Yesterday we learned how to find summer’s finest double star, Albireo. Up for a twofer? Not far from Albireo you’ll find the Coathanger, summer’s most curious star cluster. While it may only be a chance alignment of stars, it sure looks convincing. Lots of stars jam together to form a near perfect connect-the-dots upside down coat hanger.
From a dark sky site, the Coathanger, also known as Brocchi’s Cluster, is just visible to the naked eye as small, glittery patch in a vacant area of the Milky Way on binocular field of view below Binoculars really breath life into the little group, showing the distinctive coathanger shape floating upside down in a vast dark lane of interstellar dust. Four stars comprise the hook, while six or more make up the supporting bar. The effect is quite striking, and once you see it, you’ll wonder where this gem has been hiding all your life. Add a sprinkling of fainter hangers-on and you might see a couple dozen stars in total. Small telescopes reveal about 40.
The first mention of the Coathanger went out in 964 A.D. by Persian astronomer Al Sufi in his Book of the Constellations of the Fixed Stars. Much later, in the 1920s, amateur astronomer D.F. Brocchi created a map of the group to calibrate photometers, devices used to electronically measure a star’s brightness.
Brocchi’s has been the cluster’s name since, but the more informal “Coathanger” has been gaining ground for obvious reasons. If you don’t like either of those names, this swarm also goes by Collinder 399 – the 399th object in Swedish astronomer’s catalog of clusters published in 1931.
Despite its name, Brocchi’s Cluster is probably not a true, physically connected group but a chance alignment called an asterism – an unrelated but picturesque arrangement of stars. Whatever it is, every summer my friends and I like to point it out to one another. It serves as a most mundane but human connection to the night sky.