Abstract
FOR any globular cluster, an observationally estimated number ratio NHB/NRG can be compared with a theoretically estimated lifetime ratio tHB/tRG to obtain an estimate of the initial abundance of helium which characterizes cluster stars. Here NHB is the total number of stars along the horizontal branch and NRG is the number of those stars along the red giant branch that are as bright as or brighter than the horizontal branch at roughly the level of the RR Lyrae stars that lie directly above cluster turnoff. The ratio of relevant lifetimes is a moderately strong function of helium abundance1 that varies2 from about 0.4 when Y (helium abundance by mass) ∼0.1 to about 1 when Y ∼ 0.3.
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IBEN, I., ROOD, R., STROM, K. et al. Ratio of Horizontal Branch Stars to Red Giant Stars in Globular Clusters. Nature 224, 1006–1008 (1969). https://doi.org/10.1038/2241006a0
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DOI: https://doi.org/10.1038/2241006a0
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