The Caldwell Objects

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63 my eyes had aged; what I "suspected" to be visible in the year 2000 "was visible" seventeen years earlier. Of course, as Houston says, "[a]tmospheric clarity obviously played a major role for seekers of the Helix. Observers on mountains did much better than smog fighters." Here in Hawaii we don't have industrial or automotive smog, but we do have volcanic smog, or "vog." Personally, I believe that while our eyes deteriorate with age, we can balance that with the "experience factor." The 250 more you look, the more you learn how to see, and thus the more you can see. Since I can't accept the fact that I'm aging, I've decided to blame the vog. The Helix appears so large and diffuse because it is so close. Over the last 20 years, distance estimates have varied considerably, with the majority clustered around 420 light-years. John Meaburn (Victoria University, England) gave a value of 522 light-years in a 1998 paper in the Monthly Notices of the Royal Astronomical Deep-Sky Companions: The Caldwell Objects
Society. Accepting that distance, we find the nebula measures 1.8 x 1.5 light-years. The Helix's proximity also makes it unique among the planetaries, because it allows astronomers to study its small-scale structures with groundbased telescopes and extremely fine details with the orbiting Hubble Space Telescope. In ground-based images the Helix looks like a typical annular planetary nebula. The vacuous region inside the ring shows evidence of oxygen atoms being excited by ultraviolet radiation flowing out from the extremely hot (120,000° Kelvin) central star. That star shines at magnitude 13.4 (though it maybe variable), so it's a bit brighter than Pluto is when closest to the Sun. The ring itself glows with the light of excited nitrogen and hydrogen atoms. Everything looks normal until you look closely at the ring and see that it has a helical structure with two twists. But this is an optical illusion. In January 1999 a team of astronomers released a three-dimensional, globelike map of the Helix based on data from a submillimeter-wave telescope atop Mauna Kea. Close inspection of the 3-D image reveals that the Helix Nebula behaves much like a double-headed garden sprinkler. For each blob of gas that was ejected on one side of the nebula, a similar structure was ejected on the opposite side. The researchers say this behavior likely has been caused by a companion star that affects how the Helix's progenitor ejects its outer layers. The Helix is also the only planetary in which gaseous knots with cometlike tails have been observed. This does not necessarily make the Helix unique; more distant planetaries probably have similar but irresolvable features. A few hundred of the Helix's knots were first detected from the ground, but HST increased the count to 3,500. None of the knots is found close to the central star. Rather, The Caldwell Objects 63 they form a thick ring, with the most visible knots lying along the inner edge of the luminous annulus, trillions of km from the central star. At first, astronomers believed these features might be evidence of a gigantic cloud of comets like the Oort Cloud postulated to surround our Sun. But it turns out that these features are much too large to be comets. Each gaseous knot is at least twice the size of our solar system, and each tail stretches for more than 150 billion km. The gaseous knots display crescent-shaped heads that point to the central star. Meaburn says that each crescent is an ionized skin on the surface of a dusty molecular globule. And since the "comet tails" point away from the central star (which is ionizing the crescents), it's clear that each globule is being shaped by a wind from that star. In fact, we're seeing a sort of cosmic rear-end collision in which hot gas spewing from the surface of the central star rushes through a shell of colder gas that the star ejected when it was still a red giant, some 10,000 years earlier. When the hot, rarefied gas mixes with the cooler, denser gas, instability results, and the cooler cloud fragments into these gaseous knots. A close look at the HST image shows the cool ring writhing with these ghoulish "comets." The ring looks as if it's effervescing like an Alka-Seltzer tablet in water. Astronomers expect the gaseous knots, each several billion km across, to eventually dissipate into the cold blackness of interstellar space. "If this phenomenon is common among stars," says Rice University astronomer C. Robert Ο'Dell, who acquired the images, "then our galaxy could be littered with trillions of these objects." The Helix's knots are moving outward at about 15 km per second on average. This radiating array of pseudocometary pellets led Houston to coin the object's other popular nickname, the Sunflower Nebula. So how does this celestial marvel hold up 251
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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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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 241 and 242: 5 6 Pac-Man Nebula NGC246 Type: Pla
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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 257 and 258: 6 0 Ringtail Galaxy; Rataail Galaxy
Page 259 and 260: war. X-ray images taken by the Rosa
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Page 265 and 266: through the narrow haze. But give t
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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
Page 285 and 286: cosmologist Halton Arp (Max-Planck
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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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Page 309 and 310: The asymmetry remains at high power
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Page 315 and 316: times larger than the full Moon. In
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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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The Caldwell Objects

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