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The Mass of a White Dwarf
White dwarfs are extremely dense stars. They possess the mass of around half the sun's mass, yet are only slightly larger than the Earth. White dwarfs are 200,000 times denser than Earth. One teaspoon of white dwarf material would weigh 5.5 tons on Earth. Only neutron stars and black holes are denser than white dwarfs.
The Temperature of a White Dwarf
Because there is no fusion taking place within white dwarfs, they continuously cool down. Initially, surface temperatures are around 100,000 Kelvin. Not all white dwarfs are white; rather, their color is related to their temperature, acting like a visual thermometer. They appear blue at around 50,000 Kelvin, white at around 10,000 Kelvin, and yellow and red below 6,000 Kelvin. Blue white dwarfs are rare to find, as white dwarfs cool very quickly initially. Yellow and red white dwarfs are equally hard to find because they are very dim. When white dwarfs cool completely, they become black dwarfs.
The Composition of a White Dwarf
The core of a white dwarf is composed of a crystalline lattice of carbon and oxygen atoms. This cooling hunk of carbon is the end-product of the triple-alpha process, whereby helium is fused into carbon. This occurs while the star is a red giant. Stars that form white dwarfs do not have enough mass to generate the heat required to fuse the carbon into heavier elements.
What Size Stars Become White Dwarfs?
White dwarfs are formed by main sequence stars; however, with stars, size does matter. The size of stars is measured in solar masses, with one solar mass equaling the mass of the sun. Only stars that are less than four solar masses will become white dwarfs.
Formation of White Dwarfs
Stars balance their massive gravity with outward force generated by the fusion of hydrogen into helium. When stars less than four solar masses run out of hydrogen, they collapse under the force of gravity. As they collapse, they generate heat. This heat reignites the last remaining hydrogen and they expand to form red giants. During this phase, they are able to fuse their helium into carbon. The star then collapses again, blowing off its outer mass to form a planetary nebula. The core that remains in its center forms a white dwarf, slowly cooling over billions of years.
The Difference Between White Dwarfs and Neutron Stars
White dwarfs are kept from collapsing any further by the Pauli Exclusion Principle. Simply put, they do not have enough mass to force their electrons any closer, known as the electron degeneracy barrier. The maximum core mass for a white dwarf is 1.44 solar masses, known as the Chandrasekhar limit. However, cores over 1.44 solar masses will continue to collapse until they reach the neutron degeneracy barrier, forming neutron stars. Cores over three solar masses will collapse to form black holes.