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The Coathanger

Check out one of the sky’s best asterisms.
Harrington
Not long after my childhood interest in stargazing was first sparked, my parents enrolled me in a summer astronomy program at the Stamford Museum in Connecticut. Each week, we would gather in the museum’s planetarium to learn about the night sky. In between sessions, we were encouraged to view the sky both by eye alone as well as with a telescope or binoculars, if we owned one.

To help guide us at night, we were given a newsprint star atlas that showed the sky’s brighter deep-sky objects plotted among the stars. The atlas was published by American Education Publications and written by the 20th century’s preeminent amateur astronomer and author, Walter Scott Houston.

I kept that atlas with me whenever I went out at night that summer. It helped introduce me to the beauty of the binocular sky. While many targets seemed small and faint, one particular special pattern of stars, the Coathanger, drew my attention. It continues to be one of my favorite summer sights.

I’m not alone in that opinion. Recently, I received an email from Leilani Atwood. She wrote in part, “I thoroughly enjoy your Binocular Universe articles, and I happened to have a favorite object as well. In the constellation Vulpecula, there is an attractive grouping called Brocchi’s Cluster or the Coathanger.”

Have you ever seen the Coathanger? It lies inside the Summer Triangle, and it is easy to spot through binoculars, and even with the unaided eye if you know exactly where to look. Begin at Altair, the Triangle’s southernmost star. Draw an imaginary line between it and the two fainter stars set to either side. Extend that line toward the northwest for about twice its length into the neighboring constellation Vulpecula. There, you should spot a tiny flock of about 10 stars.

ASYPH0918_01

The Coathanger, also known as Brocchi’s Cluster, is not a cluster at all, but one of the sky’s prettiest chance arrangements of stars.

José J. Chambó
Atwood went on to say, “When I stare straight at the cluster with just my eyes, I can only detect a few twinkles. But through binoculars, the stars are bright and easy to count. It’s amazing how these stars happen to line up in such a way from our perspective on Earth.”

Take a careful look through your own binoculars. The Coathanger’s crossbar is drawn from six stars in a row, while another four curve away to the south, creating the hook. The wide fields of 7x to 10x binoculars give the best view, framing the Coathanger against a rich backdrop of stardust.

Historical records show that the Coathanger was known as far back as a.d. 964, when Persian astronomer Al-Sufi noted its misty appearance. Giovanni Battista Hodierna rediscovered the group in the mid-1600s, but it was skipped entirely by Messier and the Herschels. That was probably because the Coathanger spans 1°. It’s likely that the narrow fields of their long-focal-length telescopes caused them to scan right over it unrecognized. Sorry, guys, your loss! 

Of the 10 stars in the Coathanger’s upside-down outline, most appear pure white through binoculars. Two, however, may show a slight yellowish or orangish glint to observers with keen color perception. The brightest star in the pattern, 5th-magnitude 4 Vulpeculae, is an orange spectral type K0 sun, while the star directly adjacent to it in the hanger’s hook is a spectral type K5 star.

The Coathanger is also known as Brocchi’s Cluster. In the 1920s, Dalmiro Brocchi, an amateur astronomer from Seattle, became famous for drawing detailed finder charts for hundreds of stars in the American Association of Variable Star Observers program. One particular chart showed this area in detail, capturing the group in print for the first time. It was later named in his honor. That chart also led to the Coathanger being included as Collinder 399 (abbreviated Cr 399) in the 1931 catalog of scattered open star clusters compiled by Per Collinder.

But guess what? Brocchi’s Cluster isn’t a cluster at all! Data gathered in 1997 by the European Space Agency’s Hipparcos astrometry satellite showed it to be an asterism, just a chance alignment of random stars. They range in distance from 220 to 1,100 light-years away.

I was disappointed when I first read this, but it doesn’t in any way diminish the Coathanger’s sparkling beauty through binoculars. Be sure to visit it on the next clear summer’s eve.

Do you have a favorite binocular target that you’d like to share? I’d love to hear about it and possibly feature it in a future column. Do as Atwood did and drop me a line through my website, philharrington.net. 

Until next time, remember that two eyes are better than one!

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