Gloucester Area Astronomy Club

NCG 7293 – the “Helix Nebula”
By Glenn Chaple

NGC 7293, the Helix Nebula, is the nearest planetary nebula (distance ~ 450 LY) and largest in apparent size (12 by 16 arcminutes). Moreover, it’s a 7th magnitude object. An easy telescopic target? Hardly! The magnitudes listed for deep-sky objects are often misleading, and the Helix Nebula is a prime example. Were you to defocus a 7th magnitude star until the image covers half a moon diameter, you’d have an idea of the visual appearance of the Helix. In Visual Astronomy of the Deep Sky, author Roger N. Clark notes that its average surface brightness is 20.8 magnitudes per square arcsecond.

Despite its faintness, the Helix Nebula can be readily observed. On a clear, moonless night in dark-sky areas, it may be glimpsed with binoculars. In fact some keen-eyed observers in extremely remote locations have spotted the Helix with the unaided eye! The key to viewing the Helix by telescope is to use a telescope/eyepiece combination that can produce a field about one-half to a full degree across. Because of its southerly location, you’ll want to select a viewing site with an open southern horizon, free of any sky glow.

I first saw the Helix on July 31, 1981 from the clear skies of Stellafane. Through my 3-inch f/10 reflector at 30X, it appeared as a “large, tenuous glow.” Stellafane regular Peter Kandefer peeked into the eyepiece and confirmed my sighting. More recently, I had no trouble spotting the Helix with a 4-inch f/8 reflector. The key in both instances was to know exactly where to look. The accompanying finder chart pinpoints the Helix Nebula’s location in the southern part of Aquarius about 1 ½ degrees west of the 5th magnitude star Upsilon Aquarii.

The Helix Nebula offers three challenges:
1. Capture it with binoculars or small telescope
2. Discern its annular form with medium to large-sized telescopes
3. Spot its 13th magnitude central star
Are you up to the challenge? On the next clear, moonless autumn night, try your luck with NGC 7293, the Helix Nebula.

Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.

Epsilon Pegasi – the “Pendulum Star”
By Glenn Chaple

This month, we’re going to pay a visit to epsilon Pegasi (Enif), the “Pendulum Star.” It’s an optical double star comprised of magnitude 2.5 and 8.7 component stars separated by 144 seconds of arc. Pairs this wide usually don’t merit much consideration, but wait! Epsilon Pegasi has a surprise for us.

After centering this pair in the eyepiece field (60 – 100X is the recommended magnification), mentally trace a line between them. While keeping your eye at the eyepiece, gently jigglethe telescope back and forth so that the two stars move at right angles to the imaginary line. While the golden yellow primary (a K-type star) travels serenely back and forth, the little companion seems to swing wildly to and fro, like a clock pendulum. It’s one of the most amazing telescopic optical illusions you’ll ever witness.

What’s happening? According to Sir John Herschel, who was among the first to describe the “Pendulum Star,” the oscillations are due to the longer time it takes light from the faint star to affect the retina. We detect the motion of the primary a split second earlier, so the companion seems to lag behind. Rapid back and forth movement of the telescope generates the illusion of pendulum-like motion.

The Pendulum Star received plenty of recognition in astronomy guidebooks written in the late 19th and early 20th century – a time when double stars enjoyed tremendous popularity. Nowadays, with attention directed towards nebulae, clusters, and galaxies, epsilon Pegasi receives scant notice.

The finder chart shows the location of epsilon Pegasi. If you hunt down deep-sky objects by the star-hop method, you may recognize it as a pointer (with nearby theta Pegasi) to the globular cluster M15. Next time you plan to visit M15, take a moment to check out epsilon Pegasi. This star will put on a show that’s sure to dazzle!

Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.

“Chaple’s Arc”
By Glenn Chaple

Forgive me for the apparent ego trip, but this month I’m going to introduce you to an amazing little asterism called “Chaple’s Arc.” I stumbled upon the Arc in the mid-1970s while looking for the double star h1470. Instead of one double, I found four arranged in an arc 1/2o across. So smitten was I by its extraordinary appearance that I eventually wrote about it in the September 1980 issue of Deep Sky Monthly. New York amateur astronomer John Pazmino viewed the group and dubbed it “Chaple’s Arc.”

A quarter century later, I decided to introduce the Arc to a much larger audience by featuring it in my “Observing Basics” column in Astronomy. To my amazement, I saw the same group described in the British magazine Sky at Night. The writer called it the “Fairy Ring.” Uh-oh! Had I missed something?

After a little detective work and an assist from Sky and Telescope’s Sue French, I learned that the Arc had been seen by Utah amateur astronomer Kim Hyatt in the early 1990s. Like me, he found it during a search for h1470. Because he was using a larger telescope than I had, he was able to view some faint pairs that, along with my four, formed a ring of double stars. Not knowing about Chaple’s Arc, he and a friend christened it the Fairy Ring.

This summer you can view a famous Ring (M57) for the gazillionth time, or you can be one of the few to glimpse a much lesser-known Ring (the Fairy). Here’s how to find it. Using a low-power eyepiece, trace a line from eta Cygni to 25 Cygni and extend it a half degree beyond to the Arc.

For an interesting discussion on Chaple’s Arc/the Fairy ring, Google “Chaple’s Arc” and look for the Cloudynights thread in the subject. Whether you call it Chaple’s Arc or the Fairy Ring, this is one asterism that will astound and delight you.

Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.

Messier 6 and Messier 7
By Glenn Chaple

The song “Love and Marriage,” contains a line that goes, “you can’t have one without the other.” The words aptly describe the open clusters M6 and M7 in Scorpius. This cosmic “horse and carriage” lies in the southern sky above the Scorpion’s stinger.

M7 is the brighter and larger of the two. With an overall magnitude of 3.3, it spans 80’ – over twice the moon’s apparent diameter. Readily seen with the unaided eye in the absence of bright moonlight or city lights, M7 was first reported by the Greek astronomer Claudius Ptolemy nearly two millennia ago. “Ptolemy’s Cluster” is a dazzling sight in binoculars and small rich-field scopes – a striking aggregation of some 80 stars between magnitudes 6 and 10, immersed in a sparkling background of Milky Way stars. Because of its large size, M7 appears rather sparse in large-aperture scopes. Current studies indicate that M7 is 800 light-years away and is approximately 200 million years old.

Just five degrees northwest of M7 is its partner M6. This cluster, which is a magnitude fainter than M7 and one-third as large, lies just outside the glow of the Milky Way. Like M7, M6 is visible to the unaided eye and was recorded by Ptolemy. Rather than be saddled with the nick-name “Ptolemy’s Cluster II,” M6 was dubbed the “Butterfly Cluster.” The outline formed by its brightest stars does indeed resemble the outstretched wings of this insect. M6, like M7, is at its visual best when viewed with binoculars or small RFTs. With the latter, you can see about 80 stars brighter than 11th magnitude. The most luminous of the cluster’s member stars is the reddish-orange semiregular variable BM Scorpii, whose magnitude fluctuates from 5.5 to 7.0 in a cycle of roughly two years. M6 is twice as remote as M7 and half its age.

Though the splashier M7 seems to get better reviews, I found M6 to be a more attractive sight when recently observed in the 1.5o field encompassed by my 4-inch f/4 Astroscan at 35X. M6 appeared as a tight little group, while M7 seemed sparse. Oddly enough, I was more impressed by M7 when I first viewed the two clusters with a 3-inch f/10 reflector at 30X back in the summer of 1977. Compare M6 and M7 and see what you think. Now if I can just get that “Love and Marriage” melody out of my head!

Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.

Finder chart for M6 and M7 From Mag-7 Star Atlas
(Copyright Andrew L Johnson)

44 Bootis
By Glenn Chaple

Rule #1: Never write about a deep-space object you haven’t seen.

Rule#2: Ignore Rule #1.

In the early 1970s, during my tenure as a “rookie” backyard astronomer, I observed double stars with relentless abandon. My instrument of choice at the time was a 3-inch f/10 reflecting telescope, made by Edmund Scientific. For a reference, I chose the 1966 edition of Norton’s Star Atlas.

One evening, I decided to dine on double stars in the constellation Bootes. According to Norton’s, one particular pair, 44 Bootis, had a separation of 2.6 seconds of arc – close, but not impossible in a 3-inch scope. To my surprise and disappointment, I couldn’t split the pair – not that night or on subsequent evenings. Had I read Norton’s more carefully, I would have seen a note describing 44 Bootis as a binary pair that was closing. I would later learn that its magnitude 5.3 and 6.2 components were separated by a mere 0.4 arc-seconds at the time of my futile attempts.

Fast forward four decades to the present. 44 Bootis, whose 210-year orbit is highly inclined to our line-of-sight, has opened up. Orbital data indicate that its component stars are separated by 2.2 arc-seconds. Time for a feast!

I haven’t yet seen 44 Bootis, at least not double. But I’ll be outside this month trying. Although a 2+ arc-second separation is within reach of a 3-inch, I’m going “loaded for bear.” My instrument of choice this time will be a 5-inch f/12 Maksutov-Cassegrain telescope, paired with an eyepiece that magnifies at least 150X. To be safe, I’ll conduct the observation on a night of above average seeing. Instead of my reporting what I ultimately see, check out 44 Bootis for yourself, and we’ll compare notes.

Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.

Mizar, the First Double Star
By Glenn Chaple

It was my high school friend Ray Gerbi who introduced me to backyard astronomy. On a late summer evening in 1963 (yes, I am that old!), he plunked a Gilbert reflecting telescope on an open area in his back yard and proceeded to show me Saturn and a pair of fuzzy blobs of light (M13 and M31). But the highlight of the evening, an event that would give direction to my pursuits in amateur astronomy, came when Ray showed me the double star Mizar. I was totally mesmerized by the sight of two sparkling white stars shining side-by-side. For years afterward, double stars would be my favorite deep-sky objects.

The middle star in the handle of the Big Dipper, Mizar is the easiest double star for the novice to locate. It boasts historical importance as the first double star discovered. Until a few decades ago, credit for the discovery went to the Jesuit astronomer Jean Baptiste Riccioli, who reported the star’s duplicity around 1650. But a recent investigation of Galileo’s notebook reveals that a former student of his, Benedetto Castelli, saw Mizar several decades earlier.

If you study Mizar carefully with the unaided eye, you’ll see a fainter star close by. This is Alcor, Mizar’s 4th magnitude optical companion. The two appear at opposite sides of a low-power telescopic field. Look closely at Mizar. This 2nd magnitude star is attended by another 4th magnitude companion just 14 arc-seconds away. The pair lies about 80 light-years from Earth.

May brings us Astronomy Day and the opportunity to introduce backyard astronomy to the general public. If you decide to become involved in a public star party, be a Ray Gerbi and introduce your guests to Mizar. You might just inspire a future double star afficionado.

Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.

Saturn’s Moons
By Glenn Chaple

Every fifteen years, thanks to the nuances of Earth’s and Saturn’s orbits, Saturn’s rings appear edge-on to our line of sight. During a “ring plane crossing” these incredibly thin ornaments virtually disappear in all but the largest telescopes.

We might bemoan the loss of Saturn’s rings (they are a major draw at public star parties), but without the glare they produce, we can better view Saturn’s retinue of moons. 8th magnitude Titan is the easiest and can be glimpsed in a common 60mm refractor. Rhea (magnitude 9.5), Tethys (10.0) and Dione (10.2) will require 4 to 6 inch scopes. An adventurous skygazer, using a telescope in this size range, might try for magnitude 11.5 Enceladus

Now that Saturn is past opposition and well-placed in the evening sky, take the opportunity to view and identify some of those moons you’re never before seen. A moderately high magnification will increase contrast and make the fainter moons more readily visible. Carefully sketch the field, then go inside to confirm your “kills.” If you’re fortunate enough to own a copy of the RASC Observer’s Handbook 2009, turn to pages 245-248 for a graphic showing the positions of Saturn’s major moons at any date and time of the year. You can also use Sky and Telescope’s interactive Saturn moon finder. Go to www.skyandtelescope.com, scroll down to “Interactive Observing Tools,” click on “Saturn’s Moons,” then submit the date and time of your observation. You’ll need to register to obtain this service, but it’s free.

Take advantage of the current Saturn ring-plane crossing. We won’t experience another until 2025!

Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.

NGC 2362 – Open Cluster in Canis Major
By Glenn Chaple

A remarkable, yet little-known open cluster is NGC 2362 which surrounds and includes the star tau Canis Majoris. This neat little stellar gathering is located three degrees east and slightly north of delta Canis Majoris. Observing guides describe NGC 2362 as a 4th magnitude object, but this figure is misleading. Much of the cluster’s brightness is taken up by tau. The rest of the 40-odd member stars are magnitude 7 to 13, and are concentrated in an area about 6 arc-minutes across. A 5000 light year distance translates to an actual diameter of 9 light years. From this distance our sun would appear as a magnitude 15.5 speck!

What makes NGC 2362 such a visual delight is its appearance in small-aperture scopes. When I look directly at tau with my 3-inch f/10 reflector, I see a handful of tiny stars around it. When I turn my gaze to the side (averted vision) the field fairly explodes with stars. It’s an amazing transformation!

Considering the fact that William Herschel was enchanted by the beauty of NGC 2362 and that Sir Patrick Moore recently included it in his Caldwell Catalog as one of the finest non-Messier objects, this cluster deserves your attention. Before Canis Major fades into the sunset, give NGC 2362 a look-see!

Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.

Ceres
By Glenn Chaple

What makes amateur astronomy such a wonderful hobby is the diversity of activities it offers. We can make telescopes, dabble in astrophotography, or simply go outside with binoculars and notch a few Messier objects. One of my favorite astronomical pastimes is observing asteroids. There are plenty to see – I’ve glimpsed over a hundred with an ordinary 3-inch reflecting telescope.

As the name implies, an asteroid is starlike in appearance, giving it a low score on the “wow” scale. In fact, you can’t be 100% sure you actually spotted an asteroid until you return outside an evening or two later to verify that the suspected object moved relative to the background stars. If it has, you’ve notched another asteroid. It’s the astronomical equivalent of rock collecting.

The largest of these cosmic rocks is Ceres. Approximately the size of Texas, Ceres was the first asteroid discovered, having fallen to the Italian astronomer Giuseppe Piazzi on January 1, 1801. Originally considered to be a planet, Ceres was downgraded to minor planet status when more of these tiny worlds were discovered in the gap separating Mars and Jupiter. In the summer of 2006, the International Astronomical Union (IAU) formally drafted a definition of planet. Ceres was reclassified as a “dwarf planet.”

During February, Ceres will be cruising through Leo, reaching 7th magnitude around the time of opposition on the 25th. You should have little trouble spotting it as long as you use good binoculars and the finder chart on this page. Don’t be disappointed should you fail to see anything more substantial than a tiny stellar speck. Even when scrutinized with the Hubble Space Telescope or large earthbound telescopes enhanced with adaptive optics, Ceres reveals precious little surface detail.

That situation is about to change. On September 27, 2007 NASA launched the Dawn Mission. It will reach the asteroid Vesta two years from now and rendezvous with Ceres in 2015. Astronomers will at last have an in-depth look at these little worlds. Until then, binoculars or telescopes are your only ticket to asteroid adventures. Whatever you call it – asteroid, minor planet, or dwarf planet – Ceres is a worthwhile first specimen for your cosmic rock collection.

Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.

Struve 817 Orionis
By Glenn Chaple

I’m a big fan of “off-the-beaten-path” sky objects. One of my favorites is the little-known double star Struve 817 – the 817th double star catalogued by the German-born Russian astronomer F. G. W. Struve during a survey conducted between 1824 and 1827. According to a measure made in 2004 and posted in the Washington Double Star Catalog, Struve 817 consists of near-twin magnitude 8.7 and 8.9 stars separated by 18.8 ascseconds in a position angle of 73o. The separation and P.A. differ little from what Struve himself measured around the time of discovery. Double star astronomers describe stellar partnerships that show little relative motion as being “relatively fixed.” If the component stars of Struve 817 form a true binary pair, their orbital period must encompass many centuries.

What gives this relatively obscure double star a special allure is its location, less than a half degree south of the red supergiant Betelgeuse. To find Struve 817, simply aim your telescopeat Betelgeuse. A medium power eyepiece (75 to 100X works well) should capture this delicate pair shining bravely beside the dazzling ruddy Betelgeuse. It’s a startling contrast. Speaking of contrast, the Washington Catalog lists the spectra of Struve 817’s components as A5 and K. Can you make out a color contrast between the two?

Some years ago, I wrote a four-part series for Deep Sky Magazine in which I introduced my favorite 100 double stars. Included with such celebrated pairs as Mizar, Albireo, and the “Double-double” epsilon Lyrae was Struve 817. On the next crisp winter night when Orion beckons you to visit his magnificent Nebula, take a minute to travel a road less taken and try for this delightful double star.

Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.