Distribution of Elements Inside Stars

The chemical composition measured in stellar atmospheres is not necessarily the same as in deeper layers (outside the core). Indeed, for a significant fraction of main-sequence G to B types stars the discrepancies between superficial and internal abundances go from a few percent (for the coldest of these stars) to huge factors (for hot chemically peculiar stars). This is due to atomic diffusion process, which may produces elements segregation at some stages of the stellar evolution.