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Binocular Universe: Gone Fishin'

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Binocular Universe:
Gone Fishin'

October 2015

Phil Harrington

The wet quarter of the sky is made up of several constellations that are associated with water in one way of the other. These include Capricornus, the Sea-Goat; Aquarius, the Water-Bearer; Piscis Austrinus, the Southern Fish; Cetus, the Whale; and Pisces, the Fishes. All ride low in the southern sky from the vantage point of most of us living in the Northern Hemisphere. As a result, you may need to travel to a dark sky that is free of terrestrial obstructions, low-slung light pollution, and any horizon-hugging haze in order to see some of the targets explored below.


Above: Autumn star map from Star Watch by Phil Harrington.
Click the chart to open a printable PDF version in a new window.


Above: Finder chart for this month's Binocular Universe.

Chart adapted from Touring the Universe through Binoculars Atlas (TUBA)
Click the chart to open a printable PDF version in a new window.

The first stop on the tour, however, is bright enough to be spotting from most locations that have an unobstructed view to the south this month. The star Fomalhaut in Piscis Austrinus is the only star in the wet quarter that cracks the first-magnitude barrier. Because it is so low in most of our southern skies, Fomalhaut puts on quite a colorful show through binoculars, twinkling like a kaleidoscope. That's just a side effect of our atmosphere's turbulence. In reality, Fomalhaut is spectral type A3 and shines pure white.

Studies have found that Fomalhaut is surrounded by several debris disks. The outermost disk, which is sometimes referred to as "Fomalhaut's Kuiper Belt,” held a surprise that was discovered in 2008. Using the Hubble Space Telescope’s Advanced Camera for Surveys (ACS), astronomers discovered an extrasolar planet orbiting inside the debris ring. This was an historic find, as this was the first extrasolar planet to be photographed directly. Known as Fomalhaut b, the planet is thought to be less than twice Jupiter's mass.

Piscis Austrinus also holds a very nice binocular double star in its belly that very few of us have ever seen it. Dunlop 241 lies about 1.25 degrees northeast of Beta Piscis Austrini (PsA). The pair’s 5.9-magnitude primary star is separated from the 7.6-magnitude companion by 93 arc-seconds. That’s wide enough to be resolved in just any binocular. Both stars show a nice color contrast, with the brighter star a pale blue and the companion a pale yellow. Slightly defocusing the view will help enhance the colors.

Beta PsA is also a binocular binary worth attempting. You’ll find, however, it’s a much tougher nut to crack. Here, the 4th-magnitude is teamed with an 8th-magnitude companion that is a scant 30 arc-seconds away. That may be right at the limit of 10x50s if Beta is well above the horizon. But given that it rides the horizon for many readers, I’d say that is wishful thinking. Still, I hope you’ll give it a go and post your results in our on-line forum.

If you have been a binocular astronomer for long, then you know that binoculars and planetary nebulae usually do not mix. Even through telescopes, many look like nothing more than fuzzy stars, only revealing their true identity after careful examination at high magnifications. Low-power binoculars are usually left in the dust. There is one planetary nebula in the autumn sky, however, that measures half the size of the Full Moon in our sky and shines at 7th magnitude. Sounds like a perfect combination for binoculars.

NGC 7293, the Helix Nebula, is found among the dim stars of Aquarius, the Water-Bearer. Unlike so many objects listed in the New General Catalog, NGC 7293 was not discovered by the father-son team of William and John Herschel. Instead, discovery is credited to German astronomer Karl Ludwig Harding, who spotted it around the year 1824 using a 4-inch refractor at Göttingen Observatory.

Now, wait a minute! How could classic observers like Charles Messier, Pierre Méchain, and the Herschels have missed a 7th-magnitude object that is 16 arc-minutes in diameter? The answer is obvious to anyone who has ever tried to find the Helix through a long-focus telescope and narrow-field eyepieces, which is what these observers used for their discoveries. When you look for the Helix, it's just not there.

That's because simple data do not tell the whole story of the Helix Nebula. NGC 7293 is, in fact, the brightest and largest planetary in the sky. It is also the closest, at about 695 light years away. Why, then, is it so hard to see? Simply because it is so large. Imagine how dim a 7th-magnitude star would be if you defocused it until it measured the size of the Helix Nebula. It would pretty much fade into the background. The surface brightness, or brightness per square area, drops precipitously. Couple that fact with an object so large that it would more than fill the field of a narrow-field eyepiece and it's easy to understand how the Helix could be passed over and escape unseen. But aim toward the Helix through a pair of 50-mm binoculars on a dark autumn night, and its dim, ethereal disk should be visible without too much difficulty.

To find the Helix, place Fomalhaut along the southeastern edge of the field, 4th-magnitude Epsilon PsA should pop into view along the opposite side. Move 6°, or about another field of view, due north to find 5th-magnitude Upsilon Aquarii (Aqr). Finally, slide Upsilon just slightly to the east; the dim glow of the Helix should be nearly centered in view. 

Through my own 10x50s, the Helix looks slightly oval, its long axis tilted northwest-southeast. The outer edge is slightly irregular in texture, with brighter sections toward the northeast and southwest, coincident with the disk's minor axis. I can't say with any certainty that I can detect the nebula's central hole through my 10x50s, even though it is so obvious in photographs. My 11x80s and 16x70s turn the Helix into the familiar ring shape, but only with averted vision.

A second unusual planetary nebula is found in the extreme western part of Aquarius. NGC 7009 is most easily found by beginning at the triangle of neighboring Capricornus the Sea-Goat. Begin at 4th-magnitude Theta Capricorni, found along the top side of the triangle. Scan 6 degrees north-northeast to 5th-magnitude Nu Aqr, and then turn westward another degree to find you mark.

On nights of good transparency, NGC 7009 is visible as a fuzzy greenish point of light in steadily-supported binoculars and small telescopes. It stands out surprisingly well, partly due to its rather bleak surroundings, but mostly thanks to a remarkably high surface brightness (one of the highest of any planetary). This brilliance is caused by unusually strong ultra-violet radiation emitted by its central star. The greenish color results from double-ionized oxygen.

It takes at least a 6-inch telescope to reveal why this strange little enigma has been called the "Saturn Nebula." Two faint lobes protrude from either side of the cloud's round disk that are strongly reminiscent of Saturn’s rings when viewed at low power. Hence, the nickname. 

John Raymond mentions many interesting binocular sights in his book Asterisms for Small Telescopes and Binoculars (2005: Brunswick Publishing, ISBN-10: 1556182058), including several interesting stellar triangles and star chains that collectively form the Waters of Aquarius

To ride the rapids, begin at the spout of the water jar, where we find is a small triangular asterism formed by the stars Psi-1, Psi-2, and Psi-3 Aqr. Psi-1, the northernmost of the three, may look faintly orange if you defocus its image slightly. Five 5th-magnitude stars -- 103, 104, 106, 107, and 108 Aqr -- create a third set of rapids further south still. The star 103 Aqr also impresses me as slightly orangish, especially when compared to white 104 directly to its north. Two more collections of three-star rapids, formed by 98-99-101 Aqr and 86-88-89 Aqr, complete our journey southward back toward Fomalhaut. The stars 86, 88, and 89; 98, 99, and 101; and 104, 106, 107, and 108 Aqr all fall into a helix of sorts. This twisted appearance led to their being christened the DNA Strand in the Saguaro Astronomy Club’s asterism database. If you aren’t familiar with this free resource, follow the link to learn more.

These and other targets within this month’s Binocular Universe are listed in the table below.

Next month, we will continue our trek across the southern autumn sky. Till then, remember my mantra: 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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