OF THE ROGER N. CLARK

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VISUAL ASTRONOMY OF THE DEEP SKY -- -- ---- ---------- ---- -- --==-- M8 (NGC 6523), THE LAGOON NEBULA IN SAGITTARIUS, NGC 6530 R.A. JSh 04.7m, Dec. _240 20' (2000.0) Technical. The Lagoon Nebula, M8, is a beautiful and complex region of dust and glowing gas. The main nebula appears cut in two by a dark lane, which the late-19th_ century astronomy writer Agnes Clerke claimed looked like a lagoon. The nebula was discovered in 1747 by Le Gentil, but a star cluster, which is located near the center of the nebula and designated NGC 6530, was discovered by John Flamsteed as early as 1680. The nebula is estimated to be about 4500 light-years away, and the age of the star cluster to be about 2 million years, depending on the authority cited. This distance would make the nebula's outlying portions well over 100 light-years across. One of the stars that irradiates the hydrogen gas into glowing is Herschel 36, magnitude 9.5, near the nebula's brightest part. Other stars also illuminate the nebula, including 6th-magnitude 9 Sagittarii, and probably several more embedded in the nebula that we cannot see. These young stars are pushing gas away from the nebula's center with their intense radiation and stellar winds. As the hot hydrogen collides with cold dark denser patches, it flows around the and ' outlines them with bright rims. Such rims and dark patches appear in many photographs of emission nebulae. The small, very bright patch next to Herschel 36 has been named the Hourglass Nebula for its shape. It is extremely dense, as nebulae go, and only a half light-year ac oss. The heat of Herschel 36 will soon cause It to dissipate. Astronomers have calc lated th t its present situation next to the bnght star IS so unstable that the star must be much yo n ger than 10 000 years. Herschel 36 is certamly one of the youngest stars known. -- Visual. M8 can be seen with the unaided e The total magnitude of the nebula is eY ti ated as about 5, with a total size of80 by : arc-minutes. This yields a mean surface brightness of only 22.4 magnitudes per s quare arc-second. But the brightest portions a r e many magnitudes brighter. It has been ai d that the Hourglass Nebula can be seen in : telescope when the full Moon is nearby. Its surface brightness must be similar to that of the Trapezium region in M42. On the west side of the Hourglass, with Herschel 36, are three dark lanes like a miniature Trifid Nebula. Even after observing M8 many times, I never saw them because I never tried powers above about 60x. However, 80X or more will bring them into view in medium amateur telescopes. After "discovering" this dark marking, I surveyed many amateur observers and had a hard time finding anyone else who had seen it. The lesson is clear: a beautiful and easy object has been missed because M8 seems only to be explored at the lowest possible magnifications. The drawing shows the appearance of M8 through the 8-inch telescope under moderate skies. The star Herschel 36 was seen next to the small dark lane, but the bright Hourglass outline was not recorded. Probably more time should have been spent at higher magnifications. When the observation was made, I did not know of the Hourglass or the adjacent dark "Trifid" . The Hourglass Nebula has a size of about 0.8 by 0. 7 arc-minutes, with a visual magnitude of about 8. The surface brightness is close to 16 magnitudes per square arcsecond. After I knew of the existence of the Hourglass I searched for it under the excellent skies of the Colorado Rockies. In seeing of 1.5 arc-seconds the hourglass shape could be seen in the 8-inch at 334 X. The dark lanes could be seen in the 3-inch at 266x but only with difficulty. The Hourglass region was also seen, but it appeared only as a bright patch separated from the star Herschel 36. Even considering its brightness, the hourglass shape needs high power because of the nearby 9.5-magnitude star. The major dark lane dividing the nebula in two, the one from which the Lagoon name is denved, can be seen in the smallest amateur telescopes. Under good skies more of the outer region becomes visible in the 8-inch, and it A VISUAL ATLAS OF DEEP-SKY OBJECTS begins to take on the ragged-edge appearance seen in photographs. I've often thought the two bright stars superimposed on the nebula resemble eyes, the lagoon dark lane a mouth, and the star cluster on the other side, a beard on the chin of a face. The Hourglass region then appears like the brain, and the whole effect is that of a monster from some lowbudget science fiction movie. 192 193
VISUAL ASTRONOMY OF THE DEEP SKY A VISUAL ATLAS OF DEEP-SKY OBJECTS • • • . . • • • ---5' --- Photograph oj M8. South is up. (Courtesy National Optical Astronomy Observatories.) Scale 1. 8-inch f! 1 1.5 Cassegrain 28mm Kellner (82 X) 20mm Erfle (1 17 X) Drawing oj M8. Viewing Distance (cm) 25X: 115 200X: 14 50 X: 57 300 X : lO lOO X : 29 400x: 7 air mass: 1.75, faintest star: 13.0 at zenith, 117 X; no tracking 9/ 19/82 6:40-7:00 UT at Ewa Beach, Hawaii; R. Clark 194 195
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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 103: VISUAL ASTRONOMY OF THE DEEP SKY A
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 121 and 122: VISUAL ASTRONOMY OF THE DEEP SKY A
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
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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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