Nuclear Burning in High Mass Stars

In the most massive stars, a variety of processes take place, depending on the temperature in the core, and leading to the production of heavier and heavier elements.

In a 25 solar mass star, for example, nuclear burning proceeds until all of the helium and heavier elements up to iron (Fe) are formed. Iron will not burn, and so this is where ordinary nucleosynthesis in stars terminates, leaving a central inert iron core.

From the inside out, as the temperature declines, burning in successive shells builds up zones of other heavy elements. We say that the star is layered like an onion.

In contrast to the length of time that hydrogen burning takes place (the Main Sequence lifetime), the duration of these other nuclear burning phases is very short. The last phases before catastrophe are really, really short (see the table below).

Once the nuclear burning phases cease, the core can no longer hold itself up, and it implodes in a supernova.

Stages in the Life of a 25 Solar Mass Star

Burning phase Required temperature Required mean density Duration
------------- --------------------- --------------------- -------------
Hydrogen burning 4 x 107 degrees K 5 gm per cubic cm 7,000,000 years
Helium burning 2 x 108 degrees K 700 gm per cubic cm 700,000 years
Carbon burning 6 x 108 degrees K 200,000 gm per cubic cm 600 years
Neon burning 1.2 x 109 degrees K 4 million gm per cubic cm 1 year
Oxygen burning 1.5 x 109 degrees K 10 million gm per cubic cm 6 months
Silicon burning 2.7 x 109 degrees K 30 million gm per cubic cm 1 day

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