Look to the lower right of the “big M on its side,” the constellation Cassiopeia, for a wondrous sight in binoculars; the Double Cluster of Perseus.

Best appreciated when there is no moonlight, you can still spot it this weekend, although the moon reaches first-quarter, Saturday, Oct. 5. Moonless evenings return after the full moon on Oct. 13.

The Milky Way Galaxy’s spiral arms are richly decorated with sparkling star clusters. They come in a wide variety and sizes. The members are thought to have sprung from the same nebula; siblings, if you will. Like so many human family counterparts, as eons pass the clusters spread apart and the stars someday go their separate ways.

The most well known star cluster has to be the Pleiades, a glittering groups of blue-white stars close enough to our solar system to stand out to the unaided eye. Best seen in the evening in winter, you can already see the Pleiades in early October, low in the north-northeast around 9 p.m. The Pleiades cluster is high in the south around 4 a.m.

The Double Cluster is a lot farther and thus dimmer, but still can be seen as a hazy patch with eyes alone in a darkened sky. They are easily seen in binoculars and amazing in a small (or bigger) telescope using low magnification to give a wide enough field of view.

Splendid enough if there was just one cluster, but in this case there are two, side by side.

They are about 7,100 and 7,400 light years away; it takes that many years for their combined starlight to reach our eyes. (The Pleiades are about 444 light years from here.)

Individually, the Double Cluster’s stars are too faint to be seen by eyes alone; together, their combined light makes brings the cluster to a little brighter than +4th magnitude. (A star of +6th magnitude is about the dimmest you can see on a dark night in the countryside.)

Each cluster has an estimated 300 to 400 stars. The clusters are only a few hundred light years from each other.

Just imagine the starry sky from a planet in either of these clusters, or right between them!

Examining the clusters with a small telescope, you find mainly white or bluish white stars, with a few red stars sprinkled around. The clusters are catalogued as NGC 869 and NGC 884 (or H Persei and Chi Persei, respectively). The Double Cluster spans about one degree (the apparent width of the full moon is approximately one half degree).

To find the Double Cluster, first spot the five-star constellation of Cassiopeia high in the northeast, the next clear evening. They form what looks like an “M” on one side. Using the two lower stars of the “M,” make a long triangle with the Double Cluster, to the lower right. The clusters are between Cassiopeia and the star pattern of the constellation Perseus.

Keep looking up!
