Multiple Population Theory: Extreme helium population problem
  The spreads in chemical abundances inferred by recent precision observations
suggest that some or possibly all globular clusters can no longer be considered
as simple stellar populations. The most striking case is omega Cen in the sense
that its bluest main-sequence despite its high metallicity demands an extreme
helium abundance of Y > 0.4. I focus on this issue of "the extreme helium
population problem" in this review.
