A White Dwarf Star Throws A Surprise Party

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. 바카라사이트

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.

 

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