The flying horse has lost his belly button. Astronomers took it away from him a century ago.

It’s represented by the star Alpheratz. For centuries, the star was shared by Pegasus the flying horse, and Andromeda the princess. It was his navel, and her head. It was the brightest member of both constellations.

But in the 1920s, astronomers drew formal boundaries for all the constellations. So each star was assigned to a single constellation – no more sharing. Alpheratz was just on the Andromeda side of the line, so the princess kept the star.

In a way, though, the star kept its dual identity. That’s because it’s actually two stars locked in orbit around each other. But they’re quite close together – closer than Earth is to the Sun. So from their distance of almost a hundred light-years, their light blurs together to form a single point.

Both stars are bigger, more massive, and brighter than the Sun. The higher mass means they’ll live billions of years less than the Sun will. They’re also much hotter than the Sun, so both stars look almost pure white, compared to the yellow hue of the Sun.

Alpheratz is about a third of the way up the eastern sky as night falls. It’s at the left corner of the Great Square of Pegasus, which is tilted as it rises. The rest of the flying horse stretches to its right and upper right. Andromeda stretches to the lower left – the constellation that won the right to the star Alpheratz.

Script by Damond Benningfield