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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2Here's a timetable for a 25 $M_\odot$ star. https://en.wikipedia.org/wiki/Type_II_supernova#Formation – PM 2Ring Dec 16 '20 at 08:45
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1@PM2Ring You can use this in an answer :D – WarpPrime Dec 16 '20 at 14:08
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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 However, I don't believe that these are old enough to be significantly out-of-date!)
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