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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?

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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 However, I don't believe that these are old enough to be significantly out-of-date!)

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