In this study we present first results from multi-wavelength Hubble Space Telescope ( HST ) observations of the Galactic globular cluster ( GC ) NGC 2808 as an extension of the Hubble Space Telescope UV Legacy Survey of Galactic GCs ( GO-13297 and previous proprietary and HST archive data ) .
Our analysis allowed us to disclose a multiple-stellar-population phenomenon in NGC 2808 even more complex than previously thought .
We have separated at least five different populations along the main sequence and the red giant branch ( RGB ) , that we name A , B , C , D and E ( though an even finer subdivision may be suggested by the data ) .
We identified the RGB bump in four out of the five RGBs .
To explore the origin of this complex CMD , we have combined our multi-wavelength HST photometry with synthetic spectra , generated by assuming different chemical compositions .
The comparison of observed colors with synthetic spectra suggests that the five stellar populations have different contents of light elements and helium .
Specifically , if we assume that NGC 2808 is homogeneous in [ Fe/H ] ( as suggested by spectroscopy for Populations B , C , D , E , but lacking for Population A ) and that population A has a primordial helium abundance , we find that populations B , C , D , E are enhanced in helium by \Delta Y \sim 0.03 , 0.03 , 0.08 , 0.13 , respectively .
We obtain similar results by comparing the magnitude of the RGB bumps with models .
Planned spectroscopic observations will test whether also Population A has the same metallicity , or whether its photometric differences with Population B can be ascribed to small [ Fe/H ] and [ O/H ] differences rather than to helium .