Stars that are born as twins don’t always remain apart. Sometimes, they merge to form a single star. That can happen while the stars are still young and vigorous, or even after they’re “dead.”

Astronomers recently discovered the remnant of the merger of two dead stars. It’s 150 light-years away, in the constellation Phoenix.

The remnant is a white dwarf. It’s heavier than the Sun, but smaller than Earth. It’s one of the heaviest white dwarfs ever seen.

When astronomers took a good look at it, they found a few other odd things. Its outer layers, for example, contain a high level of carbon. And the star appears to be especially old.

The astronomers plugged those traits into models of how stars work. They found that the most likely explanation was a merger between two white dwarfs more than a billion years ago.

The merger probably began when one of the original stars reached the end of its “normal” lifetime. The star puffed up, enveloping its companion. That dragged the two stellar cores closer together. The star then shed its outer layers, leaving only its dead core — a white dwarf. Later, the second star went through the same process, pulling them even closer. Over billions of years, the stars shed energy through gravitational waves — causing them to spiral together.

The combined stars were almost heavy enough to explode as a supernova. Instead, though, the merged stars settled in — beginning a long “life” as a dead star.

Script by Damond Benningfield