White Dwarf

The expansion of a planetary nebula continues until the nebula fades and becomes too faint to be observable. The carbon core of the parent star, at the center of the planetary nebula, continues to glow white hot with surface temperatures of 100,000 K. Nuclear fusion in the star has now ceased; so what keeps it from collapsing further? What is balancing gravity?

It turns out that the white dwarf is so dense that the electrons themselves (which usually have plenty of room to move around freely) are close to occupying the same position with the same velocity at the same time. Electrons are forbidden to do so, and the gas, therefore, can contract no farther. Such a gas is called a degenerate electron gas, and the pressure of degenerate electrons is all that holds the star up at this point. The great astrophysicist S. Chandrasekhar first showed that stars could support themselves from further collapse in this way. This white-hot core, about the size of the earth but much more massive (about 50 percent as massive as the sun), is called a white dwarf. One teaspoon of the carbon core would weigh a ton on the earth.

Star Words

A white dwarf is the remnant core of a red giant after it has lost its outer layers as a planetary nebula. Since fusion has now halted, the carbon-oxygen core is supported against further collapse only by the pressure supplied by densely packed electrons. Their small size makes them relatively faint objects, despite their high surface temperatures.

Star Words

A white dwarf is the remnant core of a red giant after it has lost its outer layers as a planetary nebula. Since fusion has now halted, the carbon-oxygen core is supported against further collapse only by the pressure supplied by densely packed electrons. Their small size makes them relatively faint objects, despite their high surface temperatures.

As the white dwarf continues to cool, radiating its stored energy, it changes color from white to yellow to red. Ultimately, when it has no more heat to radiate, it will become a black dwarf, a dead, inert ember, saved from gravitational collapse by the resistance of electrons to being compressed beyond a certain point. Some astonomers have proposed that the carbon atoms will eventually assume a lattice structure. The stellar corpse may become, in effect, a diamond with half the mass of the sun.

This Hubble Space Telescope image (right) reveals a population of faint white dwarfs (the circled stars) in globular cluster M4 (Messier object number 4), in the direction of Scorpius. The panel on the left is a view of this region of the sky from a ground-based telescope.

(Image by Harvey Richer of the University of British Columbia and NASA)

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