OF THE ROGER N. CLARK

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--- VISUAL ASTRONOMY OF THE DEEP SKY M20 -- -------------- - (NGC 6514), THE TRIFID NEBULA IN SAGITTARIUS R.A. ISh OI.9m, Dec. _230 02' (2000.0) Technical. The Trifid Nebula is a spectacu_ lar mass of glowing hydrogen gas with dust clouds silhouetted in front of it. Discovered by LeGentil in 1747, it gained its popular name from the three main dark lanes that divide it into three bright regions. Several stars near the nebula's center provide the ultraviolet light that makes the hydrogen fluoresce. Two bright stars can be seen near the intersection of the dark lanes. Each is a spectroscopic binary, a pair of stars rotating closely around each other that betray their duplicity by the Doppler shifts in their spectral lines. The main nebula is glowing hydrogen . South of the main nebula is another cloud that is not glowing, but reflecting starlight from the bright star near its center. On color photographs the hydrogen emission nebula appears red, the reflection nebula blue. The light of the reflection nebula is scattered from very small dust grains, as small or smaller than the wavelength of light. This type of scattering affects blue light more than red, which is why a reflection nebula can appear bluer than its illuminating star. The distance to M20 is estimated at only 4500 light years, about the same as te Lagoon Nebula, M8. The stars that eXCite the Trifid's hydrogen are thought to b on . ly . about 7 million years old. The nebula s dlstmctive structure has been explained in the . following way. Radiation pressure and possibly a stellar wind from the young stars cause the hot hydrogen to expand. As it does so it encounters an intricate network of cold, dark material. It divides and flows around dense lumps in this material, leaving them sharpy outlined. Such "bright rims" are common m emission nebulae and give them much of their beauty and drama on photogra hs .. ln the case of the Trifid we seem to be vlewmg the dark masses from behind. On the other hand they may be much closer than the nebula and have nothing to do with it at all. - T' uat. M20 can be a stunning object, but ylS h k d' . this depends critically on t e s y con Ihons. 11 gnitude estimates for the nebula are hard to :ome by, perhaps because of the difficult ! ' p osed by the many eld stars. Re ent est . ma tes put it at magmtude 8.5 and Its size IS . 29 by 27 arc-mmutes; however, t h e b rIg ' h test tWO spots - are about 9 and 5 arc-minutes in . . . diameter. If the magmtude estimate IS accurate, the mean surface brightness is 24.4 mag- h 'tudes per square arc-second, although t e nt twO main parts are couple 0 f magmtu es brighter. Even so, thiS value appears too low hen compared with the surface w brightnesses of other nebulae. Spot measurements Wit ' h a photoelectric photomter re needed to better determine the nebula s brIghtness. M20 seems visible to the unaided eye under good skies, but what's seen is probably not the nebula but the many stars and clusters in its immediate vicinity. M20 can show a wealth of detail under some skies, or virtually nothing at all. In my experience, this nebula presents . a . wider range of views than any other object m the sky. In a dark environment, it is easily visible in 7 X 50 binoculars and the dark lanes can be seen through a 2.4-inch telescope at powers of 20X to 40X. Under good skies I have easily seen the dark lanes in the 3-inch finder at 31 X. Under moderate to poor skies, the nebula is difficult or impossible in any telescope. But under conditions that are just a little better, the nebula becomes easily visible in most instruments. Through the 8-inch under moderate to good skies, the dark lanes are easily seen and structure within them becomes evident at medium powers. M20 benefits greatly from high power, which many observers tend not to use on it. At 117 X through the 8-inch with a wide-field Erfle eyepiece, the nebula just filled the field of view. This magnification was best for viewing the faint outer regions. A power of 188 X was better for defining the detail in the dark lanes. The drawing was hastened by the onset of clouds; even more detail probably could have been seen if more time had been available. As noted previously, on photographs the northern portion of M20 appears red, the southern part blue. I have observed it many times without detecting color. But one occa Sion was different. This was an excellent A VISUAL ATLAS OF DEEP-SKY OBJECTS . d night in the Cascade Mountains of Washington state, when several observers easily saw magnitude 14.8 stars in 8-inch telescopes (the actual faintest star was probably right at the theoretical limit of 15.2). When I turned an 8-inch onto M20, it came as quite a surprise to see beautiful blue and red, just like in color photographs but more pastel! What a magnificent object in dark skies. 188 189
VISUAL ASTRONOMY OF THE DEEP SKY A VISUAL ATLAS OF DEEP-SKY OBJECTS -- • .. • o 5' • Photograph of M20. South is up. (Courtesy National Op tical Astronomy Observatories.) -- -- ---- ------------------------- - Scale: 0.8 arc-min/cm 8·inch fll 1.5 Cassegrain 20mm Erfle (117X) l2.4mm Erfle (188X) Drawing oJ M20. Viewing Distance (cm) 25X: 172 200x :21 50 X: 86 300 X : 14 lOOX: 43 400x: 1 1 air mass: 1.40, faintest star: 13.8 at zenith, 188x; no tracking 5/1 5/83 12:54-13: 14 UT at Barbers Point, Hawaii; R. Clark 190 191
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OF THE ROGER N. CLARK
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Published by Sky Publishing Corpora
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CONTENTS M78 (NGC 2068) NGC 207 1,
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PREFACE During research for this bo
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VISUAL ASTRONOMY OF THE DEEP SKY AB
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VISUAL ASTRONOMY OF THE DEEP SKY TH
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.. .. .. .. VISUAL ASTRONOMY OF THE
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e viewed with enough magnification
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lens and a piano-convex eye lens. A
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controlled depends on how many laye
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The Lumicon H-Beta Filter (Figure 3
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every 1 ° in declination and 4 min
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error of only one quarter the field
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Table 4.1. Ideal limiting magnitude
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5 Making drawings and keeping recor
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more detail in the future. They add
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.. .. .. ... Contrast (C) C Log(C)
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VISUAL ASTRONOMY OF THE DEEP SKY A
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--- VISUAL ASTRONOMY OF THE DEEP SK
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--- M76 (NGC 650-651), PLANETARY NE
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Page 53 and 54: -- -- M45, THE PLEIADES OPEN CLUSTE
Page 55 and 56: -- Brightest stars of the Pleiades
Page 57 and 58: M42 (NGC 1976), M43 (NGC 1982), THE
Page 59 and 60: --- --- --- VISUAL ASTRONOMY OF THE
Page 61 and 62: -- --- NGC 2023 NGC 2024, IC 434 (T
Page 63 and 64: --- --- M78 (NGC 2068), NGC 2071, D
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Page 67 and 68: --- M67 (NGC 2682), OPEN CLUSTER IN
Page 69 and 70: -- VISUAL ASTRONOMY OF THE DEEP SKY
Page 71 and 72: --- -- M82 (NGC 3034), PECULIAR GAL
Page 73 and 74: -- --- --- MI05 (NGe 3379) NGe 3384
Page 77 and 78: MI09 (NGe 3992), GALAXY IN URSA MAJ
Page 79 and 80: -- --- M99 (NGC 4254), GALAXY IN CO
Page 81 and 82: == -- VISUAL ASTRONOMY OF THE DEEP
Page 83 and 84: --- --- -- NGC 4449, GALAXY IN CANE
Page 85 and 86: -- -- --- VISUAL ASTRONOMY OF THE D
Page 87 and 88: --- -- M90 (NGC 4569), GALAXY IN VI
Page 89 and 90: --- -- M94 (NGC 4736), GALAXY IN CA
Page 93 and 94: -- VISUAL ASTRON0MY OF THE DEEP SKY
Page 95 and 96: -- --- -- VISUAL ASTRONOMY OF THE D
Page 97 and 98: -- -- VISUAL ASTRONOMY OF THE DEEP
Page 99 and 100: --- -- M83 (NGC 5236), GALAXY IN HY
Page 101: -- VISUAL ASTRONOMY OF THE DEEP SKY
Page 107 and 108: --- VISUAL ASTRONOMY OF THE DEEP SK
Page 109 and 110: --- -- VISUAL ASTRONOMY OF THE DEEP
Page 111 and 112: ---------------- 5 / --------______
Page 115 and 116: VISUAL ASTRONOMY OF THE DEEP SKY --
Page 119 and 120: -- -- VISUAL ASTRONOMY OF THE DEEP
Page 123 and 124: -- MI5 (NGC 7078), GLOBULAR CLUSTER
Page 127 and 128: --- VISUAL ASTRONOMY OF THE DEEP SK
Page 133 and 134: -- ·rl E I u L ro 5 +J Q) (f) 0 Y
Page 135 and 136: -- --- VISUAL ASTRONOMY OF THE DEEP
Page 137 and 138: VISUAL ASTRONOMY OF THE DEEP SKY NG
Page 139 and 140: '2 5 -rl E I U L CO '---' -t-J ill
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Page 145 and 146: VISUAL ASTRONOMY OF THE DEEP SKY Ta
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Page 149 and 150: -- Appendix E A catalog of deep-sky
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-- -- --- VISUAL ASTRONOMY OF THE D
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--- --- VISUAL ASTRONOMY OF THE DEE
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VISUAL ASTRONOMY OF THE DEEP SKY AP
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-- -- VISUAL ASTRONOMY OF THE DEEP
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-- APPENDIX F: OPTIMUM DETECTION MA
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------- VISUAL ASTRONOMY OF THE DEE
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--- VISUAL ASTRONOMY OF THE DEEP SK
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VISUAL ASTRONOMY OF THE DEEP SKY -
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VISUAL ASTRONOMY OF THE DEEP SKY -
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VISUAL ASTRONOMY OF THE DEEP SKY Ta
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VISUAL ASTRONOMY OF THE DEEP SKY Ta
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G VISUAL ASTRONOMY OF THE DEEP SKY
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INDEX INDEX M70 (NGC 6681), 312, 31
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OF THE ROGER N. CLARK

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