White Dwarfs
At the bottom (faint), left (blue-white and hot) of a Hertzsprung-Russell diagram one can find
a population of stars known as white dwarfs. White dwarfs have
masses similar to the Sun, but have very high surface temperatures;
20,000 K is typical for the hotter ones.
Despite their hot surfaces, white dwarfs are very faint.
Since we know the distance to several
white dwarfs accurately
(such as Sirius B, at 2.6 parsecs or 8.6 light-years), we also know
their absolute magnitudes
(luminosities); they are typically 100 to 10,000 time less luminous than the Sun.
Sizes and Densities
From the same considerations that we applied to giant stars, the luminosity L of a
white dwarf
should be proportional to its surface area A times its temperature T raised
to the fourth power,
Since white dwarfs are very hot (large T)
but have very low luminosity (small L), it follows that they must have surprisingly small
surface
areas A. In fact, they are similar in size to the Earth! Thus they are called
white dwarfs because they are very hot and therefore often white, and they are very small.
But white dwarfs typically contain
masses comparable to that of the Sun. Therefore, the density of a white dwarf is
extremely high, with a million grams per cubic centimeter
being typical. This is about 50,000 times
the density of lead.