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What Is a White Dwarf?
Simply put, a white dwarf is a star, similar to Earth's sun, that has died. When a star dies, all of its nuclear fuel is exhausted, fusion ceases to occur and there is no longer any activity inside the star. The white dwarf is one of three possible byproducts of stellar death; the other two being neutron stars and black holes. A dead star will become a white dwarf when its mass is particularly low, unlike stars with greater mass, which will supernova, collapse in on themselves and result in neutron stars or black holes.
How a White Dwarf Forms
A white dwarf begins to form when an old, giant star is in its death throes. As the star exhausts the last of its nuclear energy, it will explode, throwing its outer layers into space as though it were shedding its external material. This is known as a nova. After the star novas, only the hot, compressed core remains, which is what scientists call the white dwarf. The white dwarf will either cool down over billions of years, or its gravity will pull on another nearby star and the white dwarf will slowly accumulate dust and gas. If the white star continues to accumulate dust and gas, it will eventually nova again and the process might repeat until the star supernovas. Scientists are still in the process of understanding this kind of supernova event.
Features of a White Dwarf
Scientists agree that a cooled white dwarf is comprised mostly of carbon and oxygen and is much like a giant diamond in space. Typical white stars have approximately half the mass of the sun, yet are about the size of Earth, but 200,000 times as dense. White dwarfs are among the most dense objects in the universe, surpassed only by neutron stars. One peculiar fact about white dwarfs is that the more dense they are, the smaller they become. White dwarfs tend to have light atmospheres that are usually comprised of helium and hydrogen.
White Dwarfs and Planetary Nebulae
Although a white dwarf isn't a planet, these dead stars do create what's known as a planetary nebula during the process of death. As the dying star becomes a white dwarf it expels clouds of dust and gas into interstellar space. These clouds expand around the star's core to form beautiful circular and conical swirls of dust and gases known as planetary nebulae. Despite the name, planetary nebulae aren't directly associated with the formation of planets, although scientists believe that all planets, even Earth, are indeed formed in part from leftover dust and gases thrown off by dying stars.