Star clusters line up in the evening sky at this time of year like pearls on a necklace. As the sky gets nice and dark, they climb straight up the northeastern sky. They stretch from the bright star Capella, which is quite low; up through the “W” of Cassiopeia; then to Cygnus, the tail of the swan, high overhead.

There’s a good line of clusters because that path outlines the Milky Way – the subtle glow of the disk of our home galaxy. Most of the clusters are classified as “open.” All of the stars in such a cluster were born together, from a giant cloud of gas and dust. But as the clusters orbit the center of the Milky Way, they’re slowly pulled apart. So over time, all the stars in such a cluster go their own way.

Perhaps the highlight of this path is the Double Cluster – two clusters in Perseus, just below Cassiopeia. Under dark skies, they’re visible to the unaided eye as a faint cloud of light.

Individually, the clusters are known as NGC 869 and 884. They’re about 7500 light-years away. Combined, their stars and gas add up to about 20 thousand times the mass of the Sun. And they’re quite young as stars go – about 14 million years. At that tender age, the clusters haven’t had time to fall apart. And with their great mass, they’re likely to hold together longer than most clusters – perhaps several hundred million years.

Tomorrow: the crescent Moon and the “evening star.”

Script by Damond Benningfield