What is a white dwarf?

A white dwarf is a type of star that contains about as much matter as the Sun, but packed into a size comparable to the Earth. The majority of white dwarfs are thought to be made mostly of carbon and oxygen.

In stars like the Sun, the inward pull of gravity is balanced by the outward push of the high-temperature hydrogen in the center fusing into helium and releasing energy in the process. There is no nuclear fusion in a white dwarf. Instead, the force that opposes gravity is called "electron degeneracy pressure".

What does a white dwarf look like?

A white dwarf looks more or less like any other star - a tiny point of light.

Astronomers distinguish white dwarfs from other stars in two ways: (1) since they are faint stars, we can only see the nearby ones, and nearby stars appear to move relative to the background stars (2) they emit most of their light in the blue part of the spectrum.

For a dramatic view of a white dwarf at the center of an ejected shell of gas, take a look at this.

Who discovered the first white dwarf?

The first white dwarf ever observed is called "Sirius B" and was discovered by Alvan Clark (a telescope maker) in 1862.

How does a star become a white dwarf?

The exact process of a star becoming a white dwarf depends on the mass of the star, but all stars less massive than about 8 times the mass of the Sun (99% of all stars) will eventually become white dwarfs.

Normal stars fuse hydrogen into helium until the hydrogen deep in the center begins to run out. For very massive stars this may take only a million years - but for stars like the Sun the hydrogen lasts for 10,000 million years. When enough helium is produced from fusion, it begins to sink to the middle of the star and release some heat in the process. This messes up the internal balance of the star, and it begins to bloat into a red giant.

If the star is massive enough, it may eventually get hot enough in the center to fuse the helium into carbon and oxygen. The star then enjoys another relatively stable period, though much shorter this time. The carbon and oxygen, in their turn, sink to the middle. If the star isn't massive enough to reach the temperature needed to burn carbon and oxygen into heavier elements, then these elements will simply collect in the center until the helium fuel runs out. In the end, you have a carbon/oxygen white dwarf star.

Where can I find out more about the evolution of stars?

Try this ExploraTour: A Peek into the Lives of Stars at NCAR.

What is Chandrasehkar's limit?

Chandrasehkar's limit is the upper limit to the mass of a white dwarf star, about 1.4 times the mass of the Sun.

Why is there an upper limit to the mass of a white dwarf star?

When atomic nuclei are squeezed very close together, the low-energy orbits that electrons would normally be able to occupy around them overlap with similar orbits in neighboring nuclei - so the orbits become indistinguishable. The rules of quantum mechanics tell us that no two electrons can occupy the same orbit, so the electrons are forced into higher-energy orbits (pushed to higher speeds) just because of the density of the matter.

This quantum pressure can oppose gravity as long as the density doesn't get too high. If a white dwarf has more than 1.4 times the mass of the Sun squeezing the nuclei, there will be too few orbits available to the electrons (since they cannot travel faster than the speed of light) and the star will collapse - causing a supernova explosion.

What is the difference between a white dwarf and a red dwarf?

"Dwarf" is a general term for stars with smaller masses. Our Sun is termed a "yellow dwarf" and there are many stars with masses less than the Sun called "red dwarfs". Very low mass objects that do not have enough material to ignite nuclear fusion in the center are called "brown dwarfs".

The color of a star is an indication of the temperature at its surface. Very hot objects emit more blue-white light, while cooler objects emit more red light.