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Type Ia supernova (SNe Ia) have become popular in recent years as luminosity distance indicators in cosmological experiments to study the dark energy; however, the progenitor systems, explosion mechanisms, and environments of these rare, luminous events are still poorly understood. Most SNe Ia are believed to result from the thermonuclear explosion of a single carbon-oxygen white dwarf, which accretes matter from a non-degenerate binary companion star until it reaches the Chandrasekhar limit (1.4 solar masses). I will present observations of an unusual SN Ia, 2007if, from the Nearby Supernova Factory, a wide-area SN Ia search and follow-up effort which ran from 2003 to 2008. The extreme brightness of the SN and the low, slowly-changing expansion velocity of the Si II 6355 absorption feature suggest that SN 2007if was probably a merger of two white dwarfs with a total mass of 2.4 ± 0.2 solar masses, well in excess of the Chandrasekhar limit and at the edge of practical limits allowed by evolution of stellar systems resulting in binary white dwarfs. SN 2007if is a member of an emerging subclass of overluminous SN Ia events which are thought to have supermassive progenitors, and its properties help to clarify the physical nature of the subclass. These SNe do not obey the usual light curve relations for luminosity standardization, and may be merely the most extreme representatives of a broader subclass of events which must be standardized separately or excluded from cosmological Hubble diagrams. |