White dwarf

From Aetilc

A white dwarf is the fate awaiting average-sized stars. It is the core of a dead star left to cool for eternity.

Nuclear fusion is the merging of two hydrogen nuclei into one helium nucleus, with the release of a tremendous amount of energy in the process. This occurs in the early stages of every star's life. It fuels the star and provides an outward pressure that acts as a balance to the star's tremendous gravity. In the absence of fusion, gravity takes over and causes a star to collapse upon itself. The larger the original star, the smaller a white dwarf it becomes. The reason for this pattern is that larger stars have stronger gravitational fields, which produce a more complete collapse.

An average-sized star will spend the final 10% of its life as a red giant. In this phase of a star's evolution, the star's surface temperature drops to between 1,727 and 3,727°C and its diameter expands to 10 to 1,000 times that of the Sun. The star takes on a reddish color, which is what gives it its name.

Buried deep inside the star is a hot, dense core, about the size of Earth. The core makes up about 1% of the star's diameter. The helium left burning at the core eventually ejects the star's atmosphere, which explodes off into space as a planetary nebula (gas and dust cloud). All that remains of the star is a glowing core, a white dwarf.

The term white dwarf is a bit misleading. The core starts out white, but as it cools it displays a range of colors—from yellow to red. When all heat within the core has escaped, the body ceases to glow and becomes a black dwarf. Billions of white dwarfs exist within a galaxy, many of them now in the form of black dwarfs. These cold, dark globes, however, are next to impossible to detect.

[edit] Words to Know

• Black dwarf: Cooling remnants of a white dwarf that has ceased to glow.
• Nebula: Cloud of interstellar gas and dust.
• Nuclear fusion: Reaction involving the merging of two hydrogen nuclei into one helium nucleus, releasing a tremendous amount of energy in the process.
• Red giant: Stage in which an average-sized star spends the final 10 percent of its lifetime; its surface temperature drops and its diameter expands to 10 to 1,000 times that of the Sun.