Stellar fusion

[dagaz]

I've been thinking about the process of nuclear fusion inside stars and have a few questions-

1) If two hydrogen atoms fuse together to form one helium atoms, where do the neutrons come from? Do they come from free neutrons or isotopes of hydrogen or some other source?

2) Assuming the answer to 1 is deuterium and tritium, doesn't the scarcity of these isotopes limit the amount of amount of fusion that can take place inside a star?

3) It is said that as the star approaches the end of its life it starts to undergo helium fusion reactions, if this is so why is there not a higher abundance of Beryllium?

[dagaz]

I just found the answers to my first two questions here. I have read a brushed-over version of this before (Simon Singh's "Big Bang"), but because it left out so many details it raised more questions than it answered.

The thread I linked to above, however, is the most concise and articulate explanation of this process I have come across, which makes me wonder why the poster was banned after more than a thousand posts.

Interestingly, the thread did not come up in a forum search, but after I had already posted came up in the Similar posts feature at the bottom of my thread.

[Jay-qu]

well keep asking those questions

You say you got the first to answered, so as for the 3rd: when 2 helium nuclie, 4protons 4nutrons, fuse they make Beryllium-8, which is unstable, but if there is enough pressure in the star a 3rd helium nuclie will fuse with the Beryllium and turn it into stable carbon-12

[dagaz]

Originally Posted by Jay-qu

You say you got the first to answered, so as for the 3rd: when 2 helium nuclie, 4protons 4nutrons, fuse they make Beryllium-8, which is unstable, but if there is enough pressure in the star a 3rd helium nuclie will fuse with the Beryllium and turn it into stable carbon-12

Thanks for that.

When you say that Beryllium-8 is unstable, who long does it remain in this state for, i.e. what is the window for fusion with the third helium nuclei? Also is the window open long enough for potentially two Beryllium nuclei to fuse together into oxygen?

[Jay-qu]

its half life is 0.00000000000000007 seconds so its not around very long, which means it needs a very high pressure so that the chance of collision in this time is great enough for continued fusion. No doubt it is possible for oxygen to also form, but an even greater pressure would be needed because the electrostatic repulsion increases with the size of the nucleus.

[Tim_Lou]

perhaps under such high gravitional influences, the half life would be greater in our frame of references...

[Jay-qu]

I was thinking this, but in the frame that counts (the suns) it is a very small half life.

[Qfwfq]

All the elements were generated in the stars.

[Jay-qu]

Originally Posted by Qfwfq

All the elements were generated in the stars.

you think? what about primordial hydrogen, or the results of a H-bomb?

[Tim_Lou]

Originally Posted by Jay-qu

I was thinking this, but in the frame that counts (the suns) it is a very small half life.

yeah and the sizes or the space between molecules will be different than what we see outside (distorted)...anyway its all equivalent.