 Binocular Universe:
The Coathanger
September 2009
Phil Harrington
The
three brilliant stars of the Summer Triangle -- Vega, Deneb, and Altair -- ride
high in the sky this month, framing the gentle glow of our Milky Way galaxy as
it stretches from horizon to horizon. If
there is a better way to spend a late-summer's eve than sitting back and slowly
scanning this hazy band of light through binoculars, I don't know it.
Even the least expensive binoculars out there will reveal swarms of
distant stars.
One
of my favorite star-bunches inside the Triangle 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 Altair and the two fainter stars set to
either side. Extend the 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.
Once
you find it, take a careful look through your binoculars.
Does its shape remind you of anything?
If not, go back inside, look in your bedroom closet, and then try again.
That's right, you have found the sky's Coathanger.
The Coathanger is drawn from six stars aligned in a straight line that
form the coathanger's cross bar, and another four that curve away to create the
hook.
Historical
records show that the Coathanger was known as far back as 964 AD, when the
Persian astronomer Al Sufi noted its misty appearance.
The group was rediscovered by Giovanni Batista Hodierna in the mid-1600s,
but it was skipped entirely by Messier and the Herschels.
That was probably because its large apparent size caused them to scan
right past it with their long focal-length telescopes and narrow-field
eyepieces. Sorry guys, your loss!
You
may also know the Coathanger by a second nickname, Brocchi's
Cluster. Back in the 1920s, Dalmiro Brocchi, an amateur astronomer from
Seattle
,
Washington
, became famous for drawing detailed finder charts for hundreds of stars in the
American Association of Variable Star Observers' (AAVSO's) 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 it being included as Collinder
399 (abbreviated Cr 399) in the 1931 catalog of scattered open star clusters
compiled by Per Collinder.
Finder chart for the Coathanger.
Chart from TUBA, www.philharrington.net/tuba.htm
But
guess what? Brocchi's Cluster isn't a cluster at all! Much more recent data
gather by the European Space Agency's Hipparchos satellite suggest the stars are
just an asterism, a chance alignment of random stars that lie anywhere from 220
to 1,100 light years away. I find that realization very disappointing, but
regardless of whether the Coathanger is a true star cluster or a celestial
fraud, it is still a wonderful sight through binoculars. Seven- to 10x glasses
give the best view, showing the Coathanger surrounded by a rich field of
stardust.
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The Coathanger and surroundings, as seen through the
author's 10x50 binoculars.
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Of
the ten 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, labeled as 4 Vulpeculae and shining at magnitude 5.1, 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
drawing also shows a phantom open cluster; that is, a phantom in my 10x50 and
16x70 binoculars. NGC
6802 lies just east of the Coathanger, but I have never been able to see any
hint of it through anything less than 80-mm binoculars, and then only under very
dark skies. While 50 stars are
compressed within its 5' boundary, the brightest only shines at 11th magnitude.
Most are fainter than 13th magnitude, placing them well beyond the reach
of 100-mm giants. Instead, they
show just the faintest wisp of blended starlight in the form of a slightly oval
smudge. Can you spot this
challenging target?
If
NGC 6802 proved to be too faint, try your luck with M71,
buried within the Arrow, Sagitta. Since
its discovery in 1775, M71 has proven to be a difficult object to classify.
Many older references still designate it as a very dense open cluster
similar to M11 in Scutum, but most modern authorities agree it is a globular.
Regardless of its true nature, binoculars disclose only a subtle glow set
a very pretty field of glittering stardust.
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M71 in Sagitta, as seen through the author's 10x50
binoculars.
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Have a suggestion for a fun binocular target that you'd
like to share? E-mail me at phil@philharrington.net. Until we get together again next month, enjoy
the beauty of the late summer sky. And
remember that two eyes are better than one.
About
the Author:
Phil
Harrington, author of Touring
the Universe through Binoculars, is currently completing a new
observing guide of challenging observing targets for Cambridge University
Press, which will be published in late 2010.
Visit his web site at www.philharrington.net
Phil Harrington's
Binocular Universe
is copyright 2009 by Philip S. Harrington.
All rights reserved. No
reproduction, in whole or in part, beyond single copies for use by an
individual, is permitted without written permission of the copyright holder.
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