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Binocular Universe: Hunting Big Game!
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Binocular Universe: Hunting Big Game!
Over the last few months, we have explored planetary nebulae and galaxies hidden in the far southern sky. This month, we head back north to tackle others gracing the December sky that are sure test your mettle. Be forewarned, as the title of this installment says, we are going after some big game.
Above: Autumn star map from Star Watch by Phil Harrington.
Above: Finder chart for this month's Binocular Universe.
Chart adapted from Touring the Universe through Binoculars Atlas
Our first target is no less than M76, the Little Dumbbell Nebula in Perseus. This is a real toughie, and probably only in the realm of giant binoculars 70mm and more in aperture and 16x or more magnification. That's because this planetary nebula shines at only 10th magnitude (some references say it's as faint as 12th magnitude, but we'll be optimistic) and tiny! That makes it one of the faintest Messier objects.
Here's how I have found it in my 16x70s, by using the stars of Andromeda as a guide. Scan along the Princess's stars northeastward from Alpheratz, the star marking the northeast corner of the Great Square, pass Mu Andromedae, to 51 Andromedae and Phi Persei, a pair of dim naked-eye points. Aim toward Phi, where you should see it and a dim, orange star immediately to its north. M76 completes a right triangle with these two stars, the orange sun marking the right angle itself.
Those 16x70s show M76 as a tiny, faint point of fuzzy light, which at first glance may look like an ordinary star. Moving up to my 25x100s, this so-called star "just doesn't look right." Instead, it looks slightly elongated approximately north-to-south. Some observers remark that it looks just like a celestial peanut!
Discovered by Pierre Mechain in 1780, M76 is a classic example of what astronomers call a bipolar planetary nebula. In these nebulae, it is believed that a disk of obscuring dust not only hides the central star from view, but also channels material into two exhaust plumes rather than streaming away evenly. The result is a planetary nebula that appears to blossom much as a butterfly extends its two wings, which has led some to call them "butterfly" planetary nebulae. This two-part appearance also led to M76 being assigned two entries, NGC 650 and NGC 651, in the New General Catalog.
The distance to M76, like many planetary nebulae, is not well established. Estimates place it about 3,400 light years away, although published values range anywhere between 1,700 and 15,000 light years. If we accept the distance of 3,400 light years, then M76 spans about 4.5 light years.
As I mention in my book Star Watch, it's interesting to point out an important lesson here for all observers. Nearby M33, the Triangulum Spiral, is listed in most references as 6th magnitude, which sounds as though it should be fairly bright and easy to find. It is neither. Now, contrast that with M76, listed at magnitude 10-ish. Should be faint, right? While it is, many observers report that M76 is actually easier to find than M33! Why? The answer is "surface brightness." M76 has a brighter surface than M33, and is therefore actually easier to see. The point of all this is never be misled by an object's listed magnitude. In some cases, the listed magnitude makes an object sound much brighter than it actually appears, while in others, it misleads observers into thinking that an object is fainter.
As I was preparing to submit this installment for publication, I received an e-mail from a reader, Scott Harrington (no relation, that I know of anyway!). He wrote that while trying to view M76 through his 8x56 binoculars, he stumbled upon a curious asterism of six 8th- and 9th-magnitude stars that collectively look just like a horseshoe. It measures about 30 arc-minutes across and lies a few degrees north of M76. He explained further that "I have to figure it must have fallen off Pegasus as he flies through the sky!" I love it. Scott e-mailed both me and Sue French to see if either of us had ever seen or heard of it before. Neither of us has. It's also not listed in the latest version of the Saguaro Astronomy Club's extensive database of asterisms. So, good for you, Scott! Let's call it Harrington's Horseshoe, but to be very clear, that's SCOTT HARRINGTON, not me.
Above: This chart, created and provided by Scott Harrington, shows the location of his Horseshoe asterism with respect to M76. North is up.
If M76 was a bit much, then try your luck with Andromeda's NGC 7662, nicknamed The Blue Snowball. NGC 7662 lies in the western portion of the constellation. Scan the area for three 4th-magnitude stars: Lambda, Kappa, and Iota Andromedae. They a bent row lined up roughly north-to-south, some 16 degrees north-northwest of Alpheratz (Alpha Andromedae), the star shared by Andromeda and the Great Square of Pegasus. From Iota at the southern end of that row, you should spot 6th-magnitude 13 Andromedae about 2 degrees further west. NGC 7662 awaits less than half a degree to 13's south-southwest. While it will appear stellar through binoculars, its blue-green tint should make it identifiable among the other, mostly white stars. Defocusing ever so slightly often enhances its soft coloring.
On our way to M76, we passed north of the star Almach (Gamma Andromeda). To the naked eye and handheld binoculars, Almach looks like a single star. But with high magnification, its true nature begins to show. Turning my 25x100s its way, I see what German physicist Johann Tobias Mayer discovered in 1778, that Almach is a binary star. And not just any binary star, but one of the most colorful pairings in the entire sky. The brighter in the pair, Gamma-1, is a yellow star, while its fainter partner, Gamma-2, is blue. Some 10 arc-seconds separate them.
Can binoculars resolve a pair of stars that close? It all comes down to magnification. The resolving ability of a telescope is dictated by aperture, assuming quality optics. The low magnification of binoculars, however, usually makes magnification more of the determining criterion. Assuming the observer has 20/20 vision, the minimum resolution value for a given pair of binoculars can be estimated by dividing its magnification into 240. Using this, we find that a pair of 10x binoculars has a resolution threshold of 24 arc-seconds, while 20x binoculars can resolve 12 arc-seconds, and so on. I can make out the duality of Gamma Andromedae in my 25x monsters, but not in my 16x pair. How about you? Can you better the "240 Rule?"
I've saved the toughest for last. NGC 891 in Andromeda is probably the most photographed edge-on spiral galaxy in the entire sky. But it's very difficult to see visually, especially through common binoculars. To give it a go, look for it just 3.5 degrees east of Almach. That's where William Herschel was looking on October 6, 1784, when he discovered this gem. As we gaze toward NGC 891, we are seeing what our own Milky Way would look like from a sideways vantage point 30 million light years away. Like our galaxy, NGC 891 shows a pronounced central bulge and a distinctive opaque lane of cosmic dust encircling its outer perimeter. Those distinguishing features, so clear in photographs, remain hidden through binoculars. Indeed, simply find NGC 891 is difficult enough. It shines at 10th magnitude, but the fact that its light is spread over a thin, 13'x3' profile, which is then further diminished by the dark dust lane running its length, greatly complicates things. For me, it takes the magnification and light-gathering prowess of my 25x100s to confirm it. But maybe you can do better!
Now if all this has proven just too difficult, take heart. Notice what is centered on this month's chart? None other than M31, the Andromeda Galaxy! I can't possibly talk about all the difficult targets scattered in this part of the sky and ignore this magnificent spiral. Swing your binoculars its way and you'll immediately see a soft, oval blur highlighted by a prominent core. Only the central part is bright enough to pierce severe light pollution, but from a dark location, the full span of the broad spiral-arm disk becomes visible. M31 spreads its arms a full 5° -- that's as wide as ten Full Moons stacked end to end!
Also keep an eye out for its two neighbors, dwarf elliptical galaxies M32 and M110. M32, the smaller and brighter of the pair, is a visible as a small, almost stellar patch of light due south of M31's core. M110 is larger, but fainter, and therefore more difficult to see. Look for it to the north of M31's core, about twice as far as M32. M32 looks almost perfectly circular, while M110 appears more oblate.
As you can see from the list below, there are many other targets in this corner of the late autumn/early winter sky to explore tonight with your binoculars. Some are easy to see, some are hard. Why not give each a try?
Questions, comments, suggestions? Let’s talk! Post them in this column’s discussion forum or please e-mail them to me using the link above. So, until next year, tell your friends that for stargazing, two eyes are better than one.
About the Author:
Phil Harrington is a contributing editor to Astronomy magazine and author of 9 books on astronomy. Last month, his first book, Touring the Universe Through Binoculars, just marked 25 years in print. Visit his web site at www.philharrington.net to learn more.
Phil Harrington's Binocular Universe is copyright 2015 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.