Astrophysics - triple alpha core Temp

[supersam]

Homework Statement

Explain why stage 1 and 2 of the triple alpha process will not being until the core Temp of the star is around 10^8K.

Homework Equations

4He+4He = 8Be (step 1)
8Be+4He = 12C (Step 2)

The Attempt at a Solution

I "think" I need to work out the nuclear energies... However I am a little stuck on how to do this. This is a marked assignment so I'd appreciate a nudge in the right direction, rather than any form of a solution.

I have a suspicion that I need to use T^v, where v = (EG/4kT)^1/3 - 2/3; where EG is the gammon energy. However I would like just a heads up if this is the right idea.

Thanks

 

[SteamKing]

Homework Statement

Explain why stage 1 and 2 of the triple alpha process will not being until the core Temp of the star is around 10^8K.

Homework Equations

4He+4He = 8Be (step 1)
8Be+4He = 12C (Step 2)

The Attempt at a Solution

I "think" I need to work out the nuclear energies... However I am a little stuck on how to do this. This is a marked assignment so I'd appreciate a nudge in the right direction, rather than any form of a solution.

I have a suspicion that I need to use T^v, where v = (EG/4kT)^1/3 - 2/3; where EG is the gammon energy. However I would like just a heads up if this is the right idea.

Thanks

Perhaps you should review some of the basics of the triple alpha reaction first before going down the rabbit hole:

http://en.wikipedia.org/wiki/Triple-alpha_process

It takes a while before a star with a hydrogen core eagerly fusing away accumulates enough helium to interfere with that reaction. And when hydrogen fusion is interrupted, what happens in the core?

 

[supersam]

Okay - looks like I might have been getting ahead of myself. So the Helium "burning" starts due to huge temperatures when the core of the star collapses. Which in turn creates Beryllium and then Carbon. However I am still not exactly sure how the 10^8K value is determined. I can understand that the energy comes from the Gravitational Potential energy - but how do you show that 10^8K is the magic number?

 

[SteamKing]

Okay - looks like I might have been getting ahead of myself. So the Helium "burning" starts due to huge temperatures when the core of the star collapses. Which in turn creates Beryllium and then Carbon. However I am still not exactly sure how the 10^8K value is determined. I can understand that the energy comes from the Gravitational Potential energy - but how do you show that 10^8K is the magic number?

Look at the conditions necessary for helium fusion to take place after hydrogen fusion is no longer able to support the core against collapse.

http://en.wikipedia.org/wiki/Helium_flash