Introductory Astronomy: White Dwarfs

White Dwarf stars are the stellar remnant (or final product) of low and mid mass stars. After a star has converted as much of its matter into energy as possible, it begins to slowly contract. This makes the star very hot and very faint, which places it on the lower left of the HR diagram: the White Dwarf region. The star continues to contract until the force of gravity is balanced by the degenerate pressure of the electrons. At this point, it is known as a white dwarf.

Degenerate electron pressure halts the collapse so the white dwarf maintains a constant radius. Since no new energy is being produced, it begins to cool off slowly. Since the radius is constant and the temperature is decreasing, the luminosity-radius-temperature relation tells us that the star slowly dims as well (on the order of billions of years). Therefore, the path a white dwarf star would trace on the HR diagram would proceed diagonally down and to the right (decreasing luminosity and temperature). The light emitted from a white dwarf is due only to its residual heat, since it has no internal energy source. Once a white dwarf has radiated off all of its residual heat, it emits no light and is referred to as a black dwarf.

It can be shown that the radius of a white dwarf can be determined from its mass. The more massive a white dwarf is, the smaller its radius will be (since a more massive star has a higher gravity and is therefore compressed to a smaller size). So, if we consider stellar cores that are more and more massive, we know that they will be smaller and smaller in size. Eventually, we can imagine a white dwarf with a mass that is so large that its radius would have to be zero. We cannot have white dwarf stars which are more massive than this because we cannot have radii less than zero. This mass limit is known as the Chandrasekhar limit. It states that the largest mass that a white dwarf can have is 1.4 solar masses. In general, most White Dwarf stars have masses of about 0.6 solar masses and are about the size of the Earth.

The Stellar Graveyard , an example of white dwarf stars. For more information on these White Dwarf Stars, click here.
A Cataclysmic Variable , a white dwarf and normal star binary system. In this illustration, the normal star looks like a balloon that is losing air and the white dwarf is at the center of the swirling disk of material. To learn more about white dwarfs in cataclysmic systems like this one, click here.