  Recent work, based on data from the Hubble Space Telescope (HST) UV Legacy
Survey of Galactic Globular Clusters (GCs), has revealed that all the analyzed
clusters host two groups of first- (1G) and second-generation (2G) stars. In
most GCs, both 1G and 2G stars host sub-stellar populations with different
chemical composition. We compare multi-wavelength HST photometry with synthetic
spectra to determine for the first time the average helium difference between
the 2G and 1G stars in a large sample of 57 GCs and the maximum helium
variation within each of them. We find that in all clusters 2G stars are
consistent with being enhanced in helium with respect to 1G. The maximum helium
variation ranges from less than 0.01 to more than 0.10 in helium mass fraction
and correlates with both the cluster mass and the color extension of the
horizontal branch (HB). These findings demonstrate that the internal helium
variation is one of the main (second) parameters governing the HB morphology.
