The Caldwell Objects

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28 28 NGC 752 Type: Open Cluster Con: Andromeda RA: 01 h 57.6 m Dec: +37° 50' Mag: 5.7 Diam: 75' Dist: 1,200 light-years Disc: William Herschel, 1786 (though probably preceded by Giovanni Batista Hodierna, before 1654) W. H ERSCHEL: [Observed 21 September 1786] Avery large coarsely scattered cluster of very [bright] stars. Irregularly round. Very rich. Takes up ½ 0 , like a nebulous star to the naked eye. (HVII-32) GC/NGC: Cluster, very large, rich in stars, stars large [bright] and scattered. AFTER LOOKING AT THE DATA LISTED ABOVE you might well conclude that open cluster NGC 752 is one Caldwell object that Charles Messier should have noticed on his nightly comet hunts. However, the star-speckled object seemingly escaped his careful gaze, even though it is brighter than 6th magnitude and has an apparent diameter more than 1½ times that of the full Moon. The cluster is visible to the naked eye as a soft glow about 5° south-southwest of the golden 2nd-magnitude telescopic double star Gamma (γ) Andromedae. You can also use our galaxy's spiral companions in the Local Group as signposts; NGC 752 lies about 8° north-northeast of the famous Pinwheel Galaxy (M33 in Triangulum) and 14° east-southeast of the great Andromeda spiral (M31). With such competition vying for amateurs' time and attention, it's no wonder the relatively nondescript open cluster continues to go virtually unnoticed. Nevertheless, NGC 752 is a noteworthy attraction. Certainly no binocular observer will be disappointed after turning his or her gaze to it under a dark sky. Because of its large apparent diameter and splash of bright 8th- to 12thmagnitude suns, telescopic views of NGC 752 will be best in a rich-field instrument at very low power. Charles Messier, who observed under the smoggy skies of 18th-century Paris, probably didn't have a chance of spotting this dim glow with the unaided eye. Furthermore, the telescopes he used to seek comets didn't afford the low power required to tighten up this loose aggregation of bright stars and make it conspicuous. Messier employed small, sim- 112 Deep-Sky Companions: The Caldwell Objects
pie refractors and inefficient speculum-mirror reflectors, generally at magnifications ranging from 60x to 138x. Roger Clark states that the optimum magnification for detecting NGC 752 is 7x with a 2-inch telescope and 27x with an 8inch. Clearly Messier was at a disadvantage. I can imagine him sweeping past NGC 752 and noticing a rich field of stars but moving on because the object did not look fuzzy. Of course, it's also possible that his telescope didn't provide a large enough field of view to make NGC 752 stand out as a discrete entity. Brent Archinal has acquired an interesting naked-eye perspective on this cluster. While he was attending the Black Forest Star Party in north-central Pennsylvania, he set out to look for M33 with the naked eye — a time-honored test of sky darkness and transparency "This is definitely one of the darkest sites I've seen in the eastern U.S.," he said, "second only to Spruce Knob, West Virginia. Something I noticed that might be worth pointing out is that if one tries to casually observe M33 with the unaided eye, it is easy to confuse it with NGC 752, which is in the vicinity and much easier to see." Unlike most open clusters, which are tens or hundreds of millions of years old, NGC 752 is about 2 billion years old. This places NGC 752 among the oldest open clusters known, though it is much younger than 5-billion-year-old NGC 188 (Caldwell 1). Most of the 77 or so measured stars apparently populating NGC 752 are F-type subgiants that have moved off the main sequence and are evolving toward gianthood. An 18-yearlong radial-velocity survey has confirmed that 15 red giants are members of the cluster. Rosat detected 49 X-ray sources in the cluster region, seven of which have been tentatively matched up with visible cluster members; three of these latter ob-jects are short-period binaries, one is a blue strag- The Caldwell Objects 28 gler, and one a so-called rapid rotator. These discoveries all paint a picture of a cluster whose stars have had considerable time to follow various evolutionary paths. NGC 752's very open nature itself hints at the cluster's seniority. Unlike a globular cluster, whose hundreds of thousands of stars exert a mutual gravitation that keeps them moving in a tight pack, an open cluster tends to loosen as it ages. Stars lining the cluster's edge are lost to the gravitational pulls of passing stars, molecular clouds, and other clusters. These "lost" suns blend into the populated stellar streets of the Milky Way, where their identities eventually fade beyond recovery. At 23x in the 4-inch NGC 752 seems to fill the field of view but, as alluded to above, it is hard to know where the cluster's boundaries end. The cluster has no central concentration; in fact, I see a curious double nature to it, each section seemingly a very wide open cluster of its own. To me these groupings look like celes- 113
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DEEP-SKY COMPANIONS The Caldwell Ob
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DEEP-SKY COMPANIONS The Caldwell Ob
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Contents Foreword by Patrick Moore
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to be more difficult than others, s
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years of thought and observation ar
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XII position, type, angular size, a
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sky and share its splendors. There
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CHAPTER 1 About This Book O N ONE S
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times vog can back up and cause a s
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when there is moisture in the air.
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Herschel's original observations, p
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we use today and to one another? I
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SIMBAD (Set of identifications, Mea
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Caldwell 26. The objects in the Cal
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tion near the edges of the charts).
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nights of observing. Unless otherwi
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CHAPTER 2 The Caldwell Objects 1 1
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Because of its location far from th
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as strongly energized as would be e
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suggested by the patchiness of NGC
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4 NGC 7023 Type: Reflection Nebula
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astronomers found direct evidence o
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and his contemporaries. Why then wa
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6 Cat's Eye Nebula NGC 6543 Type: P
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Eighth-magnitude NGC 6543 is locate
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7 NGC 2403 Type: Mixed Spiral Galax
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according to theory, the gas densit
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8 Ghost’s Goblet NGC 559 Type: Op
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One other thing adds to the cluster
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RA: 22 h 56.8 m Dec: +62° 37' Mag:
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nebula is a near impossibility, eve
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1 0 The Horseshoe Cluster NGC 663 T
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heart of the cluster, dividing it i
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appears to be the edge of a rapidly
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1 2 NGC 6946 Type: Mixed Spiral Gal
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to the west by a row of similarly b
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the south-southwest. NGC 457, howev
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Page 87 and 88: 15 outer shell falls upon the retin
Page 89 and 90: "knots" have also been noted by exp
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Page 93 and 94: 1 7 NGC 147 Type: Elliptical Galaxy
Page 95 and 96: the center. A faint outer halo is a
Page 97 and 98: 1 9 Cocoon Nebula IC 5146 (nebula)
Page 99 and 100: trickle into the pool surrounding t
Page 101 and 102: 2 0 North America Nebula NGC 7000 T
Page 103 and 104: egion's size and mass extreme when
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Page 107 and 108: 2 1 Box Galaxy NGC 4449 Type: Irreg
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Page 119 and 120: RA: 03 h 19 m 48 s Dec: +41° 30.7'
Page 121 and 122: Through the 4-inch, NGC 1275 is les
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Page 125 and 126: 26 Silver Needle Galaxy NGC 4244 Ty
Page 127 and 128: 2 7 Crescent Nebula NGC 6888 Type:
Page 129: Observatory, England) still lists a
Page 133 and 134: 29 NGC 5005 Type: Mixed Spiral Gala
Page 135 and 136: s h a edges. r p However, Christian
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Page 139 and 140: NGC 7331 is extended much farther t
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Page 143 and 144: sweep the telescope in a north-sout
Page 145 and 146: highly suspect by some. However, in
Page 147 and 148: 33 Network Nebula; Bridal Veil Nebu
Page 149 and 150: A quarter-second later, the star en
Page 151 and 152: under the dark skies of Seagrave Ob
Page 153 and 154: 35 NGC 4889 Type: Elliptical Galaxy
Page 155 and 156: magnitude 12.2 some 18' to the east
Page 157 and 158: 36 NGC 4559 Type: Mixed Spiral Gala
Page 159 and 160: Comae Berenices, the obvious northe
Page 161 and 162: 3 7 NGC 6885 Type: Open Cluster Con
Page 163 and 164: pressed, stars of magnitude 6 to 11
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Page 177 and 178: 4 0 NGC 3626 Type: Lenticular Galax
Page 179 and 180: 40 NGC 4546 in Virgo, NGC 4138 in C
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Northern Hemisphere, and people cel
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magnitude field star 2' to the nort
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this state of affairs.) The Hyades
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4 2 NGC 7006 Type: Globular Cluster
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each square arcminute of its disk s
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4 3 Electric Arc NGC 7814 Type: Spi
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hook as my "train route." Start at
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230 billion Suns and a physical dia
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4 5 NGC 5248 Type: Mixed Spiral Gal
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of the field. Those living under da
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1" in a mere four days. Clearly, La
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porarily) even the most practiced o
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stragglers are stars that appear to
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4 8 NGC 2775 Type: Spiral Galaxy (S
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with a CCD camera — discovered a
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this wreathlike network of dust and
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fuzzy patch of light with the unaid
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known as NGC 2244; and NGC 2239 sho
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At a glance the two stars at the cl
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On four plates taken with the Bruce
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years. A total of 43 star-cluster c
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5 2 NGC 4697 Type: Elliptical Galax
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northeast of the sixth star in the
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53 The Spindle NGC 3115 Type: Lenti
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and the 100-inch telescopes atop Mo
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5 4 NGC 2506 Type: Open Cluster Con
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emerge, followed by a diffuse outer
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perfection than is actually the cas
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jets, like a stream of water from t
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5 6 Pac-Man Nebula NGC246 Type: Pla
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the triple-alpha process, wherein t
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5 7 Barnard's Galaxy NGC 6822 Type:
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to show how the galaxy's western re
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core and limber arms, which, he wro
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esembles Jupiter . . . '" Smyth's f
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years" and suggests amateurs monito
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6 0 Ringtail Galaxy; Rataail Galaxy
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war. X-ray images taken by the Rosa
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W. Sinnott's NGC 2000.0 has a visua
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62 Milkweed Seed Galaxy NGC247 Type
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through the narrow haze. But give t
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was more readily seen in binoculars
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Society. Accepting that distance, w
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any large telescope that can provid
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ought it to our attention in the Fe
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inch the cluster shows a pyramid of
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NGC 253 is the brightest member of
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magnitude stars, one just to the ea
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6 6 NGC5694 Type: Globular Cluster
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you see its core dancing? Its starl
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cosmologist Halton Arp (Max-Planck
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and kinked. All this detail is wrap
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telescope. But seeing it takes more
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streamers begin to appear, but they
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tounding, because NGC 6302 is a mag
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70 Southern Pinwheel NGC300 Type: S
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would John Herschel suddenly fault
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Dunlop discovered NGC 55 with his 9
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even when crossing the meridian. Co
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"nebulous clusters" (Class II); and
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galaxy's disk and halo. NGC 2477 ap
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condensed, almost to the very margi
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The asymmetry remains at high power
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other popular nickname, the Eight-B
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longed look, the object displays a
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times larger than the full Moon. In
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emains of a globular cluster that w
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he classified them accordingly. NGC
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NGC 6231). Under averted vision the
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7 7 Centaurus A NGC5128 Type: Pecul
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Harvard College Observatory's 24-in
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at 16 million light-years. If you s
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78 NGC 6541 Type: Globular Cluster
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seems to fragment into tiny stars,
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sense, evinced by any known cluster
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12th-magnitude double star. The clu
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original with Bayer, but goes back
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Omega Centauri is generally regarde
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glitter all come to mind as the clu
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shattered remains of their diminuti
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It's a nice even disk of l0th-magni
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with the fiery reddish light of hyd
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nebulosity when he viewed the regio
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the two jets of radio-wave-emitting
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pair of tweezers. A fairly bright s
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suggest that our universe may be on
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8 5 Omicron Velorum Cluster IC2391
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Nicolas Louis de Lacaille, however,
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much sky as does the full Moon, tho
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timates, making it 2 billion years
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NGC 5822. Don't be surprised if you
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89 S Normae Cluster NGC 6087 Type:
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fact, if you look about 7' southwes
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lowing night fell clear, when Hersc
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NGC 2867 is a nitrogen-rich planeta
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halfway between M37 in Auriga (200
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9 2 Eta Carinae Nebula GC 3372 Type
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anch of cold, dark material. Indeed
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stretch 16 billion km from the cent
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the object was known to some civili
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east, and a dim lip of gas to the s
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1 0 1 NGC 6744 Type: Barred Spiral
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have been identified in NGC 6752's
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ightest spiral arm(s) to the north,
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his maiden voyage of discovery in 1
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simply takes much too much effort t
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very wide double of near-equal inte
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went out and looked at the cluster
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NGC 2516 is 60 million years young
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and contains some 1,890 members, wh
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9 8 Coalsack Cluster NGC 4609 Type:
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it near the Swan? There is a still
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with the naked eye is a challenge.
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1 0 0 Lambda Centauri Cluster Colli
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position updates to (2000.0) R.A. 1
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Eta Carinae Nebula. But remember th
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9th magnitude. Of the latter, 23 ha
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103 1 0 3 Tarantula Nebula; True Lo
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southern-sky explorer Abbe Nicolas
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ters it has had thus far with the g
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ly east of NGC 362 we find the magn
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the eye ever inward and takes your
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1 0 5 Southern Butterfly NGC 4833 T
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stars"). Lacaille also likened NGC
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is about 7' in length, and nearly 2
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evealed a significant blue-straggle
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ightest stars shine around 4th magn
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egion looks cold and disturbing, no
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The galaxy received some incidental
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This entry is obviously in error, b
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7 N G C 4 0 4 T y L p e n : G t a i
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pointed her 7x35 binoculars toward
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13 NGC 288 Type: Globular Cluster C
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your telescope on Eta Sculptoris an
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19 N G C 6 2 8 1 T y O p e C : n l
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C Name Type Con. R.A. Dec Mag. Size
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C Name Type Con. R.A. Dec Mag. Size
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Appendix B Why Messier Did N o t i
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A typical pattern for evening hunti
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Earth turns, the cluster would clim
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Initially Herschel made his living
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of the heavens with it. For his swe
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tube's front end. This meant that t
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Star in Collo Ceti" about the star
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years, Herschel would observe some
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fluid, but added that ages of obser
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Appendix D Photo Credits in order o
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A Abell426, 101 Acrux, 388, 392, 39
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index Caldwell 44, 173, 176-178 Cal
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and nebulae, 271, 274, 362 and open
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Hevelius, Johannes, 72, 81, 452 Hip
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M103, 49-50, 58-59 M110, 77 MACHOs.
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NGC 3324, 369 NGC 3372 (Caldwell 92
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Perseus Cluster, 101 Phi Cas Cluste
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Τ Tarantula Nebula (Caldwell 103),
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The Caldwell Objects

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