Jianrong Shi

In Andromeda's stellar stream, a long ribbon of stars ripped from a small galaxy during a collision with a larger one, astronomers spotted a star that outshines all its neighbors. It pulsates, swelling and contracting every 89 days, like a heart beating in rhythm. Such stars are called Type II Cepheids, and this one is the brightest known.

Imagine three witnesses who saw the same event. Two tell the same story, the third tells a different one. Whom do you believe? This is exactly the situation in astronomy when measuring carbon in the star J0815+4729, an ancient object with a record-low iron abundance (carbon is one of the key elements used to infer the properties of the…