A White Dwarf Star Throws A Surprise Party

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Supernovae explosions herald the „death“ throes of stars after they have consumed their necessary supply of nuclear-fusing fuel, and have gone raging into that good night. Frequently, the supernova progenitor is a massive star that contains an extremely heavy iron-nickel core that weighs in at 1.4 times the mass of our Sun. However, smaller stars, like our Sun, do not perish in the terrible beauty and fury of a supernova blast like their more massive stellar siblings–at least, not when they are solitary, lonely small stars like our own. Alas, when a small Sun-like star „lives“ in a binary system with another still-„living“ star, it is a wild party about to happen. In December 2018, astrophysicists using NASA’s Chandra X-ray Observatory, announced that they have detected a bright X-ray outburst from a star inhabiting the Small Magellanic Cloud (SMC). The SMC is a small nearby satellite galaxy of our Milky Way, and it is located almost 200,000 light-years from Earth. A combination of X-ray and optical data suggest that the source of this outburst of radiation is a white dwarf star that may be the fastest growing white dwarf ever observed. This new study of the white dwarf named ASASSN-16oh provides a valuable explanation for what are called supersoft X-rays that were detected emanating from this intriguing „dead“ star. The discovery was made by the All-Sky Automated Survey for Supernovae (ASASSIN).

Unlike our Sun, most stars do not „live“ in isolation. Most of our Galaxy’s stars are members of multiple stellar systems–such as binary systems, that contain a closely dancing stellar duo. If the two stars are sufficiently near one another, and one of the stars is a dense white dwarf, its powerful gravity can sip up the material from its still-„living“ companion star–and victim.

White dwarfs are dense stellar ghosts that are about the same size as Earth, but contain a mass that is equal to that of our Sun, compressed into a small volume. Thus, the gravity at the surface of these „dead“ stars is strong enough to suck up matter from a luckless, still-„living“ companion star.

In about 5 billion years our own Sun will run out of its necessary supply of nuclear-fusing fuel and–following its swollen red giant stage–will shrivel up and shrink, evolving into a considerably smaller, dimmer white dwarf star. Our future Sun, at this stage, will only be about the same size as Earth, and because its matter has been packed into such a small volume, its surface gravity will be several hundred thousand times more powerful than that of Earth. However, our Sun will never go supernova because it has no companion star. Our Sun is destined to perish with great beauty and relative peace. In its white dwarf stage, our Star will be surrounded by a beautiful, multicolored shroud of shimmering, glimmering gases that were once its outer layers. Such stellar shrouds are freqently referred to as the „butterflies of the Universe“ by astronomers as homage to their great beauty.

The new study is based on observations conducted by astronomers using both Chandra and the Neil Gehrels Swift Observatory. The study reports on the discovery of the distinctive X-ray emission emanating from ASASSN-16oh, which is actually a binary system, composed of a duo of white dwarf stars. The important discovery involves the detection of soft (low energy) X-rays, created by gas at temperatures of several hundred thousand degrees. In dramatic contrast, higher energy X-rays reveal phenomena at tempertures of tens of millions of degrees. However, the X-ray emission from ASASSN-16oh is considerably brighter than the merely soft X-rays manufactured by the atmospheres of normal stars. This places ASASSN-16oh in the special category of a supersoft X-ray source.

White Dwarf Stars

As a doomed, elderly small Sun-like star nears the grand finale of its nuclear-burning phase, it casts off its outer material–that becomes its surrounding and beautiful planetary nebula. Only the „dead“ star’s core remains to tell the sad story of its former sparkling existence. The core becomes the searing-hot white dwarf, with a roasting temperature above 100,000 Kelvin. If the star is a lonely one, like our Sun, and is not accreting material from a victimized nearby binary stellar sibling, the white dwarf will continue to cool down over the next billion years–or so. A multitude of nearby, youthful white dwarfs have been spotted as sources of soft, low-energy X-rays. Recently, both soft X-ray and extreme ultraviolet observations have been used by astronomers in their quest to understand the composition and structure of the thin atmosphere possessed by these stellar ghosts.

A typical white dwarf star is about 200,000 times as dense as Earth. This makes white dwarfs the second-densest collection of matter, surpassed only by neutron stars. Neutron stars are the city-sized relics left behind by stars that are more massive than our Sun. A teaspoon full of dense neutron-star stuff weighs as much as a large pride of lions.

White dwarf stars cannot create internal pressure derived from the release of energy from nuclear-fusion. This is because fusion has ceased, and internal pressure is necessary to keep the still-„living“ star bouncy against the merciless pull of its own relentless gravity. All stars, regardless of their mass, must maintain a precious balance between the two battling forces of radiation pressure and gravity. Gravity wins in the end, when fusion ceases, and it compacts the doomed star’s matter inward until even the electrons that make up a white dwarf’s atoms are squashed together. Under normal circumstances, identical electrons (meaning those with the same „spin“) cannot occupy the same energy level. Because there are only two ways that an electron can spin, only two electrons can occupy a single energy level. The term for this, used by physicists, is the Pauli Exclusion Principle. In the case of a normal gas, this isn’t a problem. This is because there aren’t enough electrons dancing around to fill up all the energy levels completely. However, in the case of a white dwarf star, the density is much higher, and all of the electrons are smashed much closer together. This is termed a degenerate gas. This basically means that atoms are filled with electrons. In order for gravity to compress the white dwarf star further, it must force electrons to go where they are unable to go. Once a star is degenerate gravity is unable to compress it further. This is because quantum mechanics states that there is no more available space to be taken up. Therefore, the white dwarf star manages to survive. This tiny dense stellar relic does not survive because of internal fusion, but by quantum mechanical principles that prevent it from experiencing complete collapse. Quantum mechanics is the mathematical study of the mechanics of subatomic particles.

Degenerate matter exhibits some very weird properties. For instance, the more massive a white dwarf star, the smaller it is. This is because the more mass a white dwarf possesses, the more its electrons must be squashed together in order to maintain sufficient outward pressure to support the extra mass. However, there is a limit on the amount of mass a white dwarf star can have. Subrahmanyan Chandrasekhar (1910-1995) discovered this limit to be 1.4 times solar mass. This is appropriately termed the Chandrasekhar Limit. Chandrasekhar was an Indian-American astrophysicist who spent his professional life in the United States. He was awarded the 1983 Nobel Prize in Physics, along with the American nuclear physicist William Fowler (1911-1995) for „theoretical studies of the physical processes of stars.“ The Chandra X-ray Observatory was also named after Chandrasekhar.

With a surface gravity of 100,000 times that of Earth, a white dwarf star’s atmosphere is weird. This is because the heavier atoms in its strange atmosphere sink, while the lighter ones remain at the surface. Some white dwarfs have been found to possess almost pure hydrogen or helium atmospheres–hydrogen is the lightest atomic element, and helium the second-lightest. In addition, the relentless force of the white dwarf’s powerful gravity pulls the atmosphere close around it to form a very slender layer. If this same rather bizarre phenomenon occurred on Earth, the top of the atmosphere would be below the tops of New York City skyscrapers.

Scientists propose that there is a crust about 50 kilometers thick beneath the strange atmosphere of many white dwarf stars. At the bottom of this still-hypothetical crust there would be a crystalline lattice composed of carbon and oxygen atoms. Since a diamond is crystalized carbon, one might make the comparison between a cool carbon/oxygen white dwarf and a very large diamond.

A Stellar Surprise Party

For many years, astronomers have proposed that supersoft X-ray emission from white dwarf stars is manufactured as a product of nuclear fusion within the searing hot and extremely dense layer composed of hydrogen and helium nuclei. This very volatile material accumulates as a result of infalling matter, originating from an unfortunate companion star, that somersaults down onto the surface of the vampire-like white dwarf. This triggers a nuclear fusion explosion similar to that of a hydrogen bomb.

However, ASASSIN observations revealed there is more to it than that. The supersoft X-ray stellar binary system was initially discovered by this automated survey, which is a collection of about 20 optical telescopes scattered around the world that automatically survey the entire sky every night searching for supernovae blasts and other transient events. Astronomers then used Chandra and Swift to spot the supersoft X-ray emission.

„In the past, the supersoft sources have all been associated with nuclear fusion on the surface of white dwarfs,“ commented study lead author Dr. Tom Maccarone in a December 4, 2018 Chandra-Harvard Press Release. Dr. Maccarone is a professor in the Texas Tech Department of Physics & Astronomy who led the new paper published in the December 3, 2018 issue of the journal Nature Astronomy.

If, indeed, nuclear fusion is the cause of the supersoft X-rays from ASASSN-16oh then it should have been triggered by an explosion and the emission should have come from the entire strange surface of the white dwarf star. However, the optical light does not increase rapidly enough to be the result of an explosion and the Chandra data reveal that the emission is originating from a region smaller than the entire surface of this bewitching and bewildering white dwarf star. In addition, the source is a hundred times fainter in optical light than that of white dwarfs known to be experiencing nuclear fusion on their surface. These observations, plus the lack of evidence for gas swirling away from the white dwarf, provide strong arguments against fusion having occurred.

Therefore, none of the signs of nuclear fusion are present. For this reason, the authors of the paper present an alternative scenario. As with the fusion explanation the white dwarf is gravitationally pulling matter away from an unlucky companion star, in this case a red giant. During this process, termed accretion, the gas is gravitationally pulled onto a large disk encircling the white dwarf–and it becomes hotter, and hotter, and hotter, as it spirals toward the dense white dwarf. The gas then tumbles onto the „dead“ star. This produces X-rays along a belt where the disk touches the star. The rate of inflow of matter through the disk varies by a large amount. When the material begins to flow more rapidly, the X-ray brightness of the system grows much higher.

„The transfer of mass is happening at a higher rate than in any system we’ve caught in the past,“ added Dr. Maccarone in the December 4, 2018 Chandra-Harvard Press Release.

If the white dwarf keeps stealing mass from its victimized companion red giant star it will pay for its crime. This is because it will reach a mass limit and „go critical“– blowing itself up in a Type Ia supernova blast. A Type Ia supernova is an event that was used to discover that the expansion of the Universe is accelerating. The team of astronomers‘ analysis indicates that the white dwarf is already unusually massive. For this reason, the scientists think that ASASSN-16oh may be relatively close to going supernova.

„Our result contradicts a decades-long consensus about how supersoft X-ray emission from white dwarfs is produced. We now know that the X-ray emission can be made in two different ways: by nuclear fusion or by the accretion of matter from a companion,“ study co-author Dr. Thomas Nelson (University of Pittsburgh, Pennsylvania) commented in the December 4, 2018 Chandra-Harvard Press Release.

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Source by Judith E Braffman-Miller

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