How Stars Freeze and Shrink

WASHINGTON (ISNS) -- Stars are enormous glowing balls of gas emitting light and heat, until they reach the “senior citizen” age status and become White Dwarfs.

Although astronomers have known that White Dwarf stars cool over time for many years, a nuclear physicist has recently made computer models to watch these stars freeze. He’s also been able to determine the melting temperature, age, and composition of their “star crystals”.

Charles Horowitz, a professor at Indiana University in Bloomington explains that his research should allow astronomers to infer the amount of carbon and oxygen in the interior of White Dwarfs, “helping to show how nuclear reactions in stars make the carbon in our bodies and the oxygen that we breathe.”

A white dwarf star actually freezes first in the center, where the density is highest, to become a solid mixture of carbon and oxygen -- so packed that the elements form a crystal-like arrangement. Then the rest of the star gradually freezes from the inside out until the whole star becomes frozen.

In their experiments, Dr. Horowitz and his associates used a computer model to study the behavior of tens of thousands of carbon and oxygen ions. By adjusting the temperature to keep about half of the controlled environment solid and the other half liquid, they were able to determine the exact melting temperature of material in a White Dwarf.

White dwarf stars freeze for two major reasons, Horowitz said: cooling and shrinking. White dwarf stars start out hot, but don’t produce new heat like younger stars do. White Dwarfs steadily cool down over time as their stored heat very slowly radiates away. Also, when the star gets to a certain size its gravity pulls the star’s material in to create a high pressure. Gravity is strong because a white dwarf has a large amount of mass -- about as much as the sun -- in a very small size -- about the size of the earth. The high pressure increases the density of the material and this makes it much easier to freeze.