Type Ia supernovae are thermonuclear explosions of white dwarfs that astronomers use as "standard candles" to measure distances. But among them is a rare subtype, 2003fg, which shines too brightly and can skew the measurements. For a long time, it was unclear why they are like that. A new study has for the first time examined twenty host galaxies of such supernovae and measured the conditions at the very explosion sites.

It turns out that 2003fg-like supernovae prefer the metal-poor outskirts of galaxies. At the explosion sites, the oxygen abundance is on average only 0.68 times the solar value. This is important: if the explosion came from a single rapidly spinning white dwarf, it would need a young, metal-rich environment. But the observations show the opposite: metal-poor, old outskirts. So most likely, two merged white dwarfs explode. Now astronomers can take this peculiarity into account to measure distances and dark energy more accurately.