Astronomers believe that a supernova occurs when a star explodes at the end of its life. Supernovae are widely used as “standard candles” in cosmology, and observations of type 1a supernovae provided the first evidence that the expansion of the universe was accelerating - one of the most important advances made in physics in the past decade. However, astronomers disagree about the exact origins of these outbursts.

Supernovae are categorised according to their spectra: type-Ia events are recognized by the presence of elements such as silicon and sulphur, and the absence of hydrogen. However, Hamuy and colleagues have now detected hydrogen emission from SN2002ic, which otherwise has a spectrum that matches those of several other well-known type-Ia supernova.

Hamuy’s group believes that its discovery provides new support for one of the two main theories of the origins of type-Ia supernovas. The ‘single-degenerate’ model says that a white dwarf – the burnt-out remnant of a star – gathers material, including hydrogen and helium, from an ordinary companion star until it is massive enough to explode. Although popular for other reasons, this idea had suffered from the absence of hydrogen in the spectra of any previously observed type-Ia supernova.

But not everyone agrees with their interpretation. “At first glance the observation appears to support a single-degenerate scenario in which the white dwarf accretes from a normal companion”, write Mario Livio and Adam Reiss of the Space Telescope Institute in the US in a preprint just published on (astro-ph/9308018). “However, the opposite may be true, and the observations may support the merger of two white dwarfs as the cause for type-Ia supernovas”.

Although recent calculations have cast doubt on the double-degenerate model preferred by Livio and Riess, it can explain the absence of hydrogen in most type-Ia spectra. Moreover, even though they disagree with the interpretation of Hamuy and co-workers, Livio and Riess add that the detection of hydrogen emission from a type 1a supernova is a “landmark discovery.”