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Binocular Universe: North for the Spring
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There's light at the end of the tunnel. The moment we've all been waiting for is almost here. Spring. SPRING!
As we begin to say farewell to Orion, Taurus, Canis Major, and the rest of the winter enclave for another year, we welcome the stars of Leo, Ursa Major, and the rest of the spring collection. While not as bright as the winter stars, the spring sky carries with it many exciting targets to which we can raise our binoculars.
Above: Spring 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
We begin with two of my
favorite galaxies, M81 and M82. Nowhere else in the binocular sky
do we see two such diverse galaxies set so close to each other. You may also
know M81 as Bode's Galaxy for its discoverer, German astronomer Johann Bode,
who found it in 1774. Five years later, Pierre Méchain rediscovered the galaxy,
with his colleague Charles Messier adding it to his growing list of cometary
imposters shortly thereafter.
Studies conclude that M81 spans about 90,000 light years and lies 12 million light years away. As seen from Earth, it shines at magnitude 6.9 and measures 10x21 in size. That brings it well within range of binoculars, even with moderately light-polluted suburban skies. In fact, under ideal conditions, some eagle-eyed observers have reported seeing it without any optical aid at all! But for us mere mortals, M81 appears through binoculars as an oval smudge of grayish light surrounding a brighter stellar core.
Finding M81 in the first place, however, can be a bit of a chore. Heres how I do it. The galaxy lies at the end of a long line extending to the northwest from Phecda through Dubhe, both in the bowl of the Big Dipper. The galaxy and Phecda are about equal distances from Dubhe, but set directly opposite one another. Looking through your binoculars, star hop (more like star bound) from Phecda to Dubhe, then on toward M81. You will pass a arc of faint stars along the way before coming to a small right triangle, with the star 24 Ursa Majoris marking the right angle. The triangle lies just to the northwest of M81.
M81 is thought to be smaller than our Milky Way, but it is still massive enough to serve as the central character in a group of some 34 galaxies known appropriately as the M81 Group. Of those, M82 is a real attention getter.
Lying half a degree to the north, neighboring M82 was also discovered by Bode in 1774 and added to Messier's list after Méchain rediscovered it in 1779. Nicknamed the Cigar Galaxy for its stogy-like shape, M82 is often cited in books as the quintessential example of an irregular galaxy. Turns out those books are wrong. All that changed when the discovery of two spiral arms was announced in a paper entitled The Discovery of Spiral Arms in the Starburst Galaxy M82, which appeared in the July 2005 issue of Astrophysical Journal. Examining images of M82 taken at near-infrared wavelengths, authors Y. D. Mayya, L.Carrasco, and A. Luna noted that we are seeing M82 nearly edge-on. Between that challenging orientation, the high disk surface brightness and presence of a complex network of dusty filaments in the optical images are responsible for the lack of detection of the arms in previous studies.
Some people can see M82 easily through binoculars, while others miss it completely. Even two observers standing side-by-side may have two totally different experiences seeing M82. It seems that spotting M82 through binoculars depends as much on a binocular's exit pupil as it does on sky darkness. In fact, I will go out on a limb and say that the exit pupil is even more important than sky darkness when it comes to seeing M82.
To check out this theory for my book Cosmic Challenge, I conducted a simple test using three different pairs of 50-millimeter binoculars from my suburban backyard. Since all three had the same aperture, so their light-gathering areas were identical. Each was mounted on a tripod to minimize shaking. I also waited until both galaxies were high in the sky to eliminate as much sky glow as possible. First, I raised my old 7x50s (7.1-mm exit pupil) their way. M81 was clearly visible, but I could do no better than suspect M82's existence with averted vision. I next switched to my 10x50 binoculars (5-mm exit pupil). Sure enough, M81 was again clearly visible, as was M82 with averted vision. Finally, I tried a pair of 16x50 binoculars (3.1-mm exit pupil). The field background was noticeably darker, but sure enough, M82 was also more obvious.
The reason for the steady improvement is two-fold. First, by decreasing the exit pupil, the brightness of the background sky also decreased, improving image contrast. At the same time, the higher magnification increased image size, which also helps to bring out low-contrast targets. Give it a try yourself if you can and see if your results mirror mine. I'd love to hear what you find.
Moving southward into the Big Dipper's Bowl, test your skills by looking for the planetary nebula, M97. Nicknamed the Owl Nebula for a pair of "wide eyes" visible through larger telescopes and in photographs, M97 is actually very easy to locate, but paradoxically, hard to see because of its low contrast and surface brightness. But I have seen it through my 10x50s under dark skies, so don't be afraid to give it a go.
To spirit out the Owl, aim toward Merak, the southwestern star in the Bowl. There, take aim at a dim star that lies about a degree to the east-southeast. The Owl's small disk is just to its east. Through most binoculars, it will show as a faint, oval glow. Use all the tricks to help make it out, such as mounting your binoculars on a tripod or other support, and using averted vision. The Owl will eventually show itself to those who persevere.
Méchain discovered M97 in 1781, with Messier viewing it later that same year. Although it may look like a simple disk even in photographs, the Owl is actually a very complex object. Like more planetary nebulae, the Owl is believed to be shaped like a torus, or a donut. We are viewing it slightly off center, so the "eyes" of the Owl are actually the comparatively empty donut holes.
The Owl is perched less than one degree to the southeast of our next target, the galaxy M108. Both are in the same field of view, so once you find M97, don't move! Look for M108 about halfway between Merak and M97.
Here is a true challenge object for all but the biggest binoculars. While M108 is technically bright enough to be seen through 10x50s, it won't exactly "wow" you. Even peering through giant binoculars, all you will see is a pencil-thin smudge of grayish light. The long, thin shape is caused by M108's nearly edge on tilt from our earthly vantage point. Studies reveal that M108 is actually a broad-armed Sc spiral, even we can never see its spiral arms.
Historically, M108 was one of those add-ons to Messier's original catalog, which originally ended with M103. But there is no doubt that he did see it. In fact, Pierre Méchain discovered M108 in February 1781. It was not until 1953, however, that Harvard astronomer Owen Gingerich, researching Messier's own hand-written notes, found the reference and, nearly two centuries later, inserted it to the catalog.
Okay, we have one more Messier object to go this month. Center your attention on Megrez, the star that joins the bowl of the Big Dipper with the handle. Without shifting your aim, look 1° to the northeast for 6th-magnitude 70 Ursae Majoris (unlabeled on the chart here). Got it? Now glance less than a third of a degree farther northeast for a lone 9th-magnitude star. That's M40.
What? M40 is just a star? In most 50mm and smaller binoculars, that's probably all you'll see. My 16x70s and 25x100s, however, resolve what Messier must have seen through his telescope: a pair of 9th-magnitude stars separated by about 50 arc-seconds.
Admittedly, that's not much better. That's because M40 was a cataloging mistake! The trouble began in 1660, when the German astronomer Johannes Hevelius recorded a "nebula above the back" of Ursa Major. Try as he might to duplicate the observation a century later, Messier could only find "two stars, very close together and of equal brightness, about 9th magnitude...it is presumed that Hevelius mistook these two stars for a nebula." Even though he recognized it as just two stars, Messier still included it as his catalog's 40th entry in 1764 for reasons unknown. In 1863, Hevelius's "nebula" was again rediscovered, this time by Friedrich August Theodor Winnecke at Pulkovo Observatory in St. Petersburg, Russia, who subsequently included it as the fourth listing in his double star inventory, entitled "Doppelsternmessungen" (Double Star Measurements). As a result, M40 is often cross-listed as Winnecke 4.
Much more recent investigations suggest that the two stars are not physically related at all. Therefore, despite the pair's similar appearance, M40 is merely an optical double rather than a true binary system. But no matter. If you're on your way to a Messier certificate, such as that offered by the Astronomical League, it's another notch in your belt toward your award.
As you can see from the list below, there are many more targets within this month's Binocular Universe also vying for your attention. See how many you can find and post your results in this article's discussion forum.
And if you have suggestions for future targets, by all means post them there, as well. Till next month, remember that 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, including Touring the Universe Through Binoculars. Visit his web site at www.philharrington.net to learn more.
Phil Harrington's Binocular Universe is copyright 2016 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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