which are big balls of tightly packed neutrons no larger than a big city.  When stars above about eight solar masses run out of fuel to burn, they explode in what is called a supernova. What remains can collapses into a neutron star.
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----------------------------------  2156  - Magnetars and pulsars
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-  Magnetars and pulsars belong to a class of objects called neutron stars, which are big balls of tightly packed neutrons no larger than a big city.  When stars above about eight solar masses run out of fuel to burn, they explode in what is called a supernova. What remains can collapses into a neutron star.
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-  Pulsars are dead stars that have burned up their hydrogen and helium and collapsed into themselves at a core of neutrons.  They become pulsars because when they collapse they maintain their conservation of angular momentum spinning at incredible speeds, thousands of revolutions per minute.  At their poles they spew out intense beams of radio waves and X-rays.
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-  Magnetars have close cousins called pulsars.  Pulsars are stellar corpses that serve as the radio lighthouses of the galaxy. Spinning around several times a second, they flash the galaxy with a beam of radio waves.  Magnetars are similar, but they flash X-rays, and at a slower rate, about once every 10 seconds. They also occasionally let out a burst of gamma rays.
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-  There are about 1,500 known pulsars, but less than a dozen firmly identified magnetars. What makes magnetars special is their magnetic field, which is thousands of times stronger than that of normal pulsars and billions of times stronger than that of any magnet on Earth.
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-  These magnetic fields can be measured by observing how quickly the spin of the magnetar slows down. A rotating magnet gives off energy, and the greater the magnetic field, the faster the energy loss. Magnetars exhibit rapid deceleration, which implies a huge magnetic field.   After 10,000 years a magnetar will slow down enough to totally turn off its X-ray flash.
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-  It takes a very massive star, some 30 to 40 solar masses to produce a magnetar.  This progenitor star lives 5-6 million years before it explodes , creating the magnetar in its ashes. Massive stars die young. Our middle-ages Sun, by comparison, is about 4.6 billion years old, and will live another 4.5 billion years.
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-  Astronomers used to think that really massive stars formed black holes when they died.  But in the past few years we've realized that some of these stars form pulsars rather than blackholes, because they go on a rapid weight-loss program before they explode as supernovae.
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-  At the very end of its life, the star likely lost 90 percent of its mass, which would make it skinny enough to become a neutron star, as opposed to a black hole.
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-  In our galaxy there are only about 10 neutron stars that are massive enough and at the right age to be magnetars right now. There could be many more "dead" magnetars already in the galaxy, however.
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-  Observations of "starquakes" have allowed scientists to estimate the thickness of a neutron star's crust for the first time.  Neutron stars are very dense objects that mark the endpoints of the lives of some stars.  Using a technique similar to seismology here on Earth, researchers estimated that the crust of a highly magnetic neutron star, a "magnetar," is nearly 1 mile thick and made of material so tightly packed that a teaspoonful of the stuff would weigh about 10 million tons on Earth.
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-  A neutron star forms when an ancient star several times more massive than our Sun runs through its entire stock of nuclear fuel. The star collapses under the weight of its own gravity and explodes in a cataclysmic event called a supernova.  The blast ejects most of the star's mass into space, leaving behind a dense, rapidly spinning core about the size of a small city and roughly 1.4 times more massive than our Sun.
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-  Magnetar's are neutron stars whose magnetic fields are thousands of times stronger. Astronomers have detected only about a dozen such stars. A magnetar's magnetic field is equivalent to about a hundred trillion refrigerator magnets and so strong that it could slow a steel locomotive from as far away as the Moon.
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-  The flash resulting from a violent explosion called a "hyperflare," occurs when a magnetar's magnetic field lines become so twisted with one another that they snap. Like a tightly wound rubber band that finally breaks, the snapping releases tremendous amounts of energy, triggering a "starquake" that buckles the star's crust.
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-  Astronomers calculate the thickness of the magnetar's crust by comparing the frequencies of energy waves traveling around the star against those passing through its interior.
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-  The interior of neutron stars has been a source of great mystery and speculation for scientists. The pressure and density inside a neutron star core is thought to be so great that it could harbor exotic particles called quarks not found since the moment of the Big Bang.
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-   The stars' interiors could contain these quarks which are the building blocks for protons and neutrons. Even the most powerful particle accelerators on Earth can't muster up the energies needed to reveal free quarks.
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-  Astronomers have discovered two neutron stars orbiting each other once every 2.4 hours and spiraling inward toward an eventual dramatic collision. The finding suggest such intense events are far more common than was thought.  Astronomers hope to detect elusive "gravitational waves," which should be spawned in the final seconds prior to binary neutron star mergers.
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-  A neutron star is already a stellar corpse. It is formed when an aged star explodes and as much material as what's in our Sun collapses into a region the size of a city. A teaspoonful, brought to Earth, would weigh a billion tons or so. Neutron stars are stuffed almost entirely with neutrons, neutral subatomic particles that can huddle extremely close together.
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-  Only six neutron-star pairs, called binary systems, are now known.  Previous studies of other pairs have shown that these exotic dance teams spiral toward each other and must eventually crash and unite, possibly becoming a blackhole. Einstein theorized that space-warping gravitational waves, caused by two accelerated masses in orbit, are the reason for this orbital decay.
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-  As Einstein's theory has it, any pair of neutron stars should begin a detectable death chirp moments before they merge.  One minute before the stars merge, their orbit has shrunk to a size of only a few hundred miles, and the two neutron stars move around each other some 30 times each second, producing strong gravitational waves with that same frequency  of 30 cycles per second.
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-  In the last minute before the merger, the orbital frequency increases rapidly, from 30 to 1,000 times per second.  The strength of the gravitational wave emission increases simultaneously.
When the waves reach Earth, their effect would be to displace the oceans by an amount roughly 10 times the diameter of an atomic nucleus. That is too small to notice on an ocean liner.
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-  There are several projects around the world designed to detect these otherwise unnoticed waves. Among the most prominent is the Laser Interferometer Gravitational Wave Observatory (LIGO).
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-  The observatories must be very sensitive. Gravitational waves are said to be similar to light waves. Both propagate through space at different frequencies, radiating outward like ripples on a pond. But gravitational radiation is much weaker than electromagnetic radiation, which includes light, radio waves and X-rays. This is because the fundamental force of gravity is weaker than the fundamental electromagnetic force.
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-  The Sun is a middle-aged star about 8 light-minutes from us. Its tantrums, though extremely small compared to the magnetar explosion, routinely squish Earth's protective magnetic field and alter our atmosphere, lighting up the northern night sky with colorful lights called aurora.
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-  Solar storms also alter the shape of Earth's ionosphere, a region of the atmosphere 50 miles up where gas is so thin that electrons can be stripped from atoms and molecules become ionized and roam free for short periods. Fluctuations in solar storm radiation cause the ionosphere to expand and contract.
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-  A neutron star is the remnant of a star that was once several times more massive than the Sun. When their nuclear fuel is depleted, they explode as a supernova. The remaining dense core is slightly more massive than the Sun but has a diameter typically no more than 12 miles.
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-  Millions of neutron stars fill the Milky Way galaxy. A dozen or so are ultra-magnetic neutron stars, magnetars. The magnetic field around one is about 1,000 trillion gauss, strong enough to strip information from a credit card at a distance halfway to the Moon.
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-  When massive stars go supernova they produce a magnificent nebula. But if the star is not massive enough to produce a blackhole, it usually leaves behind a neutron star.
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-   A neutron star crams as much mass as our Sun into a sphere just 10 miles across. Squeezing out the empty space that makes up most of the Sun's volume, neutron stars leave only naked atomic nuclei.
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-  This cosmic neutron star rotates and has an intense magnetic field and a thin crust of iron nuclei packed into a crystalline lattice . At about 44 trillion gauss, the magnetic field is 1,000 times stronger than that of an ordinary neutron star. By comparison, the Earth's magnetic field is a tame 0.6 gauss, and a refrigerator magnet, a feeble 100 gauss.
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-  The most a human being can normally expect to be exposed to in his life is about 100,000 gauss from a magnetic resonance imager MRI.  A field of 1 billion gauss would turn you into magnetized mush.
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-  Since the star's magnetic field is a drag on rotation, it slows the rotation. The losses are almost imperceptible, about 1 part in 100 billion.  But that represents a lot of energy since it's braking such a compact yet massive object. In the span of about 10,000 years it slows down to become an  X-ray Pulsar.   Only six of these are known to date.
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-  Under the magnetar theory however, one way that energy is released is when the diamond-like crust suddenly cracks, shifting and pumping energy into the ionized gases trapped around the magnetar.  The result of the starquake arrives at Earth as brilliant gamma-ray flares. On Aug. 27, 1998, such an outburst ionized as much of the Earth's outer atmosphere as the Sun would at high noon.
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-  Quarks are thought to be fundamental building blocks of matter. But they have never been observed alone, instead always existing together as the components of other matter. If they were liberated inside a star, they could theoretically be compressed into a smaller sphere.
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-  Massive, dying stars vibrate like giant speakers and emit an audible hum before exploding in one of nature's most spectacular blasts.  A new model developed suggests that sound waves, not ghostly particles called neutrinos, deal the final blow to stars before they become supernovas.
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-  Supernovae are powerful stellar blasts that briefly outshine entire galaxies and radiate more energy than our Sun will in its entire lifetime. Only a star that is between 10 to 25 times more massive than our Sun can become a supernova. After it has burned for 10 to 20 million years or so, the star runs out of fuel and develops a dense iron core about the size of Earth.
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-  The iron core grows until its density becomes so great that it collapses under its own weight. The core contracts, but then almost immediately springs back again. This sudden rebounding action generates a shockwave that speeds outward.  It is this departing shockwave that triggers the supernova explosion.
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-  The problem, however, is that in even the best computer simulations, the shock wave isn't powerful enough on its own to break through the dense layers of superheated gas that envelops the core.  In the models, the shock wave stalls as if muffled by a blanket and the supernova explosion never occurs.
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-  Astronomers began experimenting with the idea that ghostly subatomic particles known as neutrinos might provide the extra power boost needed to complete the blast.  Neutrinos have no charge and are nearly massless. They are produced in vast quantities during the final stages of a massive star's life and stream out of the star's inner core. It was thought that these escaping particles might carry enough energy out of the core to the star's outer layers to complete the explosion.
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-  But even when scientists incorporated the outflow of neutrinos into their computer simulations, it still wasn't enough to produce consistent supernovas.
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-  Instead of neutrino's heating up the material behind the shock,  maybe acoustic power could be doing it. The material on the inside is oscillating like a very, very strong speaker and sending out energy via sound.   The vibrations become so energetic that they create sound waves with audible frequencies in the range of 200 to 400 hertz, or around middle C. 
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-   In this scenario, the sound waves replace neutrinos as energy carriers.  The sound waves propagate out through the material and heat it up.  It acts in a way similar to the way neutrinos would act but with more efficiency.
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-  Pulsars were first discovered  in 1967 by Bell Burnell who strung 120 miles of wire to create a 4.5 acre radio telescope in Cambridge , England.  His "instrument" detected a radio signal barely rising above the background noise on his recordings. 
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-  Further study found the same signal in 10 percent of his recordings arriving 4 minutes earlier each day.  These precisely timed radio signals were 1.3373 seconds apart.  Stars could not change brightness that quickly.  The source had to be an unfathomably dense, small object.
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- Today we know the source to be two orbiting neutron stars whirling around each other every 7.75 hours.  Their orbits are shrinking by 3.2 millimeters per circuit as they loose energy to radiating gravitational  waves.    The two neutron stars are scheduled to collide in just 300,000,000 years.
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 -  Other Reviews available on pulsars and magnetars:
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-  1431 - Pulsar motion is being observed to learn if gravity behaves differently around Neutron stars.  Will gravity waves move the pulsars with the passing wave?  
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-  1397  -  Ordinary Matter should be called Ordinary Space.  The matter part is almost negligible.  Almost all of solid matter is empty space.  It is not solid at all.  What makes it feel solid is the electromagnetic force.
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-  1396  -  High school students discover a Pulsar.
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-  1376  - How can Pulsars have planets?  The Earth as the first planet to be discovered.  And, it just happens to be the right size, the right temperature, and orbiting the right star.  How lucky can you get?  Math: How to calculate the distance to a star.
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-  1331  -  How Neutron Stars become Pulsars?  Eight supernovae explosions have been recorded witnessed by human naked eyes.  Spin rates of pulsars slow down as the drag of the strong magnetic field causes a loss of spin energy. 
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-  1327  -  The fastest spinning star?  The neutron stars is spinning so fast it would fly apart except for the fact that its surface is solid and harder than a diamond.  Math:  If the neutron star has a radius of 10 kilometers and is spinning at 716 rotations per second how big was it when it started?
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-  November 8, 2018.     
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 ---------------------   Thursday, November 08, 2018         -------------------------
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