Summer Secrets
These beautiful open clusters in Scorpius — M6 and M7 — are but two of many fine sights visible in the heart of the Milky Way. But other lesser-known clusters deserve a look too, as explained in the accompanying text. Click on the image for a closer view of M6 and M7.
Courtesy Akira Fujii.
There is never a shortage of deep-sky objects. Whatever the season, the sky holds more than enough of these delights to keep you busy all night, every night — if you take the time to search them out with good charts and referemce books. In the summer evening sky, almost every direction we look offers something interesting.
The area of Ophiuchus and Serpens is a turmoil of stars mixed with clouds of bright and dark nebulosity. Surprises in this part of the sky are common. One example is the open cluster IC 4756 in the eastern half of Serpens (Cauda). It's one of the largest such objects in the heavens, appearing more than 1° across and just a little smaller than the Beehive cluster, M44, in Cancer. Some 80 stars between 7th and 12th magnitude are evenly scattered across IC 4756's diameter. It appears as a patch of the Milky Way to the unaided eye. Binoculars or a finder will easily reveal its individual stars.
The most northern galactic cluster in the sky, NGC 188, is also one of the oldest. It's located just 4° south of Polaris. NGC 188 is 15' across, so use low power. It contains 150 stars, most of which are fainter than 13th magnitude. On fine nights I see it as a ghostly glow in the 4-inch Clark refractor.
Inside the Cepheus pentagon is a more challenging object, NGC 7142. John Herschel described it accurately as "a large, rich, loose cluster of stars of magnitude 10 or 11." To me it seemed an evenly spread layer of small stars. NGC 7142 doesn't show in a 2-inch finder, and I generally search with the main telescope after plotting the cluster on a detailed chart. High magnification helps after this cluster has been found.
Open cluster NGC 7510 in Cepheus is about 9th magnitude and 3' across. Of it, a Canadian amateur Dunstan Pasterfield writes, "Very easy to find, an attractive object framed by surrounding stars. It has an unusual shape, like a very thin arrowhead that is slightly bent at the tip. About 7–10 stars — delightful thing."
The sprawling open cluster of stars known as Melotte 111 (and sometimes Collinder 256) is one of binocular astronomy's loveliest sights. It fills your entire field of view with sparkling stars ranging from 5th to 10th magnitude. The triangle of bright stars in which the cluster is centered are Beta Regulus (lower right), Epsilon Virgo (lower left), and Alpha Canes Venatici (top).
Courtesy Akira Fujii.
Finally, what open cluster is visible to the naked eye, has no Messier or NGC number, and was called "gossamers spangled with dewdrops" by the 19th-century astronomy popularizer Garrett P. Serviss? Almost every skygazer has seen this group at one time or another, and its official designations are
Melotte 111 and
Collinder 256. The answer will surprise many, for the cluster is the shimmering haze of 5th- and 6th-magnitude stars we call
Coma Bernices. It's a real cluster and not just a change alignment of stars. There are about 80 members scattered across 5° of sky. Coma Bernices is only 260 light-years away and is one of the nearest open clusters. Therefore its stars appear well separated. If they are a bit too faint for your naked eye, a simple 2x or 3x opera glass gives a wonderful view.
