The Moon barrels through the Pleiades star cluster this evening. It’ll pass directly in front of the cluster, briefly blocking most of its stars from view.

The Pleiades is the most famous of all star clusters. It’s also known as the Seven Sisters, but under dark skies – with no Moon in the way – you might actually see nine stars or more.

But that’s only the beginning. The cluster contains more than a thousand stars. In fact, it was the first cluster to be recognized as a cluster – a group that’s moving through the galaxy together.

That recognition came in 1767. John Mitchell, a clergyman and scientist, was looking at several tightly packed groups of stars. He studied the Pleiades in the greatest detail. And he calculated that there was only a one-in-500,000 chance that the grouping could be random. Instead, something had to be holding the stars together.

His idea was confirmed when astronomers measured the motions of the cluster’s stars. They’re all moving in the same direction, and at the same speed.

Today, we know that’s because they were born together, from a single giant complex of dust and gas. They’re bound to each other by their mutual gravitational pull. But they won’t stay together. The cluster is being pulled apart by the gravity of the rest of the galaxy. So the Pleiades probably will dissipate in about 250 million years – with its member stars going their own way.

Script by Damond Benningfield