D.4b Fusion off the main-sequence, nucleosynthesis and supernovae

Nucleosynthesis off the main-sequence

Nuclear fusion in main-sequence stars converts hydrogen to helium by either the proton-proton chain or the carbon-nitrogen-oxygen cycle. For almost every other element in our universe, the main source is post-mainstream stars - either the atmospheres of red giant stars or in the heart of supernovae. Elements up to iron can be produced through nuclear fusion in increasingly massive stars. Elements heavier than iron rely on the s process or the r process. In the s (slow) process, isotopes gain mass by absorbing neutrons, and then decay by beta decay to form new elements. This takes place over millennia in a stellar atmosphere. In the r process, the same neutron capture occurs - but in the extremely dense neutron flux generated by a supernova explosion there is no time for beta decay and much heavier elements can be built.

Great blog post on this topic: https://www.geraintflewis.com/post/on-the-matter-of-matter

Type Ia and II supernovae

Any supernova explosion is an extremely violent event, in which a single star can shine as brightly as a galaxy. However, there are different types of supernovae.

Type 1a: This is the binary system supernova we have looked at previously. A white dwarf accretes material from a nearby companion star, until it has more than the Chandrasekhar limit (about 1.4 solar masses) at which point electron degeneracy pressure can no longer overcome gravity and the star explodes. As this always happens at the same mass, this type of supernova is of predictable luminosity and therefore makes a good standard candle.

Type II: This is standard core-collapse supernova. A star at the end of a short but exciting life leaves a core with a mass greater than the Chandrasekhar limit, thus resulting in a type II supernova explosion.