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Binocular Universe: Houston’s Triangle
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Sixty-nine years ago this month, the face of amateur astronomy changed forever. A seed was planted in September 1946 that would blossom into a decades-long infatuation with deep-sky observing among amateurs everywhere. Without any fanfare at the time, a man named Walter Scott Houston took over the reigns of a small column in Sky & Telescope magazine called Deep-Sky Wonders. Another prolific observer, Leland Copeland, originated the column and penned it for four years. But it was under Houston's watch that it evolved from a modest tabulation of three or four suggested objects into an engaging dialogue that would capture the imagination of readers for nearly half a century.
Above: Summer star map
from Star Watch by Phil Harrington.
That first column written by Houston, a mere dozen lines in length that belied this amazing man's tale-spinning nature, listed three objects in the late summer sky -- M57, M27, and M11 -- that I like to think of as Houston's Triangle. All three are visible through common binoculars, although they present three different levels of difficulty.
Above: Finder chart for this month's Binocular Universe.
Chart adapted from Touring the Universe through Binoculars Atlas
Of the three, M11, the famous Wild Duck Cluster in Scutum, is the easiest to see through binoculars. To find it, trace Aquila the Eagle's diamond-shaped body from Altair to the curve of its tail-feather stars, Lambda and 12 Aquilae. Together with Eta Scuti, Lambda and 12 form a three-star arc that curves right toward M11. More than 1,000 stars belong to this open cluster, making it the densest of its type among the Messier objects and a real showpiece in telescopes. Through binoculars, M11 actually looks more like an unresolved globular cluster than an open cluster. Except for a solitary 8th-magnitude point buried within, all of M11's suns shine between 11th and 14th magnitude, too faint to be resolvable with common binoculars (it's very doable if you have keen eyes and 20x binoculars).
Next comes M27, nicknamed the Dumbbell Nebula for its telescopic resemblance to a weightlifter's barbell. Even the view through 10x50 binoculars begins to reveal its unusual shape. M27 carries the distinction of being the first planetary nebula ever discovered, by Messier himself in 1764, and only one of four listed in his famous catalog. Located about 1,250 light years away, M27 is one of the biggest and brightest planetary visible in northern skies.
While it is easy to tell M27 apart from the surrounding stars through binoculars, locating it in the first place can be another matter. That's because its home constellation, Vulpecula the Fox, is nearly impossible to make out against the background Milky Way field. Rather than scratch your head in frustration, do as most others do and start your search in neighboring Sagitta the Arrow. Even though Sagitta is also very small, its four main stars create a distinctive pattern that stands out more prominently. Concentrate your aim on Gamma Sagittae, the star marking the Arrow's eastern tip. Look about half a field due north of Gamma for a triangle of stars formed from 14, 16, and 17 Vulpeculae. M27 will look as a smudge of grayish light less than half a degree southeast of 14 Vulpeculae, the easternmost star in the triangle. Even 7x binoculars will show the nebula's box-shaped glow floating in a field strewn with stardust. Catching a hint of its hourglass form, however, will take more magnification that most handheld binoculars can muster.
Lastly, we have M57, the Ring Nebula in Lyra. M57 lies twice the distance from us as M27, and as a result, appears much smaller. While M27 measures 8'x6' across, the Ring is just 1.4' x 1' in size. Even though M57 at magnitude 8.8 is bright enough to be visible through most binoculars, it will look like nothing more than another faint star.
The challenge to binocularists is to figure out which faint star is actually the Ring Nebula, since its tiny donut-shaped disk needs at least 25x to be resolved. Although that task sounds daunting at first, as luck would have it, the field surrounding M57 actually simplifies the task. Center your aim along the bottom of Lyra the Lyre's rectangular frame, halfway between the star Sulafat (Gamma Lyrae) marking the eastern corner and Sheliak (Beta Lyrae) at the western corner. M57 is found almost exactly halfway between the two. To confirm that you are looking in the right place, M57 is the last point in a diagonal line of five faint stars that starts just north of Sheliak (Beta). If you can see all four real stars in that line as well as a slightly fuzzy point just beyond, then you've hit M57.
There are plenty of targets within Houston’s Triangle. Let’s revisit some old favorites, as well as stop by others for the first time.
For instance, if you scan from M27 to M57, you’ll pass the star Albireo, which marks the beak of Cygnus the Swan. Albireo is a showpiece double star through small telescopes, with a 3rd-magnitude golden primary star accompanied by a 5th-magnitude sapphire-blue companion. Although their colors aren’t as apparent through binoculars, both stars can be spotted through steadily held 10x50s.
Continuing past Albireo, you will also pass the globular cluster M56. Charles Messier himself discovered this distant swarm of 100,000 or so stars on January 19, 1779. M56 is nestled among a well-populated field of stars, southeast of a conspicuous asterism of stars shaped like the number 7. Binoculars show M56 as a fuzzy 8th-magnitude glow punctuated by a brighter core, a view reminiscent of Messier’s own. He described it as a "nebula without stars, having little light.”
Of all the deep-sky targets within the triangle, the brightest by far is the Coathanger asterism. Also known historically as Brocchi’s Cluster (even though its non-cluster nature is well established) and Collinder 399, the Coathanger stands out nicely through any pair of binoculars. It’s always well worth a revisit. The outline is drawn from six stars aligned in a straight line that marks the Coathanger's cross bar, and another four that curve away to create the hook.
Since we are discussing Walter Scott Houston this month, it’s appropriate that the Coathanger be included. Scotty first introduced the Coathanger to me, and likely countless others, thanks to a small write-up in an old newsprint star atlas he created more than half a century ago.
From M27, look 2° due west to 5th-magnitude 12 Vulpeculae. Can you spot a faint glow just to the north? That’s the open cluster NGC 6830. Some two dozen stars call this cluster home. My 10x50 binoculars only show the faintest hint of the cluster. It’s much more obvious my 16x70s; in fact, I can even spot a few of its brightest stars, but only with averted vision.
Finally, we have Stock 1.
This is one of those objects that usually goes unnoticed because it covers such
a wide field. Spanning 1 degree across, only 19 stars actually populate the
group. But the area contains many others that appear to belong by chance
line-of-sight. Stock 1 spans 52' across, but these imposters cause the cluster
to appear half again larger. Stock 1 is about 3.5 degrees southeast of Albireo
and 2 degrees east-northeast of Alpha Vulpeculae. Look for a distinctive,
though faint, trapezoid of four 7th- and 8th-magnitude stars toward the group's
eastern edge. These suns are centrally located within the cluster. The
cluster's brightest true member is 7th-magnitude HD 184928, northwest of the
trapezoid. A 6.5-magnitude star looks to be the brightest, but is not a true
Until we get together again next month, I hope that you get many opportunities to enjoy the beauty of the late summer sky. And always remember that two eyes are better than one.
About the Author:
Looking for more targets to enjoy through your binoculars? Read Phil's article Tour 10 Fall Binocular Treats in the August 2015 issue of Astronomy magazine.
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