An article written by: Lee Sonogan
“The universe today is 13.8 billion years old. By 22 billion years, the Sun will have finished its main-sequence lifetime and will have become a white dwarf. The Andromeda galaxy will have crashed into the Milky Way.”
― Neil deGrasse Tyson, Welcome to the Universe: An Astrophysical Tour
A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. Degenerate matter has unusual properties. For example, the more massive a white dwarf is, the smaller it is. This is because the more mass a white dwarf has, the more its electrons must squeeze together to maintain enough outward pressure to support the extra mass. It is very dense: its mass is similar to that of the Sun, while its volume is similar to that of Earth. Stars like our sun fuse hydrogen in their cores into helium. White dwarfs are stars that have burned up all of the hydrogen they once used as nuclear fuel.
Fusion in a star’s core produces heat and outward pressure, but this pressure is kept in balance by the inward push of gravity generated by a star’s mass. When the hydrogen used as fuel vanishes, and fusion slows, gravity causes the star to collapse in on itself. When they reach the end of their long evolutions, smaller stars those up to eight times as massive as our own sun, most likely become white dwarfs.
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Because the size of the remnant core is not big enough, the core will fail to produce temperature required for fusing carbon atoms. The required temperature is extremely high at 1 billion Kelvin. Though the temperature fails to reach that high, the core is still hot at around 100,000 Kelvin and just hangs out there in the vast universe and goes through gradual cooling. While the core sits there and cools gradually, it keeps emitting faint white light along with low-energy, soft X-rays. This explains the name White Dwarf.
Black Dwarfs are formed from White Dwarfs. However, it is not practically possible to find a Black Dwarf. Why? Scientists say that the age of the whole Universe as we know today is 13.8 billion years. A White Dwarf can take hundreds of billions of years to cool down and become a Black Dwarf. Since our universe itself is not old, practically no Black Dwarfs have been formed till date.
With a surface gravity of 100,000 times that of Earth, the atmosphere of a white dwarf is very strange. The heavier atoms in its atmosphere sink, and the lighter ones remain at the surface. Some white dwarfs have almost pure hydrogen or helium atmospheres, the lightest of elements. Also, gravity pulls the atmosphere close around it in a very thin layer. If this occurred on Earth, the top of the atmosphere would be below the tops of skyscrapers.
Scientists hypothesize that there is a crust 50 km thick below the atmosphere of many white dwarfs. At the bottom of this crust is a crystalline lattice of carbon and oxygen atoms. This space stuff can also go supernova. If the white dwarf is in a close binary system of stars, the high gravitational pull will drag in extra matter. As the density becomes more over its limit, it will cause the supernova.
About 97% percent of the Milky Way will become white stars. Do not worry about it though. It would take millions of years to actually happen. When it would eventually happen, we would get a few neutron stars that would outlast everything remaining in the Milky Way. There are only 8 known white dwarfs out there in the closest 100 star systems. Space has so many interesting things in it and the white dwarfs are one of them.
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