The helium abundances of ω Centauri

Recent Hubble Space Telescope color magnitude diagrams (CMDs) of omega Centauri, which clearly show three sequences in the region of the main-sequence turnoff together with a double lower main sequence, are explained in terms of three populations having canonical heavy-element abundances ([Fe/H]=-1.7, -1.2, and -0.6), reasonable proportions and ages, but with surprising helium abundances. In particular, the CMD morphology of the second population ([Fe/H]=-1.2) is very well represented with a helium abundance much larger, by DeltaYsimilar to0.12, than that of the first ([Fe/H]=-1.7), which may hold the key to an understanding of the double lower main sequence.