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Double Cluster in Perseus on October evenings

Here’s the relationship between the prominent M or W shape of Cassiopeia and the Double Cluster in Perseus.

Double Cluster in Perseus

The Double Cluster in Perseus consists of two open star clusters near each other on the sky’s dome. Amateur astronomers know them as h and Chi Persei. The two clusters reside in the northern part of the constellation Perseus, quite close to the constellation Cassiopeia the Queen. If you have a dark sky and find Cassiopeia – which is easy, because the constellation has a distinctive M or W shape – be sure to look for Perseus, too. Then just scan with your binoculars between them. The Double Cluster – a breathtaking pair of clusters, each containing supergiant suns – will be there.

These two star clusters are thought to be located some 7,500 light-years away. They’re separated from one another by a few hundred light-years. It’s amazing that we can see these stars at all across this great span of space. We know they must be bright stars, intrinsically, or we wouldn’t be able to see them. Each cluster contains a few hundred stars, and, indeed, these stars are young, hot supergiant suns that are many thousands of times more luminous than our sun.

Astronomers tell us that the Double Cluster lies within the Perseus arm of the Milky Way galaxy. However, our solar system resides in the inner part of the Orion arm. Therefore, when we look at the Double Cluster, we are looking through our local spiral arm and all the way to the next spiral arm outward from the galactic center.

The two star clusters making up the Double Cluster are also called NGC 869 (h Persei) and NGC 884 (Chi Persei).

Star chart showing Cassiopeia and Perseus constellations with some stars labeled.
Cassiopeia is easy to find with its shape of an M or W. The constellation Perseus follows Cassiopeia across the sky.
Dense star field with line drawn from Cassiopeia to two small fuzzy white dots close together.
The constellation Cassiopeia points to the Double Cluster in the northern tip of the constellation Perseus. Faintly visible to the unaided eye on a dark night, it’s better viewed with optical aid. The Double Cluster nearly marks the radiant of the Perseid meteor shower. Image via madmiked.

How to find the Double Cluster

To locate the Double Cluster, find the W- or M-shaped constellation Cassiopeia the Queen. If your sky is dark enough, you will be able to see the graceful pattern of Perseus the Hero nearby. Scan between them with binoculars to find the Double Cluster. Or … draw an imaginary line from the star Navi (Gamma Cassiopeiae) through the star Ruchbah (Delta Cassiopeiae), and go about three times the Navi/Ruchbah distance to locate the Double Cluster.

At mid- and far-northern latitudes, the Double Cluster is circumpolar – above the horizon every night of the year at any hour of the night. If you are further south (but still in the Northern Hemisphere), try looking for the Double Cluster in the evening in autumn or winter.

Just remember … the Double Cluster is harder to see when it’s close to the horizon. If you can’t spot it between Cassiopeia and Perseus, wait until later at night, or later in the year, when it’s higher in the sky.

For general reference, the Double Cluster is high in the sky when the Big Dipper is low, and vice versa. Because the Big Dipper is lowest in the northern sky on late autumn and early winter evenings, the Double Cluster is highest in the northern sky at these times. The Double Cluster is pretty much always visible at evening except in late spring and summer.

The Double Cluster rates among the most magnificent deep-sky objects not to be included in the famous Messier catalog. Of course, Charles Messier (1730-1817) was looking for deep-sky objects that could be mistaken for comets. He must have thought nobody would see this pair of glittery clusters as a comet in the sky.

Although considered a deep-sky jewel, the Double Cluster is visible to the unaided eye in a dark country sky. If you zoom in on them with binoculars or a wide view telescope, you’ll see them as two glorious star clusters. They’re an easy target through a telescope and will wow your friends!

The position of h Persei is Right Ascension: 2h 19m; Declination: 57o 9′ north

The position of Chi Persei is Right Ascension: 2h 22.4m; Declination: 57o 7′ north

Star field with two separate tight groupings of multiple stars.
View larger. | The Double Cluster in Perseus, via Tom Wildoner at The Dark Side Observatory in Weatherly, Pennsylvania.
Two bunches of stars, each with several dozen, in star field.
Double cluster in Perseus. Photo via Greg Hogan in Kathleen, Georgia. Photo taken in 2015.

Bottom line: On an autumn or winter evening, scan between Cassiopeia and Perseus for the magnificent Double Cluster. At a distance of some 7,500 light-years, the stars in these two clusters are young, hot supergiant suns that are many thousands of times more luminous than our sun.

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