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Unformatted text preview: eac%on becomes more vigorous and nearly all the 3He is destroyed. ques%on 1b
3(b) 4He 16O 12C Like H and He, O and C abundances remain at their photospheric level down to the tachyocline due to convec%ve mixing, then “bump up” in the radia%ve zone due to gravita%onal seJling (there is a slight nega%ve slope in the rela%ve abundances down to 0.2 R). In the fusing core, 4He increases for the reasons given in the previous problem. 12C declines sharply at it is consumed by the rela%vely fast and rela%vely low Coulomb barrier reac%on 12C + 1H -‐> 13N in the CNO cycle, but not replenished by the higher Coulomb barrier 15N + 1H -‐> 12C reac%on, nor by the triple alpha process 4He + 4He + 4He -‐> 12C due to that reac%on’s low probability at low temperatures (remember, reac%on rates for triple alpha scale as T41!). 16O abundance decreases only slightly in the center of the Sun due to the rare 16O + 1H -‐> 17F CNO reac%on; the high temperature (high...
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