OF THE ROGER N. CLARK

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-- VISUAL ASTRONOMY OF THE DEEP SKY A VISUAL ATLAS OF DEEP-SKY OBJECTS M51: M51 (NGC 5194), THE WHIRLPOOL GALAXY IN CANES VENA TICI NGC 5195 R.A. 13h 29.9m, Dec. 47° 12' NGC 5195: R.A. 13h 30.0m, Dec. 47° 16' (2 000.0) Technical. M51 was discovered by Charles Messier in 1773, and it was the first galaxy recognized as having spiral characteristics, by Lord Rosse in 1845. It a d NGC 5 95 are a pair of interacting . galaxies. M 1 IS estimated to be three times as massive as its companion, or about 100 billion times as massive as our Sun. This pair of galaxies is only about 13 million light-years away. M5l is tilted 60° from edge on, but is somewhat distorted because NGC 5195 has stretched it into an elliptical shape. The small galaxy is passing the larger one, with tidal forces modifying the appearance of both. Computer simulations have been able to reproduce the structure of each galaxy. NGC 5195 passed M51 many millions of years ago and has bent one spiral arm away from its normal spiral pattern; this is the arm that appears to trail after the smaller galaxy. In photographs the spiral pattẹrn .afM51 is nothing less than spectacular, hlghhghted by complex star clouds and right and d rk nebulosity. Some of the hlghest-resolutlOn photographs even sho ,,: a fe individual . supergiant stars. On the mner Side of the sp ral arms are long, thin, dark dust lanes shOWing great complexity. The arms can be traced for about one-and-one-half turns, and can be detected as close as 15 arc-seconds from the nucleus. The nucleus appears about 2. 7 a· seconds, or approximately 500 light-years, m diameter. NGC 5195 has a bright nucleus no more than 140 light-years across, and r dio waves are emitted from a region several times larger than this. Apparently many hot youn stars populate the nucleus, intermingled With heated to 30 000 °C and dust. This galxy IS classed as an irregular, though it is beheved to be evolving into a barred spiral. -- Visual. The M51 system can be an excellent . ht in amateur telescopes, and it may show slg piral structure. M51 is visual magnitude Its s . . 5 5 d· 8 I and 10 by . arc-mmutes m lamenT, ·hile NGC 5195 is magnitude 11.0 and 2.0 : 1.5 arc-minutes . in size. Hẹre. ag in, mean Y rface brightness IS a better mdlcatlOn of the :sibility f the two.objects. M51 has a mean surface brigh tness of 21.1, and N GC 5195 20.8 magnitudes per square arc-second. Although NGC 5195 is 3 magnitudes fainter, ·ts slightly higher mean surface brightness akes it seem about the same brightness and size as the central region of M51. The visibility of M51 's spiral structure in amateur telescopes is ana1yzed in detail in Chapter 6. The smallest telescope required is about 6 inches. Amateur John Mallas reported hints of spiral structure in a 4-inch refractor, though he remarked that the observation may be spurious since he was quite familiar with photographs of the object. The view through the 8-inch telescope is shown here for three different observing conditions and levels of observer experience. The first drawing, made in 1983, was with good skies after the research for this book was complete. The detail that could be seen with careful observation was phenomenal. Some details have never been reported before with medium-size amateur telescopes so far as I know. The two spiral arms were reasonably easy at all powers. At high power (334x) a hub was visible around the central core and was slightly brighter at its east side, appearing like a rim. After observing this galaxy many times without detecting detail in the companion, it came as a surprise to see structure in NGC 5195. It had a bright, starlike nucleus, but at 188x and higher, there was a decrease in brightness and then an increase away from the nucleus. Apparently a dark dust lane surrounds the nucleus. The next day I searched observatory photographs for this feature. This central part of the galaxy, as well as the hub of M51, are commonly overexposed. When a properly exposed photograph was found, the dust lane was quite apparent. It is quite satisfying to find something in the sky that one had not seen before, and then prove the observation was not spurious . . Compare that drawing with the one made 10 197 1. Less was seen then even though the 177 sky was better (the limiting magnitude was at least a half magnitude fainter) and my observing experience was already good. But the concepts in this book had not been realized at that time. First notice that the highest power used in 1971 is the lowest used in 1983. That had a large effect on what was seen. The observing time was substantial in both cases, but in 1971 no details were detected in the companion galaxy or the central hub. The two main spiral arms were seen in 1971 but were more distinct in the 1983 observation. Under moderate or poorer skies, a medium-size telescope may not show even a hint of spiral structure. Such a view is shown in the third drawing, made on June 20, 1982. The contrast in such skies is so low that no matter what power is used, virtually no detail can be seen. A hint of spiral structure was possibly seen, but higher powers showed the same or less, even in the brighter central regions. A larger aperture in such skies would show more detail - but a small telescope in the country often outperforms a large one in the city. Sky quality, not telescope size, is the most important factor for seeing detail in deep-sky objects. Under good to excellent skies, large amateur telescopes will begin to show dark lanes as well as bright knots in the· spiral arms. 176
-- -- VISUAL ASTRONOMY OF THE DEEP SKY A VISUAL ATLAS OF DEEP-SKY OBJECTS • -- 5' -- Photograph oJ M51 (the main galaxy), with its smaller companion NGC 5195. South is up. (Courtesy National Optical Astronomy Observatories') Drawing oJ M51 and NGC 5195, its smaller companion. Compare with the drawings on the next two pages. The reasonsJor the differences are described in the text. Scale: 1.2 arc-min/cm 8-inch f! 1 1.5 Cassegrain 20mm Erfle (117x) 12.4mm Erfle (188 X) 7mm Erfle (334X) Viewing Distance (cm) 25 X: 115 200 X: 14 50 X: 57 300X:lO lOO X : 29 400 X: 7 Drawing Method 2 air mass: 1.11, faintest star: 14.3 at zenith, 188X; no tracking 5/1 1/83 8:15-8:54 UT at Waianae ranch ' Hawaii· , R. C1ark 178 179
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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 SKY A
Page 45 and 46: VISUAL ASTRONOMY OF THE DEEP SKY A
Page 47 and 48: --- VISUAL ASTRONOMY OF THE DEEP SK
Page 49 and 50: --- M76 (NGC 650-651), PLANETARY NE
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: -- --- -- VISUAL ASTRONOMY OF THE D
Page 99 and 100: --- -- M83 (NGC 5236), GALAXY IN HY
Page 109 and 110: --- -- VISUAL ASTRONOMY OF THE DEEP
Page 111 and 112: ---------------- 5 / --------______
Page 113 and 114: --- -- VISUAL ASTRONOMY OF THE DEEP
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 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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VISUAL ASTRONOMY OF THE DEEP SKY Ta
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-- Appendix E A catalog of deep-sky
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--- -- VISUAL ASTRONOMY OF THE DEEP
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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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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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