James -
This seems to be a pretty popular question recently
- it's been asked by 4 different people in the last 2 weeks alone!
You're right that "white dwarf" is a term used in astronomy, the study
of space. In fact, a white dwarf is actually a type of star. To
understand what a white dwarf is, though, you'll have to know a little
bit about the life cycle of a star, like our sun.
A star
starts when you have a really big cloud of stuff in space that gets
pulled together because of gravity. As all this stuff gets pulled
together, it becomes very dense (tightly packed) and very hot. It
actually becomes so hot and so dense that many of the hydrogen atoms in
the star's center begin to combine, forming helium. When these atoms
combine (by something called "nuclear combustion", they release a lot
of energy - both as light and heat. Stars like this (like our sun) are
very bright and very hot.
After a while, the star starts to
run out of hydrogen in the middle. The area of nuclear combustion moves
outward from the middle, making the star bigger and bigger. This is
called a "red giant". (This will happen to our sun, eventually, and
when it becomes a red giant it will be big enough to actually engulf
the earth).
A star may actually get to be a red giant twice,
because after the hydrogen is used up, many stars are hot enough to
burn up all the helium that they've just made, making carbon. This
creates a second layer of combustion that moves out from the middle.
But the stars aren't hot enough to burn up carbon, so the process stops
here.
Without the energy that comes from nuclear combustion,
the star has no way to resist the pull of gravity, which pulls the star
into a tighter and tighter ball. This is when the star becomes a "white
dwarf". White dwarf stars aren't very bright, since they don't have the
nuclear energy of a younger star to light them up. But they are
very
dense. They are about the same size as the Earth, but are so tightly
packed that they weigh about 1 million times as much as the Earth! In
fact, a single teaspoon from the center of a white dwarf star can weigh
as much as 5 tons!
The only reason that we can see white dwarf
stars at all is because of the heat-energy that is left over from their
younger days. As they gradually cool down, they become dimmer and
dimmer. Eventually, they can become what's called a "black dwarf",
which is basically just a white dwarf that's gone completely dark.
This
picture taken by a group from the University of British Columbia, using
the Hubble telescope, shows some really good pictures of white dwarf
stars. The white dwarfs are the tiny dots with the circles around them:
For more information on white dwarf stars, here's some other sites you can check out:
ScienceNet's 'What is a White Dwarf?' Another White Dwarf Site -Tamara
(published on 10/22/2007)
