The fraction of second generation stars in globular clusters from the

The majority of globular clusters show chemical inhomogeneities in the composition of their stars, apparently due to a second stellar generation in which the forming gas is enriched by hot-CNO cycled material processed in stars belonging to a first stellar generation. Clearly this evidence prompts questions on the modalities of formation of globular clusters. An important preliminary input to any model for the formation of multiple generations is to determine which is today the relative number fraction of ‘normal ’ and anomalous stars in each cluster. As it is very difficult to gather very large spectroscopic samples of globular cluster stars to achieve this result with good statistical significance, we propose to use the horizontal branch (HB). We assume that, whichever the progenitors of the second generation, the anomalies also include enhanced helium abundance. In fact, helium variations have been recently recognized to be able to explain several puzzling peculiarities (gaps, RR Lyr periods and period distribution, ratio of blue to red stars, blue tails) in HBs. We summarize previous results and extend the analysis in order to infer the percentage in number of the first and second generation in as many clusters as possible. We show that, with few exceptions, approximately 50 per cent o