If you look up the details of W Ursae Majoris, you’ll find that its two stars are about a million miles apart. The way astronomers figure that distance, though, is from the centers of the two stars. When you measure the distance between their surfaces, the stars are a whole lot closer. In fact, they’re touching. That makes them a contact binary – one of thousands discovered so far.

Many stars move through the galaxy with one or more companion stars. Their distances from each other vary greatly. Some can be light-years apart. But if they’re born close to one another, they might eventually spiral together. That might be caused by magnetic fields, the exchange of gas between the stars, or some other process.

W Ursae Majoris shows how that plays out. One of its stars is a little bigger, heaver, and brighter than the Sun. The other is about half the Sun’s mass. They’re in such close contact that they share their outer layers of gas. That makes them about the same temperature and color.

But they’re not the same brightness. As the stars orbit, once every eight hours, they cross in front of one other as seen from Earth. So the system’s brightness varies – the result of sibling stars in a tight embrace.

W Ursae Majoris is in the great bear, which includes the Big Dipper. The dipper is high in the sky at nightfall. W Ursae Majoris is well to the upper left of its upside-down bowl, and is visible through binoculars.

Script by Damond Benningfield