- 2805 - WHITE DWARF - stars. The stars in the sky may seem ageless and unchanging, but eventually most of them will turn into White Dwarf Stars. This is the last observable stage of evolution for low- and medium-mass stars. These dim stellar corpses dot the galaxy, leftovers of stars that once burned bright.
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--------------- 2805 - WHITE DWARF - stars.
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- Stars have lifetimes just like us. Their life times depend on their size. The bigger the star the shorter the life. The smaller the star the longer it lives.
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- Main-sequence stars, like our the Sun, form from clouds of dust and gas drawn together by gravity. How the stars evolve through their lifetime depends on their mass. Our Sun will eventually become a White Dwarf.
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- The most massive stars, with eight times the mass of the Sun or more, will never become White Dwarfs. Instead, at the end of their lives, they will explode in a violent supernova, leaving behind a Neutron Star or Blackhole.
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- Low- to medium-mass stars, such as the Sun, will eventually swell up into Red Giants. After that, the stars shed their outer layers into a ring known as a planetary nebulae. The core that is left behind will be a White Dwarf, a star in which no hydrogen fusion occurs.
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- Smaller stars, such as Red Dwarfs, don't make it to the red giant state. Red dwarfs take trillions of years to consume their fuel, far longer than the 13.8-billion-year-old age of the universe, so no red dwarfs have yet become white dwarfs.
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- When a star runs out of fuel, it no longer experiences an outward push from the process of fusion and it collapses inward on itself. White dwarfs contain approximately the mass of the Sun but have roughly the radius of Earth.
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- This makes white dwarfs among the densest objects in space, beaten out only by neutron stars and black holes. The gravity on the surface of a white dwarf is 350,000 times that of gravity on Earth. That means a 150-pound person on Earth would weigh 50 million pounds on the surface of a white dwarf.
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- White dwarfs reach this incredible density because they are collapsed so tightly that their electrons are smashed together, forming what is called "degenerate matter." The former stars will keep collapsing until the electrons themselves provide enough of an outward-pressing force to halt the crunch.
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- The more mass, the greater the pull inward, so a more massive white dwarf has a smaller radius than its less massive counterpart. Those conditions mean that, after shedding much of its mass during the red giant phase, no white dwarf can exceed 1.4 times the mass of the sun.
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- When a star swells up to become a red giant, it engulfs its closest planets. But some can still survive. NASA’s Spitzer spacecraft revealed that at least 1 to 3 percent of white dwarf stars have contaminated atmospheres that suggest rocky material has fallen into them. Those the fate of the closer planets.
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- New research suggests that the Milky Way's preponderance of positrons could come from a specialized type of supernova from colliding low-mass white dwarfs . This is an explosion that is difficult to detect, but rich in an isotope that generates this kind of antimatter.
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- Many white dwarfs fade away into relative obscurity, eventually radiating away all of their energy and becoming so-called “black dwarfs“, but those that share a system with companion stars may suffer a different fate.
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- If the white dwarf is part of a binary system, it may be able to pull material from its companion onto its surface. Increasing the white dwarf's mass can have some interesting results.
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- One possibility is that the added mass could cause it to collapse into a much denser neutron star. A far more explosive result is the “Type 1a supernova“. As the white dwarf pulls material from a companion star, the temperature increases, eventually triggering a runaway reaction that detonates in a violent supernova that destroys the white dwarf.
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In 2012, researchers were able to closely observe the complex shells of gas surrounding a Type 1a supernova in fine detail. The white dwarf may pull just enough material from its companion to briefly ignite in a “nova“, a far smaller explosion. Because the white dwarf remains intact, it can repeat the process several times when it reaches that critical point, breathing life back into the dying star over and over again.
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- Astronomers recently spotted perhaps the strangest white dwarf star yet It was a dead star the spins twice a second, sucking down material from a nearby companion as it goes.
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- When stars like the sun die, they heave off their outer atmospheres into space. After the fury dies down, only the core, a white-hot ball of carbon and oxygen, is left behind. That ball, no bigger than planet Earth, is supported not by the normal nuclear fusion inside living stars, but by the exotic quantum force known as degeneracy pressure.
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- But most stars do not live alone; most have siblings. And those stars can orbit in silent watchfulness as their companion ends its life in a blaze, leaving behind the corpse that is a white dwarf. Over time, that companion can either begin the final stages of its life itself, or spiral in too closely, close enough to begin a destructive dance as they orbit one another.
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- When this happens, material from the white dwarf's companion can wind up on the surface of the white dwarf, building a thick layer of hydrogen around its carbon-oxygen body. In this situation and with enough time and enough material, a cataclysm can occur. A flash of nuclear fusion is created by the intense pressures in the atmosphere. This flash of energy releases in a blast of radiation, visible from light-years away.
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- These events used to be called "novas”. Recently a team of astronomers spotted one of these novas. Called a unique “cataclysmic variable star’ dubbed “ J2056“. This is a binary system sitting about 850 light-years away from Earth It is known as an "intermediate polar" cataclysmic variable star.
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- White dwarfs are full of charged particles, like most things in the universe. They are also relatively small and spin pretty quickly. The quickly spinning charged particles generate magnetic fields, which fan out far beyond the surface of the white dwarf and affect how the material from its companion star actually makes it onto the surface of the white dwarf.
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- If the white dwarf star's magnetic fields are weak, the hydrogen from its companion star settles into a nice, regular disk of accretion, steadily feeding onto the white dwarf. If the magnetic fields are strong, they funnel the gas into streams that wrap around the white dwarf and strike the poles, like a super-charged aurora borealis.
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- However, if the magnetic fields are middling, not too weak, but not too strong, we get what is known as "intermediate polar." The word "polar" here refers to the structure of the magnetic field itself.
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- In this case, the magnetic fields aren't strong enough to completely disrupt the formation of an accretion disk, but they are beefy enough to tangle up the gas near the white dwarf. This prevents a regular, smooth flow of gas, causing the white dwarf to flicker and flare irregularly and unpredictably.
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- “J2056 white dwarf” is an example of this intermediate polar system, which means that gas from its companion star can form an accretion disk around the white dwarf, but it has trouble actually making it to the white dwarf's surface. This white dwarf is only capable of accumulating about the equivalent of Earth's atmosphere every year, which as these systems go isn't all that much.
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- J2056 isn't emitting a lot of X-ray radiation, which is also atypical of these kinds of systems. Lastly, J2056 is spinning. Fast. In fact, it's the fastest-known confirmed white dwarf, clocking in at a rotation period of roughly 29 seconds per complete revolution.
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- So how did J2056 get so fast? It could be that the configuration of its magnetic fields are just right and therefore able to pull material down onto its surface in quick spurts, accelerating the white dwarf like a stellar carousel. But its magnetic fields aren't strong enough to slow down the rotation through electromagnetic interactions with the surrounding accretion disk.
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- Still, the relative dimness of its X-rays and the supremely fast orbit of its companion remain to be explained. The companion star orbits once every 1.76 hours
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- J2056 could represent a brand-new class of cataclysmic variable stars, or it could be just a complete oddball. Either way, understanding how it works could help us to understand how magnetic fields operate around white dwarfs, which is important for understanding how they live and how they are born.
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- Astronomers always have more to learn. If you can’t keep up take notes.
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- August 30, 2020 2805
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