The Caldwell Objects

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60 & 61 W HILE SWEEPING WITH HIS POWERFUL RE- flector through Corvus, the Crow, on February 7, 1785, William Herschel chanced upon a nebula unlike any other he had seen. Of it he wrote: "Large opening with a branch, or two nebulae very faintly joined." What an utterly simple yet profound comment. It's as if Herschel were describing two butterflies dangling from a branch in some romantic embrace. How innocent he seems to us today, when we look at his records — we who live in a time of astrophysical sophistication. In his day Herschel had no way of fathoming what he had discovered. He could only describe what he saw and ponder its significance in the light of other faint and diffuse objects he had seen (no pun intended). Imagine his response had he suddenly learned the incredible truth: that he had stumbled upon two extragalactic cannibals, two savage galaxies ripping each other apart — each disemboweling billions of stars from the other and flinging them across several hundred million light-years of space, where they continue to shine like luminous entrails. Such is the case with NGC 4038 (Caldwell 60) and NGC 4039 (Caldwell 61), the finest and most popular pair of interacting galaxies in the heavens. NGC 4038 and NGC 4039 are also the brightest of the five galaxies plotted in the second edition of Sky Atlas 2000.0 near the star 31 Crateris (which lies within the modern borders for Corvus). To find the galaxy pair, look 3¾° southwest of magnitude-2.6 Gamma (γ) Corvi and about ¾° north and ever so slightly 240 east of 31 Crateris. In photographs the galaxies look like two tadpoles bonded together at their eastern ends. Two long "tails" extend from the bridge connecting the tadpoles, one curving to the north, then east, the other to the southeast, then southwest. Computer simulations show that such filaments form during close encounters between spiral galaxies. When two spirals collide, their constituent stars do not smash together like cars; the distances between the stars in each galaxy are too great for that to happen. But a battle of tidal forces does ensue, and ultimately it disrupts each galactic system. Furthermore, while gravity yanks each galaxy a-part, gas clouds collide, triggering star formation. Then, as the galaxies' respective nuclei move apart from one another, tides pull billions of stars from each spiral's disk. As a result, the galaxies lose mass and, with it, some of the gravitational pull that once held each of them together. The orphaned stars flee unimpeded into space, forming two taffylike streamers. Yet the battle has not ended. Like two warriors gathering strength, the galaxies separate, then orbit one another, until their mutual gravities force them back into battle once again. Today we see NGC 4038 and NGC 4039 in such a second round of engagement. Not until some time in the dim and distant future, perhaps a billion years from now, will the war end, with the two galaxies merging into a single chaotic stellar swarm. A report in the Astrophysical Journal for April 1, 1997, confirms that massive star formation has been triggered by this extragalactic Deep-Sky Companions: The Caldwell Objects
war. X-ray images taken by the Rosat satellite reveal that the most intense X-ray emission comes from each galaxy's nucleus and very knotty arms. The radiation may be coming from scores of Xray binaries and supernova remnants. Indeed, the duo has been the site of recently recorded supernova activity, with two explosions witnessed within the last century — one in 1921, the other in 1974. (Supernova hunters should keep an eye on this active The Caldwell Objects 60 & 61 galaxy pair.) Images taken with the 2.1-meter telescope at Kitt Peak National Observatory have also helped astronomers identify blue knots of star formation in the distorted galaxy disks; some older stellar populations in their tattered bulges; and a large dust complex at the apparent interface between the two disks. Midinfrared spectroscopic observations along this interface verify that starburst activity has recently occurred there. More recently, and 241
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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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Page 207 and 208: stragglers are stars that appear to
Page 209 and 210: 4 8 NGC 2775 Type: Spiral Galaxy (S
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Page 219 and 220: At a glance the two stars at the cl
Page 221 and 222: On four plates taken with the Bruce
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Page 225 and 226: 5 2 NGC 4697 Type: Elliptical Galax
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Page 229 and 230: 53 The Spindle NGC 3115 Type: Lenti
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Page 233 and 234: 5 4 NGC 2506 Type: Open Cluster Con
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Page 239 and 240: jets, like a stream of water from t
Page 241 and 242: 5 6 Pac-Man Nebula NGC246 Type: Pla
Page 243 and 244: the triple-alpha process, wherein t
Page 245 and 246: 5 7 Barnard's Galaxy NGC 6822 Type:
Page 247 and 248: to show how the galaxy's western re
Page 249 and 250: 5 8 NGC 2360 Type: Open Cluster Con
Page 251 and 252: core and limber arms, which, he wro
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Page 261 and 262: W. Sinnott's NGC 2000.0 has a visua
Page 263 and 264: 62 Milkweed Seed Galaxy NGC247 Type
Page 265 and 266: through the narrow haze. But give t
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Page 271 and 272: any large telescope that can provid
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Page 277 and 278: NGC 253 is the brightest member of
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Page 281 and 282: 6 6 NGC5694 Type: Globular Cluster
Page 283 and 284: you see its core dancing? Its starl
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Page 289 and 290: telescope. But seeing it takes more
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Page 295 and 296: 70 Southern Pinwheel NGC300 Type: S
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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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9 5 NGC 6025 Type: Open Cluster Con
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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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