OF THE ROGER N. CLARK

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VISUAL ASTRONOMY OF THE DEEP SKY -- Mi6 (NGC 6611), THE EAGLE NEBULA IN SERPENS B. A. IS h IS.Sm, Dec. _13° 47' (2000.0) A VISUAL ATLAS OF DEEP-SKY OBJECTS Technical. MI6 is another beautiful mixre of bright and dark nebulosity, not very :r from M8 and M20. All three lie in the eneral direction of the center of our Galaxy g nd proviae marvelous views to (Northern a . Hemisphere) summertIme observers. 16, however, is fainter and probably farther away than M8 or M20. Its distance has been estimated as about 8000 light-years, and the mass of its gas as about 12 000 times that of the Sun. The nebulosity has an embedded star cluster. Curiously, early observers noticed only the cluster and not the nebulosity. The cluster was discovered by P. L. de Cheseaux in 1746. Charles Messier in 1764 mentioned a weak light surrounding the stars, but he may have thought it came from unresolved stars. Later observers such as John Herschel and T. W. Webb mention only the cluster. M Like M8 and M20, MI6 consists of hydrogen gas that is illuminated and blown about by newly formed stars near its center. The expanding gas is encountering dark, denser clouds, resulting in bright rims that show intricate' and unusual detail on modern photographs. The cluster's age is estimated to be slightly less than 1 million years, with some stars only 50 000 years old. Visual. MI6 appears as a loose star cluster surrounded by a faint nebula. The nebulosity can be seen in small amateur telescopes, given dark skies, so it is a real mystery why early observers failed to report it. Did their optics scatter so much light that the glow of the stars hid it The magnitude of MI6 is given as about 6.5, but this probably includes the integrated light of the cluster as well as the nebulosity, so the mean surface brightness of the nebulosity can't be stated. MI6 is about 25 arcminutes in apparent diameter. The nebulosity is more difficult to detect than that of M20, but much easier than that surrounding Merope in the Pleiades. Professional photometry puts the surface brightness of the Merope Nebula at 21.6 magnitudes per square arc-second, but the calculated surface brightness for M20 is 24.4. However M20 appears brighter than the Merope Nebula. Photolectric photometry needs to be done to resolve the discrepancy. Through the 8-inch telescope under good skies, the general extent of the nebula was apparent but no small details such as dark patches could be seen. Even though the nebula was quite large, a magnification of 117 X was best for detecting the nebulosity; it filled the field of view of the Erfle eyepiece, showing an apparent diameter of about 60°. A power of 188x was best for detecting faint.stars in the cluster. In similar skies a 17-inch telescope showed some dark patches. Under excellent conditions some dark patches can be seen in 8- to lO-inch telescopes. 196 197
--- VISUAL ASTRONOMY OF THE DEEP SKY A VISUAL ATLAS OF DEEP-SKY OBJECTS • • • • • -- 5' -- Photograph of MI6. South is up. (Courtesy National Op tical Astronomy Observatories.) Scale: 1.2 arc-min/cm 8-inch flll. 5 Cassegrain 20mm Erfle (117X) 12.4mm Erfle (l88X) Drawing of MI6. Viewing Distance (cm) 25X: 115 200x: 14 50X: 57 300x: 10 lOOX: 29 400x: 7 Drawing Method 2 air mass: 1. 23, faintest star: 14. 5 at zenith, 188X; no tracking 8/12 /83 7:40-8:00 UT at Waianae ranch, Hawaii; R. Clark 198 199
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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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-- -- 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 103 and 104: VISUAL ASTRONOMY OF THE DEEP SKY A
Page 105: VISUAL ASTRONOMY OF THE DEEP SKY A
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
Page 141 and 142: VISUAL ASTRONOMY OF THE DEEP SKY AP
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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
Page 153 and 154: -- -- --- VISUAL ASTRONOMY OF THE D
Page 155 and 156: --- --- 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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