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Q & A: fusion limits

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Most recent answer: 05/16/2013
Q:
Hi, lost myself in the site, like it thanks. a question about stars.. I have a strong fasination regarding stars and the universe in general and would be very gratefull if you could further my understanding of it by answering this question that is eluding me. I understand that hydrogen fuses inside stars to form heavier elements, helium, carbon, oxygen ect.. but i dont understand what is special about iron and why it wont fuse. Why does the fusion process stop there? Reading many articles on stars has got me as far as knowing that the process uses energy rather than creates it? This really puzzles me because i thought fusion in stars was purely a matter of how much "stuff" you have, and the resulting pull of gravity pulling it together with enough force to fuse atoms. What makes iron exempt from this process? surely a star with 40 solar masses could fuse iron? bearing in mind i dont have a big science background! thanks for your time.
- ray (age 25)
england
A:
Great question. There are two main forces important for nuclei. There's a strong nuclear force which holds any nucleons, both protons and neutrons, together. On the other hand there's an electrostatic force which makes protons repel each other. If you had equal numbers of protons and neutrons, small nuclei would stick together due to the very short-range nuclear force, but as they grew the longer-range electrostatic force would win out and make large nuclei fall apart. There would be a mid-size which would be stable. Iron is a mid-size nucleus.
Things get a little more complicated because the proton/neutron ratio also can vary. The bigger the nuclei, the more it pays to have more neutrons, since they don't repel electrically. However, neutrons have a little more energy than protons, so a collection of pure neutrons is also unstable, being able to fly apart into protons and electrons. combing these effects, it turns out that one type of iron nucleus is in fact the most stable, meaning it is the lowest-energy way of combining its ingredients.

Mike W.

(published on 05/16/2013)

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