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IT’S an embarrassing gap in astronomers’ knowledge. Despite relying on type Ia supernovae as tools to measure the dark energy speeding up the universe’s expansion, they still don’t know exactly what causes the blasts. Now the picture has got even fuzzier.

In the standard scenario, a white dwarf pulls matter from a companion star, and this extra mass triggers a supernova. Heavy white dwarfs were thought more likely to explode, since it takes less to push them over the edge.

Now Christopher Pritchet of the University of Victoria in British Columbia, Canada, and colleagues have observed galaxies dominated by lightweight white dwarfs producing type Ia supernovae just as efficiently as those dominated by heavier white dwarfs. These cases may be down to the collision of two white dwarfs, says co-author Andrew Howell of the University of Toronto. The study will appear in The Astrophysical Journal Letters.

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