  In this study we present first results from multi-wavelength Hubble Space
Telescope (HST) observations of the Galactic globular cluster GC NGC2808 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 NGC2808 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 NGC2808 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~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.
