Stellar nuclear fusion question?

Question

When a star runs out of hydrogen to burn, it starts burning helium and so on until all of the fuel source is burned away into iron. My question is after a star has burned past its' main sequence and is burning helium, is the core temperature of the star lower? higher? the same?
It seems to me that the temperature would have to be higher in order to fuse these heavier elements.

As an afterthought - How, if the temperature must be higher for the post-main sequence reactions does the star attain these higher temperatures since it will undoubtedly be smaller from the release of so much energy?

Answer 1

Stars gain their energy through gravitational collapse.

When the collapse raises core temperatures and pressures sufficiently, nuclear reactions begin. These reactions produce a lot of energy and a lot of photons. The photon pressure is sufficient to stop further collapse of the star.

When the fusion materials are exhausted, collapse resumes. The core temperature and pressure then rise. When they are hot enough, further fusion of larger atoms occurs.

This continues up the periodic table until fusion is no longer possible.

The star then coninues to collapse to a very hot, degenerate ball. In some cases the temperature on the surface then gets high enough to cause a catastropic runaway fusion reaction of the helium on the surface, because the rate of fusion of helium is temperature dependent.

The start the explodes as a supernova. This throws off some matter (though not actually that much). The very high energy of the explosion is enough to produce heavier elements through nucleosynthesis.

What is left is a tiny but very very hot star called a white dwarf. Because it is so tiny, it does not radiate much energy (it has a small surface area) so it is quite dim.

Answer 2

Temperature is higher. Core temperature increase with core density all the way up to the white hot dwarf stars.

Answer 3

makes sence. The relative velocities of the atoms would have to be greater because they have a larger force to overcome (there are more protons which equate to a larger nuclear strong/electromagnetic force). And temperature is just a measure of the average veolicity of an atom. So larger velocities are needed to overcome the force and larger velocities means a higher temperature.

Answer 4

It is higher, though some fusion of Helium is ongoing anyway. The fusion area must be much smaller and hotter in this second stage.

Answer 5

A helium burning star is a red giant, it is hotter in the core and bigger and cooler at the surface.

Answer 6

I thought it was higher temperature, but I don't remember for sure. I am sure if you google "life of a star" you'll find a lot of information about it.

Answer 7

Go with EPIDAVROS's answer. He's got it all correct. The only thing in his explanation that won't happen with our sun is the supernova event. Not enough mass for that.

Answer 8

Temperature will high

Answer 9

We know Sol, our sun, is becoming 10% hotter each million years (I think), so I'm going with "hotter."

Answer 10

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