The stars in a Globular cluster ( GC ) have always been considered coeval and of the same metallicity .
Recently , this assumption has been challenged on the basis of spectroscopic and photometric observations , which show the existence of various generations of stars in GCs , differing in the abundances of products of H-burning at high temperatures .
The main final product of this burning is He .
It is then important to study the connections between stars properties and He content .
We consider here the about 1400 stars on the Red Giant Branch ( RGB ) observed with FLAMES @ VLT in 19 Galactic Globular Clusters ( GCs ) in the course of our project on the Na-O anticorrelation .
Stars with different He are expected to have different temperatures ( i.e. , different colours ) , slightly different metallicities [ Fe/H ] , and different luminosity levels of the RGB bump .
All these differences are small , but our study has the necessary precision , good statistics , and homogeneity to detect them .
Besides considering the observed colours and the temperatures and metallicities determined in our survey , we computed suitable sets of stellar models - fully consistent with those present in the BaSTI archive - for various assumptions about the initial helium content .
We find that differences in observable quantities that can be attributed to variations in He content are generally detectable between stars of the Primordial ( P , first-generation ) and Extreme ( E , second-generation ) populations , but not between the Primordial and Intermediate ones ( I ) .
The only exception , where differences are significant also between P and I populations , is the cluster NGC 2808 , where three populations are clearly separated also on the Main Sequence and possibly on the Horizontal Branch .
The average enhancement in the He mass fraction Y between P and E stars is about 0.05-0.11 , depending on the assumptions .
The differences in Y , for NGC 2808 alone , are of about 0.11-0.14 between P and I stars , and about 0.15-0.19 between P and E stars , again depending on the assumptions .
When we consider the RGB bump luminosity of first and second-generation stars we find different levels ; the implied Y difference is more difficult to quantify , but is in agreement with the other determinations .