The Double Cluster
Many amateur astronomers, including me, believe observers in the southern hemisphere enjoy a better collection of fine objects for small telescopes. But there are treasures in the northern skies, too, and one of the finest for a small telescope is the Double Cluster in Perseus. This sparkling cluster, which is among the youngest known in the galaxy, presents a rich array of scintillating giant stars of contrasting colors.
The Basics
• The Double Cluster was known since antiquity as a faint cloud in the northern Milky Way. It was– who else– William Herschel who discovered the nature of this object. He cataloged each of the two clusters by their now-official names: NGC 884 and NGC 869. The clusters lie in a star-clogged section of the Perseus spiral arm of the Milky Way which lies further from the center of our galaxy than the Orion arm, where we are located.
• Each cluster contains some 300 stars, some of which are immensely bright: 50,000x more luminous than our Sun. These two clusters likely formed out of the same molecular cloud only 3-5 million years ago, making them the among the youngest star clusters known. The Pleiades, by comparison, is more than 100 million years old.
• The clusters lie at RA 2h19.0m and Dec 57d08s, so they are circumpolar for many in the north, but hard to see from the deep southern hemisphere. But fear not you southerners: the lovely “Jewel Box” cluster in Crux is similar to the Double Cluster in age and beauty.
The Double Cluster (NGC 884 and NGC 869) lies at the middle of this map (click to enlarge).
A Deeper Look
• Despite their great distance of some 7,000 light years, the two clusters span 1.5-2.0 degrees of sky, so use your lowest-power eyepiece to take them all in. The view in a telescope is truly breathtaking, especially in dark sky. Try to look for colors in these young stars: sapphire blue, topaz, white, and the red glow of swelling giant stars moving towards their violent end as supernovas. When these stars explode, what a sight they will be!
• If the Double Cluster were as close as the Pleiades, they would span a quarter of the northern sky. Many of its 600 stars would shine as bright as Vega.
• Both clusters approach us at roughly 21 km/s. So in the time it took you to read this this far, these wonderful objects moved 1200 km closer to your telescope.
A Bit of History
Many find it strange that 18th-century comet-hunter Charles Messier did not include so bright and pleasing an object as the Double Cluster in his famed list.
But Messier compiled his list to map out comet-like objects he might confuse with real comets during his many sweeps of the sky. To a skilled observer like Messier, the Double Cluster could not be confused with a comet because it lies in the northern part of the sky, away from regions where comets are usually found. Plus it’s easily resolved as a dual star cluster with an appearance unlike that of a comet.
Since Messier likely believed he would never confuse the cluster with a comet, he left it off his list.
Personal View
To truly appreciate their intricate beauty, try making simple sketches of these clusters. By drawing the positions of the stars, you’ll discipline your eye to see color and pattern in the apparently random swirl of stars. Savor this object and return to it frequently. If you’re consistently bored by the Double Cluster, it might be time to give up astronomy and take up stamp collecting.
