The Amazing “Coathanger” Asterism

The night sky is full of wonders of many kinds.  One of the most common and surprising are the groups of stars known as “asterisms.” These are distinctive stellar patterns lying within a constellation or, in some cases, one made up of those from two or more adjoining constellations.

Some are so unusual and artificial-looking that they seemingly couldn’t possibly be real!  One of these is the famed “Coathanger” asterism, now well placed for viewing with your Edmund binoculars on October evenings.

Undoubtedly the best-known and most easily recognized of all asterisms is the Big Dipper. (Contrary to popular belief, it’s not a constellation—but rather part of Ursa Major, the Great Bear of the Heavens.  See the Sky Talk column for April, 2007, for more about it.)  The much smaller heavenly Coathanger appears as an upside-down coat hanger lying within the Milky Way, with six stars aligned in a straight line and a curved hook coming out of the center made up of four more.  Over a degree in length, it’s too big to fit in typical telescopic fields of view.  An exception is the Edmund Astroscan-Plus with its wide 3 degree field (that’s six Full-Moon diameters of sky!) using its 16x low-power eyepiece. Since telescopes typically invert the image, the upside-down Coathanger appears right-side-up as seen in this amazing window on the heavens.

To find the Coathanger, we use another of the sky’s asterisms—this one the huge “Summer

Triangle.”  It’s made up of the three bright, blue-white stars Vega, Deneb and Altair, positioned high in the southwestern sky at this time of the year. (Use your Scientifics Star and Planet Locator to identify them.)  The Coathanger lies about one-third of the way along a line from Altair to Vega, appearing as a bright patch in the Milky Way to the unaided eye on a dark, moonless night. It’s a delightful surprise coming across it while sweeping its location with binoculars. Altogether, there are some two dozen fainter suns visible in such glasses sprinkled across and around the cluster. However, despite appearances, this lovely sight is not a true cluster of related stars but rather a chance alignment of them lying at different distances from us.

Officially logged as Collinder 399, it’s also known as “Brocchi’s Cluster” after is discoverer.  The Collinder Catalog itself contains many other such fascinating groups of stars all over the sky—ones too wide and scattered to have been included in standard compilations like the Messier Catalog or the New General Catalogue (the NGC).  Even without a detailed star chart to identify them, simply scanning the heavens with binoculars or a wide-field telescope like the Astroscan will turn up many other asterisms. But none can match the remarkable appearance of the Coathanger itself!  With so many wonders like these available to stargazers every clear night, it almost seems as if the universe wants us to be in a perpetual state of amazement.  Even after more than half a century of viewing the sky, I still find myself saying “wow” at the views virtually every time I go out observing.

-James Mullaney

Former assistant editor at Sky & Telescope magazine and author of eight books on stargazing.