The Caldwell Objects

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107 from the south celestial pole. With 206 square degrees, the constellation ranks 67th in area. Two globular clusters lie within its boundaries: NGC 6101 (Caldwell 107) and dim IC 4499, the closest globular star cluster to the South Celestial Pole. James Dunlop discovered NGC 6101 in the 1820s and cataloged it as his 68th object. He found it to be a "pretty large rather faint round nebula, about 3½ or 4' diameter, a little brighter in the middle. There is a very small nebula on the north preceding [northwestern] side joining the margin of the large nebula." Dunlop's last statement alarmed me at first; did he chance upon a comet and not know it? But I quickly recalled John Herschel's dismay with Dunlop's catalog. After he set out for South Africa and conducted his own southern-sky survey, Herschel concluded that a "want of sufficient light or defining power in the instrument used by Mr. Dunlop has been the cause of his setting down objects as nebulae where none really exist." On photographs of NGC 6101 (like that on page 423) I have identified a distinct oval of roughly 14th-mag-nitude stars in the cluster's northwestern halo, 424 and I believe this could have manifested itself to Dunlop as a nebulous extension, especially if his optics were poor. NGC 6101's brightest stars shine at magnitude 13.5, but the cluster's horizontal-branch magnitude, which determines the cluster's resolvability, is a dim 16.6 — a challenge for a 12inch telescope. NGC 6101 is intrinsically a rather large globular, measuring 160 light-years, which places it roughly halfway between NGC 362 and 47 Tucanae (Caldwell 104 and 106, respectively) in terms of size. NGC 6101 appears small and faint to us because it is so distant (about 3½ times farther away than 47 Tucanae). It is about 13,000 light-years closer to the galactic center than to our Sun. The cluster's iron-to-hydrogen ratio is slightly below normal for a globular; each cluster member has only 1/66 as much iron as does our Sun. According to William Harris (McMaster University, Canada) its integrated spectral type is F5. A 1991 Astronomical Journal article by Ata Sarajedini (Yale University) described CCD photometry that Deep-Sky Companions: The Caldwell Objects
evealed a significant blue-straggler population in NGC 6101. It was found that these stars are more centrally concentrated than the cluster's similarly bright subgiants. This indicates that NGC 6101's blue stragglers have larger masses than their ostensible counterparts, supporting the theory that they either are binary systems or have been formed by the merging of stars. Fifteen variable stars have been discovered in the region of NGC 6101. Eleven of these appear to be RR Lyrae stars, one of the types of variables used to size up our galaxy. To find NGC 6101 start with magnitude-1.9 Atria, Alpha (α) Trianguli Australis. The cluster lies 3½° to the southwest of it. If that's too far a jump, first locate 5th-magnitude Zeta (ζ) Trianguli Australis 2° to the southwest of Atria; the globular is another 2° south of Zeta Trianguli Australis. NGC 6101 can be seen in binoculars. Look for a tight triangle of 7th- to 8th-magnitude stars whose westernmost, 7th-magnitude member appears double. The cluster should be just south of the triangle and will 107 appear as a dim glow with a slight central condensation. It's best to have some magnification handy if you want to confirm your sighting. With an 8-inch telescope sited in Wellington, New Zealand, I saw the cluster as a moderately bright sphere surrounded by a dim gray halo. The roughly 3'-wide core is fractured into three parts. The innermost section is an uneven wedge of suns. A dark lane separates this mass from a tiny cap of stars to the northwest (Dunlop's "very small" nebula?), while a chevron of starlight to the south appears to have calved off the core. Through Auckland Observatory's 20-inch Zeiss reflector, the core of NGC 6101 looks herniated. A splash of 13th- to 15th-mag-nitude suns crosses the cluster's entire face, as if some disgruntled artist shook a wet paint brush at it. The drawing here is a composite of all these views. Through his 12-inch telescope, Ernst Hartung discerned a "rather faint but very rich globular cluster... irregularly round, rising broadly to the centre, about 3' across with rays of faint stars emerging." The Caldwell Objects 425
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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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photographs. The cluster's bright "
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play begins at 8:00 p.m., when my e
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ightness by summing the contributio
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jewels. Here are sapphires, rubies,
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15 outer shell falls upon the retin
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"knots" have also been noted by exp
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tight gathering of a half-dozen rou
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1 7 NGC 147 Type: Elliptical Galaxy
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the center. A faint outer halo is a
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1 9 Cocoon Nebula IC 5146 (nebula)
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trickle into the pool surrounding t
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2 0 North America Nebula NGC 7000 T
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egion's size and mass extreme when
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The Caldwell Objects 20 87
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2 1 Box Galaxy NGC 4449 Type: Irreg
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fuse but obvious glow of patchy lig
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qualifier—the word "light" — ha
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with a sheen of light, especially a
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1003, NGC 1023, and NGC 1058 (a nic
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Way) that pop in and out of view as
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RA: 03 h 19 m 48 s Dec: +41° 30.7'
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Through the 4-inch, NGC 1275 is les
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(300,000 light-years), Pal 4 (360,0
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26 Silver Needle Galaxy NGC 4244 Ty
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2 7 Crescent Nebula NGC 6888 Type:
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Observatory, England) still lists a
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pie refractors and inefficient spec
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29 NGC 5005 Type: Mixed Spiral Gala
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s h a edges. r p However, Christian
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ulars; challenge yourself and your
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NGC 7331 is extended much farther t
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away" stars escaped the confines of
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sweep the telescope in a north-sout
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highly suspect by some. However, in
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33 Network Nebula; Bridal Veil Nebu
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A quarter-second later, the star en
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under the dark skies of Seagrave Ob
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35 NGC 4889 Type: Elliptical Galaxy
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magnitude 12.2 some 18' to the east
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36 NGC 4559 Type: Mixed Spiral Gala
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Comae Berenices, the obvious northe
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3 7 NGC 6885 Type: Open Cluster Con
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pressed, stars of magnitude 6 to 11
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of Herschel's telescope. W then e e
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see the dark needle near the third
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sion of a "thin flat ring of enormo
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(which does not appear on Sky Atlas
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not be found in planetary nebulae,
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star in a large naked-eye triangle
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4 0 NGC 3626 Type: Lenticular Galax
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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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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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Page 399 and 400: 9 5 NGC 6025 Type: Open Cluster Con
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Page 405 and 406: NGC 2516 is 60 million years young
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Page 409 and 410: 9 8 Coalsack Cluster NGC 4609 Type:
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Page 415 and 416: 1 0 0 Lambda Centauri Cluster Colli
Page 417 and 418: position updates to (2000.0) R.A. 1
Page 419 and 420: Eta Carinae Nebula. But remember th
Page 421 and 422: 9th magnitude. Of the latter, 23 ha
Page 423 and 424: 103 1 0 3 Tarantula Nebula; True Lo
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Page 427 and 428: 104 NGC 362 Type: Globular Cluster
Page 429 and 430: ters it has had thus far with the g
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Page 435 and 436: 1 0 5 Southern Butterfly NGC 4833 T
Page 437 and 438: stars"). Lacaille also likened NGC
Page 439 and 440: is about 7' in length, and nearly 2
Page 441: 107 NGC 6101 Type: Globular Cluster
Page 445 and 446: ightest stars shine around 4th magn
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Page 449 and 450: The galaxy received some incidental
Page 451 and 452: This entry is obviously in error, b
Page 453 and 454: 7 N G C 4 0 4 T y L p e n : G t a i
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Page 457 and 458: 13 NGC 288 Type: Globular Cluster C
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Page 461 and 462: 19 N G C 6 2 8 1 T y O p e C : n l
Page 463 and 464: C Name Type Con. R.A. Dec Mag. Size
Page 465 and 466: C Name Type Con. R.A. Dec Mag. Size
Page 467 and 468: Appendix B Why Messier Did N o t i
Page 469 and 470: A typical pattern for evening hunti
Page 471 and 472: Earth turns, the cluster would clim
Page 473 and 474: Initially Herschel made his living
Page 475 and 476: of the heavens with it. For his swe
Page 477 and 478: tube's front end. This meant that t
Page 479 and 480: Star in Collo Ceti" about the star
Page 481 and 482: years, Herschel would observe some
Page 483 and 484: fluid, but added that ages of obser
Page 485 and 486: Appendix D Photo Credits in order o
Page 487 and 488: A Abell426, 101 Acrux, 388, 392, 39
Page 489 and 490: index Caldwell 44, 173, 176-178 Cal
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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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