The Caldwell Objects

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96 members do extend beyond the cluster's putative edge. Perhaps this exercise simply demonstrates just how difficult it is to determine the extent of an open cluster in a rich Milky Way region. It also highlights the imprecision of supposedly "hard" astronomical data. Archinal elaborates: "First of all, there appears to be a basic definition problem here regarding what the diameter of a cluster should be. This is another one of those cases where the NGC designation really applies to something that's not quite the same thing as the modern astro-physically accepted cluster. Most of the stars are concentrated in a 20-30' diameter area, but possibly with outlier stars going out to 1°. So, depending on how one wants to define diameter, all the values cited are correct! It all just boils down to deciding whether you prefer the 'original' diameter, the approximate obvious visual diameter (my 22' seems reasonable), or the maximum diameter, to include all possible cluster members (1° or more)." Our astronomical predecessors had no way of determining the distances to the stars, so their perception of a star cluster was simply that — stars clustered together on the plane of the sky. When Lacaille discovered NGC 2516, he used a ½-inch telescope at 8x, so his view would have been similar to mine through 7x35 binoculars — a wishbone of stars with a fuzzy "bottom." Through my 7x35s about a dozen stars can be seen to 9th magnitude. So Lacaille probably did include the northwestern branch of the wishbone in his own "very close group of 10-12 stars." NGC 2516 has 103 known members, several of which are red giants: an orange, magni-tude- 6.7 K1III star (R.A. 7 h 58 m 09.8 s ; Dec. -60° 49' 00") near the core of the cluster's densest part; the 5.2magnitude Μ0ΙΙ irregular variable V460 Carinae (R.A. 7 h 59 m 37.6 s ; Dec. -60° 35' 13"); and a magnitude-5.7 K4II star (R.A. 7 h 56 m 386 18.6 s ; Dec. -60° 31' 35"). Given the historical discrepancies regarding the cluster's size, one can only wonder which of these reddish stars was Herschel's "orange-coloured star . . . in [the] middle of [this] large and magnificent cluster." At 23x in the 4-inch NGC 2516's densest part appears boxy, like the core of the Beehive Cluster (M44 in Cancer), and many streams of irregularly bright stars radiate from it. The cluster also contains several dark voids, and with a little imagination I can envision a cat getting ready to sleep on a cushion of starlight whose creases are the shadowy recesses of darkness. This cushion's brightest part is a wide crescent of mixed and colorful suns. Try defocusing the image ever so slightly, and look for a dark rosette centered on this cushion of stars. At 72x the "cat" is surrounded by three prominent pools of blackness, each of which looks like crushed black velvet. These pools are outlined by sparkling jewels. Yet they are not ink-black, for they shimmer softly, as if they were reflecting the starlight surrounding them. Use high magnification to look for a "comet tail" of dim suns flowing to the southwest. Deep-Sky Companions: The Caldwell Objects
NGC 2516 is 60 million years young and is one of seven similarly aged clusters that move through space together. The Pleiades, IC 2602 (the Southern Pleiades), and the Alpha Persei Cluster also belong to this group. NGC 2516's total mass is somewhere between 800 and 1,500 solar masses and its most populous region spans 9 light-years. The cluster lies 52 light-years below the galactic plane at roughly 96 the same distance as the neighboring (and much younger) Gum Nebula. The latter is famous for the Vela Pulsar and Vela X, a supernova remnant that emits copious radio waves. NGC 2516 is a breeding place for chemically peculiar, binary, and variable stars. Its multiple passages through Vela's dense molecular clouds appear to be triggering repeated bursts of star formation. The Caldwell Objects 387
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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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4 7 NGC 6934 Type: Globular Cluster
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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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Page 357 and 358: 8 5 Omicron Velorum Cluster IC2391
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Page 365 and 366: timates, making it 2 billion years
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Page 371 and 372: fact, if you look about 7' southwes
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Page 379 and 380: 9 2 Eta Carinae Nebula GC 3372 Type
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Page 389 and 390: 1 0 1 NGC 6744 Type: Barred Spiral
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Page 415 and 416: 1 0 0 Lambda Centauri Cluster Colli
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Page 423 and 424: 103 1 0 3 Tarantula Nebula; True Lo
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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
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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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