Hubble Space Telescope analysis of stellar populations within the
  globular cluster G1 (Mayall II) in M31
  In this paper we present a multi-wavelength analysis of the complex stellar
populations within the massive globular cluster Mayall II (G1), a satellite of
the nearby Andromeda galaxy projected at a distance of 40 kpc. We used images
collected with the Hubble Space Telescope in UV, blue and optical filters to
explore the multiple stellar populations hosted by G1. The $m_{\rm F438W}$
versus $m_{\rm F438W}- m_{\rm F606W}$ colour-magnitude diagram shows a
significant spread of the red giant branch, that divides $\sim 1$ mag brighter
than the red clump. A possible explanation is the presence of two populations
with different iron abundance or different C+N+O content, or different helium
content, or a combination of the three causes. A similar red giant branch split
is observed also for the Galactic globular cluster NGC6388. Our
multi-wavelength analysis gives also the definitive proof that G1 hosts stars
located on an extended blue horizontal branch. The horizontal branch of G1
exhibits similar morphology as those of NGC6388 and NGC6441, which host stellar
populations with extreme helium abundance (Y>0.33). As a consequence, we
suggest that G1 may also exhibit large star-to-star helium variations.
