Supernova SN 2011fe from an exploding carbon–oxygen white dwarf star

Multi-instrument detection of a nearby type 1a supernova shows that the exploding star was probably a carbon–oxygen white dwarf star in a binary system with a main-sequence companion. Identification of a supernova companion Supernova 2011fe in the Pinwheel galaxy, discovered by the Palomar Transient Factory on 24 August 2011, is the brightest type Ia supernova that's been seen from Earth for many years. Type Ia supernovae are thought to result from a thermonuclear explosion of an accreting white dwarf in a binary system, but little is known of the precise nature of the companion star and the physical properties of the progenitor system. Two new reports of observations of SN 2011fe narrow down the range of possibilities for the mystery companion. Nugent et al . present some of the earliest data ever obtained from a type Ia supernova. They find that the exploding star was probably a carbon–oxygen white dwarf, and conclude from the lack of an early shock that the companion may have been a main sequence star. Li et al . analysed pre-discovery images in the Hubble Space Telescope archives and find that no object was visible before the explosion. That rules out luminous red giants and the vast majority of helium stars as the mass-donating companion to an exploding white dwarf. Type Ia supernovae have been used empirically as ‘standard candles’ to demonstrate the acceleration of the expansion of the Universe 1 , 2 , 3 even though fundamental details, such as the nature of their progenitor systems and how the stars explode, remain a mystery 4 , 5 , 6 . There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary body could be anything from a main-sequence star to a red giant, or even another white dwarf. This uncertainty stems from the fact that no recent type Ia supernova has been discovered close enough to Earth to detect the stars before explosion. Here we report early observations of supernova SN 2011fe in the galaxy M101 at a distance 7 from Earth of 6.4 megaparsecs. We find that the exploding star was probably a carbon–oxygen white dwarf, and from the lack of an early shock we conclude that the companion was probably a main-sequence star. Early spectroscopy shows high-velocity oxygen that slows rapidly, on a timescale of hours, and extensive mixing of newly synthesized intermediate-mass elements in the outermost layers of the supernova. A companion paper 8 uses pre-explosion images to rule out luminous red