  Two "twin" Type Ia supernovae (SNe Ia), SNe 2011by and 2011fe, have extremely
similar optical light-curve shapes, colors, and spectra, yet have different
ultraviolet (UV) continua as measured in Hubble Space Telescope spectra and
measurably different peak luminosities. We attribute the difference in the UV
continua to significantly different progenitor metallicities. This is the first
robust detection of different metallicities for SN Ia progenitors. Theoretical
reasoning suggests that differences in metallicity also lead to differences in
luminosity. SNe Ia with higher progenitor metallicities have lower 56Ni yields,
and lower luminosities, for the same light-curve shape. SNe 2011by and 2011fe
have different peak luminosities (Delta M_V = 0.6 mag), which correspond to
different 56Ni yields: M_11fe(56Ni)/M_11by(56Ni) = 1.7^+0.7_-0.5. From
theoretical models that account for different neutron to proton ratios in
progenitors, the differences in 56Ni yields for SNe 2011by and 2011fe imply
that their progenitor stars were above and below solar metallicity,
respectively. Although we can distinguish progenitor metallicities in a
qualitative way from UV data, the quantitative interpretation in terms of
abundances is limited by the present state of theoretical models.
