JULY

| CHAPTER 7

Peering into the Cat's Eye

"Skies around the north ecliptic pole in Draco are rather barren," Scotty once wrote, • but a few interesting objects wait for the zealous observer." Take the bright planetary nebula NGC 6543, for example. Scotty believed that pale green planetary nebulae of its caliber were sometimes neglected by amateurs. It's easy to understand why. Some are so small they could be easily mistaken for stars. "When searching for one of these emeralds," Scotty advised, "the observer must first locate the field very carefully, and may have to examine many stars at higher power to discern the nebula's tiny disk." The Cat's Eye is different, however. Through a telescope it appears as a small bluish disk about the angular size of Jupiter, and looks like an out-of-focus 9th-magnitude star. It does offer special challenges to observers with all manner of observing aids. But, as Scotty attests, a glimpse of it through a large-aperture telescope can turn the purest scientist into a hopeless romantic.

You are all poets,” 1 told a gathering of amateur astronomers at the 1983 annual Texas Star Party. At first they reacted with silence. Then they began to agree. The common thread that binds amateurs together is a love of the grandeur and beauty of the starry deeps. While some may claim it’s the science of astronomy that interests them. I believe that deep down it is the ultimate experience of the night sky that holds the real attraction.

After my talk I received a letter from Christine Combs of Colleyville, Texas. “I ^Was introduced to NGC 6543 at the Texas Star Party.” she wrote. “The first sight ^was breathtaking... to see the bright inner star surrounded by greenish-blue neb-tilosity ... we observe with a 20-inch Dobsonian, usually with 200x. Last night I ^exPerienced anew the excitement of seeing this greenish cat eye buried deep in sky for me to wonder at. My husband, an engineer and long-time amateur. ^may give a more detailed description, but I'm the poet and always strive for words ⁰ describe the feeling aroused by these deep-sky wonders.”

The object that captured Combs's imagination is one of the most glorious plan-f^tary nebulae in the sky (Figure 7.1). Located in Draco. NGC 6543 lies only 10' ^Orn ^e ecliptic’s north pole.The ecliptic is the annual path of the Sun among the Figure 7.1 NGC 6543, the Cat's Eye (center) in Draco, is estimated to be 1,000 years old.

Its complex structure reveals the dynamics and late evolution of a dying star and its companion.

stars — a great circle on the sky, tipped 23/2° to the celestial equator. Thus NGC 6543 lies at the center of the circle traced by the Earth’s pole during its 26,000-year precessional wobble. Because of this unusual location, precession has little effect on the planetary’s right ascension and declination.

Combs’s remarks sent me hunting for my earliest observing records, made in grade school more than half a century ago. Sure enough, I had penned: ‘6543, which Astronomy with an Opera-Glass says is a planetary. I see a green star, but much wider than the other stars around it.” These notes describe views with a homemade 1-inch refractor at 40x. A larger instrument will easily reveal its nebular nature. My 4-inch Clark refractor shows it as a rather uniform bright disk about 20" in diameter, but high magnification reveals areas of irregular brightness. Photographs show the nebula has a coiled form similar to the much larger Helix Nebula in the constellation Aquarius. Its length is about 22". or two-thirds the separation between the components of the double star Beta (P) Cygni (Albireo).

NGC 6543 made a bit of astronomical history in the 1860s. Before then it was assumed that most nebulous objects were actually clusters of very faint stars, and a failure to see them as such was merely due to lack of a big enough telescope. NGC 6543 was one of these unresolved objects. Then, on August 29. 1864, William Huggins turned an 8-inch refractor equipped with a visual spectroscope toward the nebula. (He was the first person to turn a spectroscope toward a planetary nebula.) Rather than a continuous streak of light spanning the visible spectrum, Huggins discovered that the spectrum of NGC 6543 consisted of three bright lines, the strongest being in the green. Thus, “at a glance” he realized that NGC 6543 was a cloud of luminous gas and not, as some astronomers had

_ueht, a mass of unresolved stars. With this single observation came proof that

•ill deep-sky objects arc made of stars.The mysterious green lines were attrib-^111,1 | to a hypothetical element called nebulium. More than half a century would ^llU before these lines were recognized as due to doubly ionized oxygen. ^fjGC 6543 ^{offers several} challenges for the observer. One concerns the nebu-• color, and another the brightness of its central star.Through his obervations, William H. Smyth found the planetary a "very fine pale blue," while other bservers have seen it more as green. Smyth seems to have had an unusually °cute sensitivity for color; most amateurs today cannot distinguish the multitude _of shades he could. 1 suggest using a magnification of about lOOx when making color estimates.

NGC 6543’s central star shines at about magnitude 11, but the interior stars of planetarics arc more difficult to see than their magnitudes suggest. That the central star is surrounded by bright nebulosity means there exists a contrast effect, which poses a problem when attempting to judge the star’s brightness. Modern catalogs generally list 9.5 as the visual magnitude of the star. However, my own estimates tend to favor a fainter value. The central star is also bright when compared with other planetaries. With a 3- or 4-inch telescope at 30x, NGC 6543 may look like little more than a swollen star, but as Combs’s letter testifies, with a large instrument the nebula can be very impressive.

I recently hunted up NGC 6543 with my 4-inch Clark refractor. At 120x the view with my normal eye showed the planetary and its central star much as expected. With the other eye, which had its lens removed during a cataract operation, the star was brighter than the nebula! This was due no doubt to much more ultraviolet light from the hot star reaching the retina of my lcnsless eye. Deep photographs record a faint shell about 4' across surrounding the planetary, but I doubt it can be seen visually.

In 1985 I asked for reports from amateurs who examined NGC 6543 with large telescopes. One such report came from Michael Gardner of Sunnyvale, California. As a member of the Mount Wilson Observatory Association, he had viewed it with the observatory’s 60-inch reflector! It was a particularly good night, as fog had rolled into the Los Angeles basin and obscured the city lights. Furthermore, 'he fine seeing for which Mount Wilson is so noted was even better than usual.

We were observing at the Cassegrain focus of the 60-inch,” Gardner writes. “A 55-nun Plossl produced 450 power and a field of view about 6/2' in diameter. NGC ^543 was a stunning blue-green oval. The colors were like a Kodachrome. We saw ^much more structure in the planetary than the picture [S£y & Telescope, July 1985, P^agc 89] shows, and the colors were much more vivid and bluish." Gardner did not $^{ee l}^^e faint shell about 4' across that surrounds the nebula on deep photographs, ^Ul h^e was not specifically looking for it.

Often planetary nebulae will show best within a very narrow range of magnifi-^atlOn for a given telescope. Once you have located one of these glows, spend ^e time experimenting with different eyepieces (don’t forget to try a Barlow lens if you have one). Aperture masks may also improve the view. Careful exam ination of an object can sometimes reveal details not generally noted in observ ing handbooks. For example, while viewing NGC 6543 I found the central star "blazing yellow," due to its strong contrast with the nebula's blue color. I do not recall ever reading about this effect before.

The Crown Jewels

Observing is a continual learning experience. At first, time spent under crystalline starlight has all the flush and romance of a soft embrace on a warm summer's eve Wasn't that first glimpse through a telescope enchanting? As we grow and become more knowledgeable, we tend to move past the romance and begin to challenge ourselves and our visual limits. We start first with the naked eye and binoculars, then move on to modest-sized instruments, until we find ourselves delving ever deeper into the universe with ever larger telescopes. Once we have years of experience under our belts, we start seeking out celestial treasures that seemed impossible when we first started out. For more than four decades Scotty watched amateur astronomy progress in this way, as if our visible horizons and telescope technology were expanding in sync with the universe. Here Scotty shows how one of the sky's smallest constellations, Corona Borealis, can satisfy both the naked-eye novice and the monster-telescope owner.

The stars are always with us. Night after night the blaze of distant suns stretches from horizon to horizon. For many of us the fascination of a starry sky began even before grade school, as the spectacle of the heavens astonished and excited our imagination. As we tally more and more memorable hours under the night sky, the sensation is cumulative. It makes no difference whether we observe with the naked eye, a 4-inch telescope, or a 36-inch Dobsonian.

With each passing year the parchment of the sky comes to hold more information, more contentment, and more wisdom for us. We know that as we step out under the stars tonight we will be headed for a reunion with some old friends. Nevertheless, as observers we must be prepared to push beyond our accepted frontiers, perhaps even to the point of having to reject some “truths" to which we have devotedly clung.

As darkness falls on July evenings, one frontier I always turn to is Corona Borealis, the Northern Crown. The constellation rides high in the sky, aloof front the fainter stars of Bootes and Hercules. This ancient group is one of the few star patterns that span many cultures in essentially the same form.

Two objects in Corona Borealis are of special interest to naked-eye observers. Both are variable stars, but they have very different behaviors. R Coronae Borealis (Figure 7.2) is usually 6th magnitude and just visible to the naked eye under good conditions. It remains relatively constant, sometimes for years; then it can abruptly plunge to around 14th magnitude. Recovery is often slow, and

here ca^{n man}y relapses. Sometimes the star takes several years to regain its filial brightness. I check the naked-eye visibility of R Coronac Borealis every *|car night — if it's missing, out comes the telescope.

Sonic 3° to the southeast is another interesting variable. Unlike R Coronae gorealis. this one attracts the most attention tv/ten it attains naked-eye visibility. T Coronae Borealis is a recurrent nova and typically hovers around 10th magnitude. p_ut it has exploded to 2nd or 3rd magnitude. This happened in 1866 and 1946, and lesser flare-ups occurred as recently as 1963 and 1975.The next burst will likely be first spotted by an amateur who checks T Coronae Borealis with the naked eye or binoculars.

On the other end of the spectrum, the explosive growth of amateur astronomy has produced an interesting group of specialists — those who delve into the deepest of deep skies. While such people don't lend themselves to easy description, I've pieced together a rough profile based on my mail. They are individuals who observe alone. They use at least a 12-inch telescope, more typically a 17-inch or larger instrument.They are experts at getting the most from their telescopes. They keep the optics as clean as possible to reduce the scattered light that dilutes the contrast between a faint object and the sky background.They also make proficient use of the various nebula filters currently on the market.

Figure 7.2

Why R Coronae Borealis suddenly fades from view is uncertain, though it may be related to a shell of light-absorbing carbon particles surrounding the star. T Coronae Borealis is a nova and brightens periodically.

Objects in the New General Catalogue of Nebulae and Clusters of Stars (NGC) are ^Slr>all potatoes to these people. The NGC objects were discovered visually in the 19th ^Ceniury. so they lack the attraction of fainter quarry — objects that were likely discovered on photographs and many of which have never before been viewed direct ly! The popular observing guides are therefore of little use. Instead these observers turn to multi-volume catalogues with such alien code names as thc UGC, MCG, and MOL*, which are usually found only in professional observatories.

These observers make up only a tiny percentage of amateurs. Most of us p_re fer a casual stroll through the well-tended gardens of the heavens as opposed to beating new trails through thc dense celestial jungle. But every now and then it’_s fun to take on a challenge — to climb a difficult path and stand for a moment on a mountain peak.

Figure 7.3

The Corona Borealis cluster is a rich swarm of galaxies 940 million light-years distant. Observers who search it out will be rewarded with the sight of dozens of tiny gray spindles scattered across the deeps.

For instance, take veteran deep-sky observer Ronald Morales, who described his quest for the faint Corona Borealis galaxy cluster (Figure 7.3) in the May 1990 issue of Sky & Telescope (page 563). Morales explained how he keeps a “failed-to-see” list of objects that he returns to on nights when the sky is unusually dark and transparent. By doing this he has located many that he might otherwise have given up hope of finding — objects that have brought him unexpected pleasures. This is a worthwhile practice for every serious deep-sky observer, but I can’t recall ever seeing it in print before.

The Uppsala General Catalogue of Galaxies, the Morphological Catalogue of Galaxies, and the Master Optical List of Nonstellar Astronomical Objects, respectively.

Morales’s name is familiar to many readers. In addition to his own articles, I ! mentioned his observations many times in this column. He has an observa-*’^aV _olltside Tucson. Arizona, that includes telescopes with apertures up to 17%. ? hes but his favorite is a 12%-inch f/7 Cave reflector. Many years back Morales E. I spent ^{an evenin}S observing with his 8-inch reflector under the pristine ^al'\ of Arizona’s Empire Mountains. That night I had my best view ever of the ^S bulosity surrounding thc Pleiades. Morales is also coauthor of the Webb Society Deep-Sky Observer's Handbook, Vol. 6: Anonymous Galaxies, a source _of endless deep-sky challenges.

Dueling Globulars

-The northern winter sky," Scotty wrote, "sparkles with scores of open clusters that dot the Milky Way. Spring brings the realm of galaxies into view, and autumn is for hunting planetaries. But it's the warm summer nights now upon us that are perfect for viewing globular clusters." In fact, if you look at the meridian in mid-July around 9 p.m. with the naked eye and binoculars, you can spot several Messier globulars lined up from north to south, as if clinging to some sinuous celestial vine. In order, they are M92 and M13 in Hercules; M12, M10, and Ml07 in Ophiuchus; and M80 and M4 in Scorpius. A little tilt of the head east or west will bring others into view: M5 in Serpens; and M14, Ml 9, and M62 in Ophiuchus. A telescope will reveal even more globular wonders. What to do? Which ones to cover? Well, without question, the most popular globular is M13 in Hercules. But Scotty was hesitant to bestow full honors on it, for — as we next read — it appears he preferred another globular over this one.

As you read this, amateurs somewhere are looking at the splendid globular star cluster Ml3 in Hercules (Figure 7.4). Some might be comparing notes on how well various telescopes resolve the cluster into individual stars. Others are using it as a test of eyesight or sky conditions (though with a total magnitude °f 5.9, this ball of ancient stars should not be much of a challenge except from Places like the fens of southern Connecticut). M13, along with the Orion Nebula, ^,s perhaps the most frequently observed deep-sky object. Indeed, even when Halley's Comet beckoned people to look skyward during 1986. it is probably fair to suggest that more people saw M13 than thc comet itself. It is pure coincidence, '^1ut nevertheless interesting, that Edmond Halley chanced upon M13 in 1714. following year he published a paper that was the first to describe a half ^ozen “nebulous” objects in detail rather than mention them as asides to a star catalog. Of the great globular Halley wrote, “This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent.”

I*¹ J^Une 1986 I mentioned that MB’s popularity is derived not only from its ^s'^Ze and naked-eye brightness, but also from its favorable sky location. For ^ervers at mid-northern latitudes the cluster is visible much of the year. On

Figure 7.4

The great globular cluster Ml 3 contains more than 100,000 stars to 21st magnitude. The true stellar population may be close to half a million.

pleasantly warm summer evenings it passes nearly overhead where the view is through the thinnest layer of interfering atmosphere. Furthermore. M13 is conveniently placed on the line between Eta (rj) and Zeta (Q Herculis, which forms the west side of the Hercules Keystone. With such prominent stars to point the way, it’s easy to understand why even novice observers can quickly locate the cluster. M13’s association with the Keystone is so interwoven that the relatively small asterism is believed by many to be all there is to the constellation.

Small binoculars or a finder will show M13 as a pale, colorless glow with a diameter as much as half that of the Moon. If encountered accidentally while sweeping, M13 can be quite startling. A 3-inch telescope will just begin to show stars at the cluster’s edge, and a 4-inch will add more. The fact that Charles Messier never saw any with his 60x Newtonian of 4!4-foot focal length shows just how far telescope making has advanced in two centuries.

By the mid-1800s. W. H. Smyth was extolling MB’s appearance in a 5.9-inch refractor. “An extensive and magnificent mass of stars,” he wrote in 1844. "with the most compressed part densely compacted and wedged together under unknown laws of aggregation.” Smyth had read a similar comment by Wiilia⁰¹ Herschel, and this may have influenced his own observations.

Photographs, too, have influenced observers, but not always in a positive way-About 1850, while using Lord Rosse’s telescopes in Ireland, Bindon Stoney

,j _an unusual pattern of three dark rifts (Figure 7.5) radiating outward from ^lU’n the center of M13.The 19th-century observer and author Thomas W. Webb ⁿ ntions in his classic Celestial Objects for Common Telescopes that these lanes ¹¹¹ “beautifully seen by Buffham,” who used a 9-inch reflector.

afterward photography became the all-powerful tool of astronomy, lion of Stoney’s rifts essentially disappeared from the literature. I suspect 'cause the lanes did not appear on early photographs of M13. I first wrote bout the "propeller" in the July 1953 column, but no amateurs of the day orted seeing it. Every few years I would bring it up again, with similar results. It wasn’t until 1980. after I asked several more times for amateurs to hunt for them that John Borllc reported seeing the lanes with his 12/;-inch Newtonian reflector at 176x.The cosmic jest surrounding the sighting was that Bortle, tired _of reading about the folklore of the lanes in my column, set out to disprove their existence, or so he said in his letter. Sighting the lanes seems to depend upon a careful balance of aperture and magnification. Both Bortle and Dennis di Cicco commented on the importance of magnification. During the Stellafane convention in 1981, di Cicco was surprised by how easily the lanes were seen with the 12-inch f/17 Porter turret telescope at about 180x. However, even knowing their orientation and appearance, he was unable to see them at 95x with a 12-inch reflector that was set up nearby.

Jan Romer of the Delaware Valley Amateur Astronomical Association in Pennsylvania reports that he cannot see the lanes under any conditions with an

Figure 7.5

In this negative drawing depicting Ml 3 by Lord Rosse, three lines form what looks like a dark propeller near the core of globular cluster Ml3.

8-inch f/8 reflector at lOOx. He does see, however, many star chains crossing th_e cluster and also several of the star-poor areas mentioned by observer and write_r John Mallas. Most observers note that they appear best at magnifications _Of about 200x. It is exciting to think that these lanes can again be seen after a lap_Se of nearly a century in which no one reported viewing them. The feature is off_Sel to the southeast edge of the cluster.

Veteran observer Mark K. Stein may hold the record. He writes, “When I lived under the polluted skies of Louisville, I considered some of your descriptions of deep-sky objects the result of a fine, experienced observer having a slightly overactive imagination. But now in the much darker skies of Bloomington, 1 can plainly see objects I wouldn’t have attempted from Louisville.” With a 6-inch at his new location Stein has seen the dark lanes in the globular cluster M13. This feat puts him in a special class, for the lanes usually require an 8-inch or, better, a 20-inch. I have never seen them in my 4-inch Clark — but I keep on trying.

On long-exposure photographs the myriad stars in M13 spread out to a diameter of more than 20', two-thirds of the Moon’s diameter. In a 4-inch telescope only the cluster’s edge can be resolved. A 10-inch instrument, however, will show more stars than the eye can count, though the center still remains a solid glow. Sharp-sighted amateurs will notice that M13 is not circular as it appears in long-exposure photographs. Early drawings like Lord Rosse’s show dark lanes and strings of stars — all still apparent to the visual observer. Of course, the resolution of M13 depends somewhat on magnification. An 8-inch aperture at 30x will not separate the stars because the cluster’s core appears too bright. Increasing the power to 300x, however, reveals stars across the entire field. One of my most memorable views occurred when a smog layer had reduced the naked-eye magnitude limit to about 4/2. As is often the case, the seeing was very steady then. At 300x the cluster was faint, but the individual stars stood out well. The number of stars attributed to the cluster depends upon which reference you consult, but all agree that it runs well past 100,000.

The Challenger

Every deep-sky observer has a list of favorites: a planetary nebula, a test object for sky transparency, or a special area of the heavens to sweep on nights when the conditions are just right. One of my pet objects is well placed in the July evening skies, near the border between Virgo and Serpens — the remarkable globular cluster M5. Formerly M5 was included in an extension oi Libra which curled up into Serpens, and consequently it is catalogued as a Libra object in the older observers’ texts, by Smyth, Webb, George F. Chambers, and Garrett P. Serviss. Since the I AU revisions of the constellation boundaries, it has otiicial-ly been charted in Serpens (Caput).

The German astronomer Gottfried Kirch found M5 in 1702. Messier, with a very small telescope, described it as a nebula. Smyth wrote: “This superb object is a noble mass, refreshing to the senses after searching for faint objects: with

utliers in all directions, and a bright central blaze which even exceeds M3 in ncentration.” It lies at the eastern end of a short chain of three, faint, naked-^C stars: 109 and 110 Virginis and 5 Serpentis. M5 is just 0.4° northwest of 5 Serpentis and is almost Z° in diameter (Figure 7.6). I saw it with my naked eye under the clear skies of Arizona, as well as from the summit of Temple I at the Mayan ruins of Tikal in Guatemala.

Telescopically M5 is a treasure house, and every increase in aperture brings _new sights. With low power on a small instrument it is very beautiful and bright, but not resolved. Its appearance is comparable to that of M13 in Hercules. At home in Connecticut, my 4-inch Clark refractor shows the cluster with a lacy fringe of stars. Years ago a 10-inch reflector in Kansas revealed some 300 stars, and the background glow hinted at

Figure 7.6

The telescopic rival of M13 is M5, in Serpens (Caput).

It radiates with the light of a quarter million stars.

tor at Wesleyan University, I would add that M5 seemed more like a starry blizzard.

The English observer Kenneth Glyn Jones contended that M5 is second only ^to M13 in the northern hemisphere. But it may come as a surprise to learn that M5 is actually listed as being a tenth of a magnitude brighter (5.75 versus 5.86) ^ln Sky Catalogue 2000.0. Both Ml3 and M5 are of similar size, too. In fact, M5 is the brightest globular cluster in the northern hemisphere of the sky and indeed ^ls surpassed by only two southern globulars, 47 TUcanae and Omega Centauri.

^e there times when you can see M13 and not M5 with the naked eye? It would ^an tnteresting project to see what the effects of altitude have on each object. ^niay he that M13 is the better-known cluster only because it passes nearly °^Verhead for those living at temperate northern latitudes.

The Orphans of Ophiuchus

Warm summer nights in July offer us the richest regions of the Milky Way. Under a dark sky, its smoky trail of fused starlight is dappled with the hazy light of globul clusters. Not surprisingly, Ophiuchus, the sky's 11 th-largest constellation (in area) contains a plethora of these objects, including a half-dozen that belong to Messier's catalog: M9, M10, M12, M19, M62, and M107. Naturally, beginners tend to explore the Messier objects first. But Ophiuchus has a host of lesser-known sights worth scouting out. Furthermore, Scotty alerted those with wide-field telescopes to be on the lookout for the sinuous veins of dark nebulosity that empty into black lagoons of obscuring dust near many of the Ophiuchus globulars. "There is something for everyone," Scotty wrote. "Fine objects abound, and the central part of Ophiuchus, lying between the two sections of Serpens, provides a test for the observing capabilities of amateurs with modest-sized telescopes." Indeed, the Serpent Bearer is full of celestial surprises, as we shall see.

Some joys of astronomy are seldom mentioned. There is the pleasure of sifting through a musty 19th-century book with its quaint prose, knowing that it inspired countless people to look at the heavens. Hidden among the pages of another is a fine engraving of an old telescope that, carefully examined, yields ideas for today’s telescope makers. Old atlases show interesting constellation figures and star names. While many of us enjoy old books and charts, we also delight in works of recent vintage. It is all as much a part of amateur astronomy as looking through a telescope on a July evening.

Perhaps the best-known contemporary celestial cartographer is Wil Tirion of the Netherlands. In recent years his beautiful star atlases have been widely circulated. Let’s consider for a moment the right-hand half of chart 22 in the deluxe version of Sky Atlas 2000.0. Scorpius sprawls across the field as it unwinds from brilliant Antares, the Scorpion's heart — to Shaula, the stinger in its tail. For those of us living in the northern United States, the southernmost stars on this chart are, at best, difficult to see. Still, we are better off than amateurs in Canada, England, and even at the latitude of Messier’s Paris, since a good portion of the heavens charted here never rises above their southern skyline.

The Milky Way here cascades down the sky as indicated by the shades ot blue on the chart. The dust in our own galaxy blocks the view beyond, so the familiar red ovals of external galaxies are lacking from this section of Tirion s chart. However, other interesting features of the sky are apparent. The galactic equator, representing the plane of the Milky Way, rises steeply through the field. To its east is a profusion of open clusters, shown in yellow, but few lie to its west. North of Antares, especially in Ophiuchus, stars are remarkably scarce; in many places it is possible to draw a 2° circle that doesn't contain a single star down to the 8th-magnitude limit of thc chart. Yet on the other hand-the star-poor region of Ophiuchus east of Antares contains an abundance of

0ne example is the open cluster IC 4756 in the eastern half of Serpens (Cauda). It is one of the largest such objects in thc heavens, appearing more than 1° across and just a little smaller than the Beehive cluster, M44, in Cancer. Some 80 stars between 7th and 12th magnitude are evenly scattered across IC 4756’s diameter. The Herschels probably missed it because the small fields of their reflectors would have passed right over the group without revealing any concentration of stars. IC 4756 appears as a patch of the Milky Way to the unaided eye. Binoculars or a finder will easily reveal its individual stars. While large reflectors will not show the entire object at once, they will be useful when searching for unusual star chains or dark lanes.

Many years ago Glen Chaple, Jr., of Townsend, Massachusetts, “discovered” a deep-sky object in Ophiuchus; he found about two dozen stars forming a group which could be glimpsed with the naked eye. He likened the cluster to the Praesepe in Cancer, but it was not plotted on his copy of Norton's Star Atlas.The object does, however, have a name. It is IC 4665 and is listed in the Index Catalogue to the NGC. In those days of stargazing, serious deep-sky observers soon learned that no single atlas can suit all their needs. Today, of course, IC 4665 is plotted on most modern star charts.

About 1° northeast of 3rd-magnitude Nu (v) Ophiuchi is the tiny globular NGC 6517. Only 4.3 ' across and about 12th magnitude, it usually requires an 8-^or 10-inch telescope to positively identify this cluster. However, last year, working with a finder chart that was plotted from thc AAVSO Star Atlas, I was able to locate NGC 6517 with my 4-inch Clark refractor at 150x. Lower powers made ¹¹ impossible to distinguish the cluster from a faint field star.

Some 1° northeast of NGC 6517 is the 5th-magnitude starTau (t) Ophiuchi. Continuing on a straight line for 1° more brings us to NGC 6539, another glob-^U'^ar missed by the Herschels. Of roughly 12th magnitude and 2¹/?' across, it is a ^challenge for apertures less than 8 inches. My notes contain only sightings made a 10-inch reflector. And, although I considered NGC 6539 a difficult object . y^ears ago, I know that during the last decade many amateurs have observed ^W|,li telescopes as small as 8 inches.

Io ^<^^not’^lcr difficult globular is NGC 6535 in Serpens (Cauda). It, too, was over-°°ked by the Herschels. Discovered in the 19th century by the English observer John Russell Hind, it is only about I/2' in diameter and 11th magnitude. my notes made years ago in Kansas read “at least lOOx needed to establish iderT tity as a globular,” in England. D. Bl anchett, using a 3-inch aperture, found Nc 6535 to be a “faint, elusive object.”

Tiny globulars seem to flourish in this part of the sky. Without suitable fi_n<j stars it is often extremely difficult to hunt down these challenging objects 0^ exception is NGC 6366. Locating it is easy, since it is in the same field as 5th magnitude 47 Ophiuchi, which lies 17' due west of the cluster. With magnifi_c tions of around lOOx. the 4-inch Clark shows NGC 6366 to be magnitude U 5 and some 3' in diameter. It cannot be recognized as a cluster at 20x in my 5-inch Apogee telescope. At Wesleyan University’s Van Vleck Observatory j_n Middletown, Connecticut, I have used the 20-inch Clark refractor to view NGC 6366. It shows the object as some 5' across and looking more like a compressed open cluster than a globular.

Figure 7.7 The globular cluster M9 in Ophiuchus lies 30,000 light-years away, close to the center of our Milky Way galaxy. It is accompanied by the inky dark nebula Barnard 64.

Eastward in Ophiuchus is the globular M9, discovered by Messier in May of 1764. About 6' across and 8th magnitude, it stands out well against the background stars of the surrounding Milky Way (Figure 7.7). Hans Vehrenberg comments in his Atlas of Deep-Sky Splendors that with a small telescope M9 can be mistaken for a star. This is worth checking out. The cluster is near the northeast edge of a remarkable dark nebula. Barnard 64, less than '/2⁰ west oi it. With my 4-inch Clark refractor or 5-inch Apogee telescope the dark nebula is easily seen.

Most dark nebulae are difficult to “see," but here the rich background is rather uniform, and it is interesting to compare the star densities northeast and south east of M9. Try powers of about lOOx. These dark patches really show best on _tographs, and it was with photography that E. E. Barnard discovered most of ?ern earlier ^{this ccntury}'

^There are ^two gl°t^>u'^{ars on,}y ^a short hop from M9. About P/2⁰ to the north-t is NGC 6356. Although still in a rich star field, this 9th-magnitude cluster, 2' ⁶⁸ oss. is easy to identify, especially at higher magnifications. The other globular, mCC 6342, lies the same distance south and slightly cast of M9.This is more difficult only 0.5' in diameter and 10th magnitude. However, Canadian Pat Brennan notes NGC 6342 as "faint" but readily seen in a transparent sky. Northeast of NGC 6342 is another dark nebula, Barnard 259. It is not as sharply j_efined as Barnard 64 but still the stars should appear noticeably thinner here than in surrounding areas.

While checking my files I was amazed to discover that I have never written about the globular cluster M19 in Ophiuchus during the four decades of prcpar-i_no this column. It lies at about the same declination as brilliant Antares and 7/2° east of the Scorpion’s heart. The fact that Messier discovered this cluster while scanning the sky from Paris is a tribute to his observing skill. Even from the northern United States, where the cluster climbs nearly 10° higher in the sky than at Paris, M19 is not especially well placed for viewing.

John Mallas, whose location in Southern California was more suitable for examining M19, once remarked that the globular is a miniature of the great Omega Centauri cluster. M19 is 5' in diameter and a bright 6.6 magnitude. Under good skies it is easily seen in most telescope finders. The visual diameters of globular clusters depend to some extent on the size of telescope and the observer’s eyesight. Members of an observing group should compare their individual estimates of these objects’ sizes, using a micrometer, an ocular grid or reticle, or by timing transits over a crosshair in the eyepiece. The results may well be quite different for telescopes of different sizes. Similarly, apparent magnitudes may differ in large and small instruments.

Two other globulars lie within a short distance of M19. A little less than 2° east and slightly south is NGC 6293. You'll know you're headed in the right direction when you pass a 6th-magnitude star about 1° from M19. Just southwest of this star is the Cepheid variable BF Ophiuchi, which ranges between magnitude 7.5 and 8.5 during a four-day cycle. About 2' across and of magnitude 8.4, NGC 6293 can be seen with only a 2-inch aperture and shows nicely in 4- to 8-inch telescopes. And south and east of NGC 6293 are more great clouds of obscuring Material which reveal themselves primarily to the camera. However, those using l^arge-aperture, rich-field telescopes may be able to detect some of the more sharply defined boundaries here. Now return to M19. Just I/2⁰ north-northeast h is NGC 6284. It is a bit smaller and fainter than NGC 6293, but still suitable f°^{r sm}all telescopes. It is a nice cluster for beginners and searching for it provides good experience in hunting out challenging objects. Amateurs who have successfully worked down from the brighter globulars to such faint ones can feel ^Weh satisfied with their observing prowess.

More Surprises in the Serpent Bearer

Although Ophiuchus is best known for its profusion of globular clusters, few rea|j_z that the Serpent Bearer contains a wealth of planetary nebulae that can both delig^ and test the mettle of all observers. When summer nights fall upon us we naturally tend to turn our telescopes to the more famous summer planetary nebulae M57 (the Ring) and M27 (the Dumbbell), which are climbing toward the meridian after sunset. But Scotty enjoyed the challenge of dim planetaries, too. He knew how difficult it was to confirm a sighting, especially if the planetary was small and faint and the atmosphere even slightly unstable. Under such conditions stars and planetaries look the same — like tiny swollen disks. Without access to a special star chart or photograph that shows an object in its precise position relative to nearby faint stars, observers can be at a loss. So how is one to know the difference between the two? For Scotty's readers, one way was to turn to the Deep-Sky Wonders column and find out how other observers worked out this dilemma. Another way was to look at a lot of planetaries. Fortunately, the wide area of Ophiuchus contains a variety of bright ones to help train your eye to recognize these gems.

Ernst J. Hartung’s book Astronomical Objects for Southern Telescopes contains a useful suggestion for observers of difficult planetary nebulae: hold a small direct-vision spectroscope between your eye and the telescope ocular. The prism spreads each star in the field into a narrow, faint streak. But because the visible light of a planetary nebula consists mainly of a close pair of bright lines in the green part of the spectrum, the nebula’s image remains almost entirely unchanged.

In this way, even the tiny faint planetaries can easily be picked out in a crowded star field, and Hartung had much success with objects only an arcsecond or so in diameter. While this powerful technique is not new, few amateurs seem to know of it. It is well worth trying, for the effect is startling. Here is a sample of brighter planetaries in Ophiuchus, some of which are readily recognizable by ordinary viewing, while others require the visual spectroscopic method.

NGC 6369 is one of the easier kind. It is within thc reach of all but the smallest telescopes. This lOth-magnitude object lies in a sparse field, facilitating its identification. It is a perfect smoke ring about half the apparent diameter of Jupiter. Several nearby naked-eye stars help guide the way. Ron Morales m Arizona saw NGC 6369 with a 10-inch reflector at 87x as "round, green, and with the edge quite sharp.” By using a magnification of 137x he could easily make out the nebula’s dark center.

Near the eastern edge of Ophiuchus is a small V-shaped asterism known as Taurus Poniatovii (the Bull of Poniatowski). Named for an 18th-century Polish king, this obsolete constellation contains the famous double star 70 Ophiuchi- a binary with an 88-year orbital period. In 1989, the 4.3 and 6.0-magnitude com ponents were near a minimum separation of 1.5". A few degrees to the northeast _this group, on the shoulder of the Bull, is the planetary nebula NGC 6572. Vilhelm Struve, who discovered this nebula in 1825, considered it to be one of “most curious objects in the heavens.”

''"in 15 ^x ^5 binoculars NGC 6572 appears as a star, if one knows where to look, jeed the nebula is a mere 16" in diameter, and could be mistaken for a star if y'wer magnifications were used even in a telescope. Yet with only a little power

• ■ js still a beautiful little gem. Its total light is about equivalent to a 9th-magni-* de star, while its surface brightness is a hundred times that of thc Ring Nebula

• ■ Lvra! With the Clark refractor I have estimated its magnitude to be 9.5. My old 10-inch reflector showed the vivid green color of this object with any power more than 50x. It is interesting to note that older observers have described NGC 6572 as green, while the younger ones tend to call it vivid blue.

The planetary’s central star is not difficult. Estimates of its magnitude range from brighter than 10th to less than 12th. but such are the problems when trying to estimate a star’s magnitude when it appears against a bright background. There are several reports of the star not being seen with 10-inch instruments. If you search for it, remember to use the highest magnification your telescope and the night will allow. This dims the nebula’s glow and improves the contrast between it and the star. A little over 3° north-northeast of NGC 6572 is the planetary PK 38+12.1 (Cannon 3-1), which is plotted on Tirion’s charts. Once located it is fairly easy to observe its 6"-diameter, 12th-magnitude disk. It is easy to find with the spectroscopic method (described above).

Certainly within reach of a 5-inch is the planetary NGC 6309. This 11.7-mag-nitude object is a bit more than 20" long and only half as wide. Ron Morales calls this the "Box Nebula” and reports a curious gray-green color seen with a 10-inch reflector. The English observer Ed S. Barker’s description in the Webb Society handbook for nebulae mentions NGC 6309 as appearing “slightly mottled” in an 8/2-inch telescope. He also finds a faint star at each end of the nebula. What do you see?

Finally. I must share with you a letter 1 received from Canadian amateur Dunstan Pasterfield, who had an interesting idea. During his first nights of observing, he scanned the sky with binoculars and was impressed with the large, very loose open cluster NGC 6633. (Webb's Celestial Objects for Common Telescopes lists this cluster in Serpens, but today we find it in Ophiuchus, according to the official constellation borders established in 1930.) When Pasterfield got an 8-inch reflector he immediately turned to NGC 6633. It s a lovely, great, straggling thing ... of an absurd shape," he writes. “I keep ⁿⁱy eye on it annually. I’m convinced that some day something will happen in ^my’ cluster.”

^s an interesting concept: adopt a deep-sky object and learn it so well that y°^u would know at a glance if something were unusual, like a nova or stray aster-^0|d or comet in thc field. The idea is not new. Robert Evans in Australia has ^rriorized hundreds of galaxies as part of his search for supernovae, and his

novae of all lime.

But you don’t have to learn hundreds of objects. In 1979 Maryland amate Gus E. Johnson was making his annual inspection of galaxies in Virgo when h" noted that M100 didn't look right.The reason was a 12th-magnitude supernov* and the credit for the discovery went to Johnson.

Naked-eye Globular Clusters

July 1994 was a sad month for Deep-Sky Wonders readers, because it was the last time Scotty's words would appear as such in the magazine. Scotty had died the previous December 23rd, while touring Mexico. His final column is reprinted here in its entirety, and revisits some objects discussed earlier in this chapter that were particular favorites. For this installment, Scotty wrote, "Astronomy, as I have known it since about 1922, is full of encouraging surprises. Share them with me." Funny how his last offer could as easily have been his first. His life seemed to have come full circle. And as this book testifies, Scotty's spirit remains with us, and his words will continue to inspire and teach us. Besides, the measure of greatness does not end with one's life, but continues as long as one's words or accomplishments continue to affect the lives of others.

arm summer nights are a fine time to relax under a dark sky. As you lie

V V back and scan the ghostly band of the Milky Way and its environs, see how many globular clusters you can detect with the unaided eye. If you observe from mid-northern latitudes and can detect 6.5-magnitude stars, there are eight globulars to try for this month in the evening sky: M2 in Aquarius, M3 in Canes Venatici, M4 in Scorpius, M5 in Serpens (Caput), M13 and M92 in Hercules, M15 in Pegasus, and M22 in Sagittarius.

It’s not unreasonable to place the naked-eye limit at magnitude 6.5.1 make no claim for special powers when my logbook shows I’ve gone that faint, or on special nights, a magnitude fainter! The rub is that globulars are not stellar points; their light is spread over a tiny area of sky — enough to make them a real visual challenge without optical aid.

I’ll assume that 6th-magnitude M13, isolated between the stars marking the western edge of the Keystone of Hercules, is no real contest; many observers regularly use it as a test of the night’s clarity. Instead, let’s begin with M5. since my records show that amateurs have not paid much attention to this celestial splendor. Shining at magnitude 5.7 and measuring 17.4' across, this globular is easy to find; just look northwest of the star 5 Serpentis, which M5 all but hugs-You’ll have trouble, however, if you can’t differentiate the cluster from the star-

German astronomer Gottfried Kirch first sighted M5 while hunting *°^r comets in 1702. Charles Messier rediscovered it through the Paris smog si* decades later, describing it as a “fine nebula which I am sure contains no star-■ cluster remained unresolved until Siam Herschel trained his Zscopeonitinl791.My 4 inch Clark shows lacv edge of stars

_ders. The 20-inch Clark refractor _of Van Vleck Observatory in Middletown. Connecticut, transforms it ' into a dazzling mass of sparkling stardust. It is a shame to let such a brilliant summer object wither on the vine.

Nestled between Sigma (o)

Scorpii and fiery Antares is 6th-mag-

nitude M4 — a huge globular measuring 26.3' wide (Figure 7.8). While traveling through Central America, I spent many hours looking at M4 with the naked eye, binoculars, and my portable 4-inch rich-field reflector. I have seen it from the wild Peten jungle of Guatemala and at midnight from the top of the pyramids in the old city of Tikal. Always it has been rewarding. To see it with the naked eye, you must filter out the “glare” from brilliant Antares, though this is not difficult from the darkest of sites.

Through a telescope M4 is a dynamic object. Although Webb, in his Celestial Objects for Common Telescopes, refers to it as being “rather dim,” remember he viewed it from high-northern latitudes. From the Florida Keys, where the Winter Star Party is held, the huge ball of M4 is mighty impressive. While you’re in the vicinity, turn to Antares and move your telescope 0.56° to the northwest. There you’ll find NGC 6144, a 9th-magnitude globular cluster 9.3' in diameter. In the open sky this would be a most easy object, but Antares must be out of the field of view if any success is to be expected.

Antares deserves a careful glance as well. It has a 6.5-magnitude companion ubout 3" to the west. First noticed in 1819 during an occultation of Antares by fhe Moon, the companion is said to glow green, but that is an illusion. The companion is easy in a 6-inch telescope, but you'll need a steady atmosphere. If ^re having problems with glare from the primary, try using a Lumicon deep-y filter. Although I haven’t tried it myself, I hear it does the job nicely.

Seemingly swimming against a current in the Milky Way, the 5.1-magnitude lobular M22 lies about 2° northeast of Lambda (A.) Sagittarii, the top of the

Teapot. Abraham I hie of Germany apparently discovered it in 1665 while f₍₎|  1

lowing the motion of Saturn. The problem in detecting this cluster is not with j_ls brightness — which is nearly a full magnitude brighter than M13 — but with i_ls I proximity to the horizon and summer haze. It's nicely positioned, though, being I just east of 24 Sagittarii.

In 1992 Brian Skiff spied 6.4-magnitude M15 from the Texas Star Party. [f_s not terribly difficult to see with the unaided eye.The trick is to separate it f_roni a star of comparable magnitude immediately to its east. By the way. if you _0Wn a large telescope, see if you can spot the dark patch with two dustlike lanes near the cluster’s center, which Webb first reported.

Two globulars are more demanding. Shining at magnitude 6.3, M3 lies in _a very star-sparse region of sky. Furthermore, it nearly abuts a 5th-magnitude star, so you'll have to resolve the two to make a positive sighting. Overshadowed by its stunning neighbor M13, the 6.5-magnitude globular M92 in Hercules is right at our magnitude limit. It is not only challengingly faint but also isolated northeast of the Keystone.

Finally there is M2, which culminates late on July nights. At magnitude 6.5 it. too, is at our preselected limit. But I’ve had no problem sighting it. I have seen it repeatedly from the bayous of Louisiana during my stint as a celestial navigator for the Air Corps.

5^s* = Asterism; BN = Bright Nebula; CGx = Cluster of Galaxies; DN = Dork Nebulo; GC = Globular Cluster; Gx = Goloxy; Open Cluster; PN = Plonefory Nebula; ♦=Star; ** = Double/Multiple Star; Var = Variable Star

JULY OBJECTS (CONTINUED)