A. Drazdauskas

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.

A star's age is the key to the galaxy's history. But prying that key from nature is no simple task: unlike a tree, a star doesn't grow annual rings. Yet, its atmosphere holds a chemical code — the ratio of elements that slowly shifted as generations of stars died and new ones were born.