Hang Your Hat On A Coat Hanger Made Of Stars

Can you see the upside down coat hanger? The straight row of brighter stars across the center of the photo outlines the flat bottom of the hanger. A curl of four stars below the center of the line forms the curved hook. Copyright: Till Credner and Sven Kohle, allthesky.com

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.

The Coathanger is a short distance – about one binocular field of view – below or south of Albireo in the Northern Cross. Credit: Bob King

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 constellation Scorpius from Arabic astronomer Al Sufi’s book. He translated an older work by Ptolemy into Arabic and added his own comments and additions.

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.

4 Responses

  1. Richard

    Thank you for this post. I saw this for the first time by accident on Mon 9th down in St Minver, Cornwall and have been wondering what it was. The much darker sky than I am used to in Croydon made it difficult for me to identify anything I was trying looking at using my old 10 x 50 Pacer binoculars (from Boots in 1970s).

  2. Steve C

    Thanks for the description of the Coathanger. I’d heard of it mentioned a few times but I did not know what it was. I actually thought it was a name given to a certain structural part of the Milky Way. I know now, and I’ve seen it for myself so I’ll be looking out for it again whenever Cygnus is around.

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