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Type Ia supernovae (SNe Ia) have been used empirically as standardized candles to reveal the accelerating universe even though fundamental details, such as the nature of the progenitor system and how the star explodes, remained a mystery. There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary could be anything from a main sequence star to a red giant, or even another white dwarf. The uncertainty stems from the fact that no recent SN Ia has been discovered close enough to detect the stars before explosion. Here we report early observations of SN 2011fe (PTF11kly) in M101 at a distance of 6.4 Mpc, the closest SN Ia in the past 25 years. We find that the exploding star was likely a carbon-oxygen white dwarf, and from the lack of an early shock we conclude that the companion was most likely a main sequence star. Early spectroscopy shows high-velocity oxygen that varies on a time scale of hours and extensive mixing of newly synthesized intermediate mass elements in the outermost layers of the supernova. A companion paper uses pre-explosion images to rule out luminous red giants and most helium stars as companions.
Nugent, P. E.; Sullivan, M.; Cenko, S. B.; Thomas, R. C.; Kasen, D.; Howell, D. A.; Bersier, D.; Bloom, J. S.; Kulkarni, S. R.; Kandrashoff, M. T.; Filippenko, A. V.;Silverman, J. M.; Marcy, G. M.; Howard, A. W.; Isaacson, H. T.; Maguire, K.; Suzuki, N.; Tarlton, J. E.; Pan, Y.-C.; Bildsten, L.; Fulton, B. J.; Parrent, J. T.;Sand, D.;Podsiadlowski, P.; Blanco, F. B.; Dilday, B.; Graham, M. L.; Lyman, J.; James, P.; Kasliwal, M. M.; Law, N. M.; Quimby, R. M.; Hook, I. M.; Walker, E. S.; Mazzali, P.;Pian, E.; Ofek, E. O.; Gal-Yam, A.; Poznanski, D.