In a massive star (i.e. an O5 star), first hydrogen is fused into helium, then helium is fused into carbon. This process continues to neon, oxygen, and finally silicon burning. So I am wondering, how long does it take for each process to commence?


My stellar astrophysics text, Francis LeBlanc's An Introduction to Stellar Astrophysics, gives the following quantities for the stages of burning in a $25M_{\odot}$ star (citing models by Arnould & Samyn 2001). This is somewhat less massive than a typical main sequence O5-type star, and with higher masses come higher core temperatures and shorter evolutionary timescales. Nonetheless, it should give you a reasonable order-of-magnitude idea of how long it takes for these processes to play out in a massive star.

Phase Central temperature (K) Timescale
Hydrogen $6.0\times10^7$ $7\times10^6$ years
Helium $2.0\times10^8$ $5\times10^5$ years
Carbon $9.0\times10^8$ $600$ years
Neon $1.7\times10^9$ $0.5$ years
Oxygen $2.4\times10^9$ 6 days
Silicon $4.0\times10^9$ 1 days

(One major caveat is that these models are two decades old, and stellar evolutionary codes are constantly being improved. However, I don't believe that these are old enough to be significantly out-of-date!)


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