…continuedA Sampling of Star Clusters
Summer and M13
As the Big Dipper slides down into the western sky in late spring and early summer, the constellation Hercules climbs high into the eastern sky. Within its boundaries lies the best globular cluster in the northern sky, number 13 on Messier's list.
The easiest way to find it in late spring or summer is to get familiar with the distinctive crooked-H shape of Hercules. Then look for the constellation between the bright stars Arcturus in Boötes and Vega in Lyra. You'll find four stars in the central portion of Hercules that form a pattern called the keystone. Aim your binoculars at the two stars on the right (west) side of the keystone. Look for a fuzzy circular glow, like an out-of-focus star. That's M13, shining across 21,000 light-years of space.
While you can pick out M13 with binoculars, this is one object where a telescope is definitely needed to reveal its true glory. In dark skies and with a good-quality 4-inch or larger telescope, M13 will begin to resolve into hundreds of faint stars. An 8-inch or larger telescope shows M13 appearing like a splattering of sugar grains sparkling in front of a dimmer glow of unresolved stars.
This cluster, like most globulars, actually contains millions of stars collected in a large spherical cloud. M13 and another 150 known globular clusters orbit the Milky Way galaxy like bees swarming around a hive.
Eight degrees south of Albireo, the head of Cygnus, the Swan, lies an open cluster known as the Coathanger (Collinder 399). Its brightest stars test the limits of naked-eye visibility. Through small binoculars this is a striking group. Its 10 brightest stars are arranged in a pattern that resembles an old-fashioned coathanger with a curved wooden bar and a metal hook. Six stars form the bar and four more to the south make the hook.
Near the stinger of Scorpius lies M7, a real binocular treat. In 50-mm binoculars you can count about 30 stars in this large, beautiful open cluster. Larger glasses will show 60 mixed bright and faint stars. A smaller knot of bright stars, M6, can be seen northwest of M7 in the same field of view. Both are wonderful targets for a small telescope, too.