Two stars circle each other like dancers who come together and drift apart. Over time, one swells, and its outer layers spill onto its neighbor. The star that receives foreign matter looks no different from a single star, but its surface remembers everything.

Astronomers built models of the evolution of massive binary stars and noticed patterns in surface chemical composition. When a star strips gas from its companion, its surface bears telltale marks: some elements become more abundant, others less. From these marks, one can calculate how much material was transferred and what its composition was.

The method was tested on known stars. For γ Columbae, which was thought to be a star that had lost its envelope, the calculations showed the opposite: it had gained mass from its companion. That companion likely ended its life in a supernova explosion, leaving behind a compact object.

This approach helps reconstruct the past of binary systems that evolve toward supernova explosions and the formation of pairs of neutron stars or black holes. The chemical composition of a star's surface becomes a window into a history that would otherwise remain unseen.