The accelerating expansion of the Universe, and the need for dark energy, were inferred from observations^{1, 2} of type Ia supernovae. There is a consensus that type Ia supernovae are thermonuclear explosions that destroy carbon–oxygen white dwarf stars that have accreted matter from a companion star³, although the nature of this companion remains uncertain. These supernovae are thought to be reliable distance indicators because they have a standard amount of fuel and a uniform trigger: they are predicted to explode when the mass of the white dwarf nears the Chandrasekhar mass⁴ of 1.4 solar masses (M). Here we show that the high-redshift supernova SNLS-03D3bb has an exceptionally high luminosity and low kinetic energy that both imply a super-Chandrasekhar-mass progenitor. Super-Chandrasekhar-mass supernovae should occur preferentially in a young stellar population, so this may provide an explanation for the observed trend that overluminous type Ia supernovae occur only in 'young' environments^{5, 6}. As this supernova does not obey the relations that allow type Ia supernovae to be calibrated as standard candles, and as no counterparts have been found at low redshift, future cosmology studies will have to consider possible contamination from such events.

1. Department of Astronomy and Astrophysics, University of Toronto, 60 St George Street, Toronto, Ontario M5S 3H8, Canada
2. Lawrence Berkeley National Laboratory, Mail Stop 50-232, 1 Cyclotron Road, Berkeley, California 94720 USA
3. California Institute of Technology, East California Blvd, Pasadena, California 91125, USA
4. DAPNIA/Service d'Astrophysique, CEA/Saclay, 91191 Gif-sur-Yvette Cedex, France
5. LPNHE, CNRS-IN2P3 and University of Paris VI and VII, 75005 Paris, France
6. Department of Physics and Astronomy, University of Victoria, P O Box 3055, Victoria, British Columbia V8W 3P6, Canada
7. LAM CNRS, BP8, Traverse du Siphon, 13376 Marseille Cedex 12, France
8. CPPM, CNRS-IN2P3 and University Aix Marseille II, Case 907, 13288 Marseille Cedex 9, France
9. University of Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK

Correspondence to: D. Andrew Howell¹ Correspondence and requests for materials should be addressed to D.A.H. (Email: howell@astro.utoronto.ca).

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