In 1994, astronomers watched as the star V4332 in the constellation Sagittarius suddenly flared up, becoming thousands of times brighter. It hasn't settled down since: its brightness has changed several more times. These outbursts sit between ordinary novae and supernova explosions, and their cause has long been a puzzle.

New infrared observations show that the star's atmosphere contains a huge amount of the carbon-13 isotope—3.5 times more than carbon-12. Usually, stars have the opposite ratio. Such an imbalance arises when the star's inner layers, where nuclear reactions occur, mix with the surface. It's like vegetables from the bottom of a soup pot suddenly floating to the top — a sign that the soup has been thoroughly stirred.

Most likely, V4332 Sagittarii merged with a companion star. Before the merger, they were surrounded by a common gas disk, like a flattened pancake. When the stars fell into each other, the material mixed, and deep carbon-13 was flung outward. This is the first direct evidence of a merger for such outbursts, besides the famous case of V1309 Scorpii.

The repeated flares of V4332 Sagittarii are another clue. They suggest the merger wasn't instantaneous but happened in stages, like two water droplets merging, trembling and changing shape before becoming one.