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The Question

(Submitted May 26, 2010)

During stellar evolution, for small and medium stars (until 8-10 solar), during the white dwarf formation process, the star doesn't have enough mass to increase the core temperature to start carbon fusion. After the white dwarf formation process, the core matter is in a degenerate state. But in huge stars, the mass is high enough to allow the core to reach a higher temperature, starting carbon fusion, and forming other chemical elements (until iron formation, when a supernova is born). For large stars, the core is not under a degenerate state (I suppose, because the carbon burning is not a runway reaction).

So, why in the white dwarf formation process does the core matter reach a degenerate state, but it doesn't in huge mass stars? Is it because in large mass stars the carbon fusion occurs in a plasma state core?

The Answer

For a stellar core to be in a degenerate state, the density has to be extremely high, and the temperature cannot be too high. If the core exceeds a certain critical temperature (which also depends on the chemical composition), it cannot become or remain degenerate.

Now, the cores of more massive stars have higher temperatures, because you need higher thermal pressure to balance the gravity of all the matters above. When you work out the numbers, it turns out that the cores of massive stars (more than 8 times the mass of the Sun) are always hot enough not to become degenerate.

For somewhat more details, check out:

http://cass.ucsd.edu/public/tutorial/StevII.html

Hope this helps,

Koji & Georgia
for "Ask an Astrophysicist"

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