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View Full Version : Attention budding nuclear physicists


RedFred
17-07-2003, 19:57:44
Seriously.

I am reading Richard Feynman's book of lectures he gave back in 1965 called The Characteristics of Physical Law. They are aimed at 'the general public' so the handwaving he does to, for example, explain the laws of gravity without resorting to calculus is amusing. But he is extremely lucid throughout.

The tricky bit that I am having trouble understanding is on the creation of different elements from hydrogen from the nuclear fusion going inside stars. I basically understand how you can fuse hydrogen atoms together to get helium. For that matter I understand the curve of binding energy, which means that you could keep fusing various light elements together while releasing energy to get heavier ones up to about iron; conversely that you could keep splitting apart large, heavy atoms or waiting for them to decay until you got something about as light as iron.

What I don't get is how you could ever get anything heavier than iron from the fusion process. So here is what Feynman says [p.117]:
"If three helium atoms could come together to form carbon, we can easily calculate how often that should happen in a star. And it turns out that it should never happen, except for one possible accident - if there happened to be an energy level at 7.82 million volts in carbon, then the three helium atoms would come together and before they came apart, would stay together a little longer on the average than they would do if there were no level at 7.82. And staying there a little longer, there would be enough time for something else to happen, and to make other elements. If there was a level at 7.82 million volts in carbon, then we could understand where all the other elements in the periodic table came from. And so, by a backhanded upside-down argument, it was predicted that there is in carbon a level at 7.82 million volts; and experiments in the laboratory showed that indeed there is."

Sounds like some serious handwaving going on here. Can anyone explain this slightly more thoroughly?

Darkstar
17-07-2003, 22:09:53
Don't know about that... but...

In standard star lifespan, you don't really go past helium fusion unless you have massive stars. There's several orders of "massive stars" though, so it does happen occasionally. Other then those massive stars, most heavy elements are the result of nova, super-nova, and near black hole collisions between matter that is thrown into escape arcs.

You do see carbon-oxygen fusion in stars that like 50 times more massive then our sun, if I remember my astronomy correctly. This doesn't happen until the late stages of the star's life though, after it has gone through a stage where it has cast off it's outer shell, and become a white dwarf core.

And you even see neon-carbon fusion in stars that are like 120 times more massive then our sun. Above that, I haven't read of any fusion cycles. But then, those stars that massive easily turn into black holes in their later life cycles, so we have trouble find them to study them. ;)