Nuclear Fusion in Massive Stars

• The production of new elements is called Nucleosynthesis
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• In massive stars there is an ``onion skin'' of fusion shells with the outer layers dropping fuel to lower layers and heavier and heavier nuclei being cooked up as you move towards the center of the star.

  

• A very common path to building elements is via the Alpha Process where He nuclei (for historical reasons these are known as ``alpha particles'') are added to exisiting nuclei.

• Now, all the statistically likely reactions are such that the PRODUCT nuclei have less mass than the input nuclei with the ``mass defect'' being converted into energy.

  This is the way Nature likes to work - these reactions release energy

• This proceeds all the way to Iron (actually to Nickel that decays back down to Cobalt and Iron).

  For EQUILIBRIUM fusion reactions, Iron is the end of the line

  

• If a reaction lead to MORE binding energy per nucleon (or less nuclear potential energy per nucleon) then this is a reaction that occurs in equilibrium.

  Therefore light elements fuse to push elements up the curve. Similarly for the heavy elements, fission reactions tend to push elements up the curve from the right and release energy. Fusing He to make Be goes the wrong way and this reaction requires energy INPUT to make it happen. Nature doesn't care for these sorts of reactions.

• I like to plot the Potential Energy tied up in the nuclei verse atomic number to make the analogy with Gravitational potential energy clearer:

  

• So Iron (Fe) is the most stable of all the nuclei and if the temperature of the Universe were high enough, this is where everything would be headed.

• By the usual cosmic standards, this all happens very fast. For a 25 stars: