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Sometimes you find a movie ticket from three years ago in the pocket of an old jacket. You can't reconstruct the time from it, but you can figure out where the owner went. The same goes for hot subdwarfs — tiny stars in the late stages of life that shine mostly in ultraviolet. Their light is like that ticket: it tells you about the past, but reading it is tricky.

Astronomers took a sky map from the Gaia telescope, where each star leaves its own fingerprint — a spectrum split into colors. For 20,000 hot subdwarfs, they built a similarity map: the closer two stars are on this map, the more similar their spectra. The result was a map where stars lined up in two long strands. In these strands, binary systems gathered — pairs where a subdwarf orbits a regular star. Singles scattered across the map on their own.

Machine vision confirmed: if a subdwarf pulsates or flares, there's almost certainly a companion nearby. Sharing space with a regular star changes a subdwarf's fate, like a noisy neighbor in a communal apartment forcing you to live on their schedule.

The similarity map built by the algorithm became a magnifying glass for astronomers. It showed that hot subdwarfs aren't just loners living out their days. Nearly two-thirds of them are in pairs, and that's crucial for understanding where the excess ultraviolet in galaxies comes from. The ticket from the pocket has been read.