The Coathanger asterism, also known as Brocchi's Cluster and Collinder 399, is located in Vulpecula. | SkyNews
The Coathanger asterism, also known as Brocchi's Cluster and Collinder 399, is located in Vulpecula. (Michael Watson)

Summer night sightseeing: The Coathanger, Tweedledee and Tweedledum

Take a tour of the summer night sky, through the Coathanger asterism, as well as the Tweedledee and Tweedledum clusters.

The Coathanger asterism, also known as Brocchi’s Cluster and Collinder 399, is located in Vulpecula, about midway between Aquila’s brightest star Altair and the bottom two stars of Lyra’s parallelogram. In binoculars, it resembles its namesake, but upside-down.

Look for a half-dozen magnitude 6 stars arranged in a 1.5-degree-long straight line with a hook-shaped arc of four slightly brighter stars below it. You might be able to see the pattern with your unaided eyes, too. Your telescope will reveal that most of the stars are of a whitish A- and B-class type, but the two most southerly members are orange-red K- and M-class stars.

The Coathanger asterism, also known as Brocchi’s Cluster and Collinder 399, is located in Vulpecula. (Michael Watson)

The Coathanger is not a true cluster of related stars, but a trick of happenstance — their distances vary between 230 and 2,050 light-years from the Sun. Before moving on, swap in a stronger eyepiece and look 17 arc minutes beyond the eastern tip of the Coathanger’s bar for NGC 6802 (FN 105). It’s a small, dense, North-South elongated open cluster of about 50 stars.

Next, aim your binoculars halfway between Altair and the bright star Celebrai in Ophiuchus (or look with your unaided eyes, if the sky is dark and the Milky Way is prominent). Your quarry is a pair of bright, magnitude 4.6 open star clusters straddling the border between Serpens and Ophiuchus. Since their centres are separated by nearly three degrees, the duo will nicely fill your binoculars’ field of view, like a southerly version of the Double Cluster in Perseus.

The smaller, more westerly cluster NGC 6633 has been dubbed Tweedledum, while the larger cluster is named Graff's Cluster or IC 4756 (but I think I prefer Stephen O'Meara's suggested moniker of Tweedledee) | SkyNews
The smaller, more westerly cluster NGC 6633 has been dubbed Tweedledum, while the larger cluster is named Graff’s Cluster or IC 4756 (but I think I prefer Stephen O’Meara’s suggested moniker of Tweedledee). (Chris Vaughan, Stellarium)

The smaller, more westerly cluster NGC 6633 (FN 093) has been dubbed Tweedledum. It contains more than 50 prominent stars arranged in loose clumps with no defined core that stretch Northeast-Southwest across 30 arc minutes of sky. Your telescope will reveal the myriad of smaller Milky Way stars in the background — but don’t magnify the view too much!

The larger cluster is named Graff’s Cluster or IC 4756, but I think I prefer Stephen O’Meara’s suggested moniker of Tweedledee. This cluster sits inside a one-degree-wide circlet of 6- to 8-magnitude stars. Its rich core contains 70 or more 9-magnitude stars distributed in pairs and clumps. Once more, use a low magnification eyepiece and note the rich star fields around it. Can you detect the dark dust region to the east of these “twins?”

Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the universe together.

More about the summer night sky

Take a jaunt through more of the summer night sky in the July/August 2021 edition of SkyNews. In his column, “Beyond Messier,” swim the Cygnus Loop and travel the North America Nebula with Chris Vaughan!

SkyNews magazine cover, July/August 2021. | SkyNews

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