For the first time, astronomers have watched the spiralling dance performed by two stars merging into a single star. The observations, taken between 2001 and 2008, suggest a solution to the vexed problem of how rare "red novae" form. Most novae are blue and occur when material on a white dwarf star explodes. But what causes red novae has been a mystery.
In September 2008, the red nova V1309 Scorpii appeared in the Milky Way. Fortunately, it was positioned in a part of the sky being watched by the Optical Gravitational Lensing Experiment (OGLE), a Polish-run programme using data from a telescope at the Las Campanas Observatory in Chile to search for signs of dark matter and planets. As a result, the team had inadvertently captured the process that sparked the red nova.