Hobbies And Interests
Hydrogen and Helium Shells
The vast majority of white dwarfs have outer hydrogen shells --- about 80 percent --- or outer helium shells --- about 20 percent --- around their carbon and oxygen core. White dwarfs generally have very thin and heavy atmospheres in which the heavier helium sinks below the lighter hydrogen. Due to this differentiation, most white dwarfs have an outer atmospheric shell of hydrogen. Dwarf stars with helium shells are those that have no hydrogen layer and thus a solely helium outer layer.
Hot Carbon
Hot carbon white dwarfs are extremely rare, having neither a hydrogen or helium outer shell over its carbon layer. They were first discovered by astronomers Patrick Dufour and James Liebert in 2007. Astronomers have thought that they form from the most massive stars just small enough to escape the explosive fate of a supernova, in which the star violently explodes.
Pulsating Carbon
The first pulsating carbon white dwarf was discovered in 2008 by astronomers at the McDonald Observatory in Texas. It has no external atmosphere of hydrogen or helium, as do the hot carbons, yet it also pulsates about every eight minutes. Astronomers find special significance in pulsating stars because they can learn more about the star's makeup based on its variations in light output. This is similar to how geologists study the Earth's interior based on seismic waves.
Helium Core
Some rare white dwarfs have helium cores as opposed to the usual carbon and oxygen cores. They generally have only half the mass of most white dwarfs. A particular globular cluster in the Milky Way contains at least 24 of these helium core white dwarfs. Normal stars' helium cores burn out to produce carbon and oxygen, but these stars did not survive long enough for this process to take place, and thus they retained their helium cores into their white dwarf phase.