OF THE ROGER N. CLARK

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-- -- VISUAL ASTRONOMY OF THE DEEP SKY NGC 5139 (OMEGA CENTAURI), THE GREAT GLOBULAR CLUSTER IN CENTAURUS R.A. J3h 26.8m, Dec. --47° 29' (2 000.0) Technical. Omega Centauri is without doubt the grandest globular cluster in sky. I t has the larges t true diameter of any measured (620 light-years) and also "If-'jJ"lIl1 to be one of the closes t known (16 500 years). It is about 15 billion years old, Iy the age of our entire Galaxy. Omega tauri contains an estimated one million s It is one of the most massive clusters with about 500 000 times the mass of Sun. In addition, it is one of the fastest ing globulars. As a result it is decidedly tical, about 25 percent longer on one axis. are fortunate to have it as such a neighbor. Like other globular clusters, Omega tauri is deficient in heavy elements and tains no measurable gas or dust. Since cluster's stars are nearly as old as the verse, they formed out of material fresh from the Big Bang: hydrogen helium, with only small traces of heavy ments that were synthesized inside nr"\I1,rIllO generations of stars. The lack of gas and dust is explained the cluster's orbit. A detailed study of the motions of individual stars in the cluster allowed an accurate determination of the entire cluster's motion and hence its orbit around and through our Galaxy. Omega Centauri follows a highly elliptical orbit around the center of our Galaxy that brings it to within 6 200 light-years of the galactic nucleus and as far away as 21 000 light-years. Its many passes through the plane of our have swept it clean of gas. This orbit is typ!cal of other globulars. Since they have no m terstellar gas, they have not been able to produce new stars for billions of years. Omega Centauri was catalogued by Claudius Ptolemy over 1,800 years ago. In 1603 it was again listed as a star, this time by Johann Bayer, who gave it the Greek letter Omega in his sky atlas Uranometria. Edmond Halley seems to have been the first to recog high in the sky, it definitely appears larger nize the object as a cluster, in 1677. When than a star and quite fuzzy to the naked eye. V· ual. Omega Centauri is beyond com- IS Its total visual magnitude of3.65 makes p a t r brightest globular in the sky. With a It . h r: d' la meter of 30 arc-mmutes, . t e mean sunace brightness is 19.6 magmtu d es per square arcond. Only the cluster 47 Tucanae far to sec the south can come c I ose to ' Its Vlsua . I splendor. Through the 8-inch telescope under moderate skies, Omega Centaui was awe . spiring even when very,low m the sky. At ;owers of more than 100 X it was resolved to the center. Countless stars filled the field of The star density increases only slowly toward the center. The cluster's bright central portion is 6 to 7 arc-minutes across, and the maximum extent observed through the 8- inch was about 13 arc-minutes. On deep-sky photographs Omega Centauri is well over one degree in diameter, though visually when high in the sky it appears about 30 arc medium-power Erfle eyepieces. minutes across. East of the center are two U-shaped strings of stars with the Us connected at their bottoms. Farther east, and above and below the double U, are two strings of stars that extend farther eastward still. The southernmost is an arc about 3.5 arc-minutes long. In the accompanying drawing, only a few of the brightest tions. Plotting thousands of stars by eye is stars are in their geometrically correct posi hardly feasible! The brightest stars in Omega Centauri are of 11 th magnitude, so the cluster can be at least partially resolved with telescopes as small as 2 inches, when it is high in the sky. These bright stars are red giants, and their color may be detectable in large amateur telescopes. Omega Centauri contains over 160 known variable stars, second only to M3. The well-known globular M13 (NGC 6205) in Hercules appears miniscule in comparison with Omega Centauri. From low northern latitudes, such as in Hawaii, M13 and Omega Centauri are above the horizon at the same time, so the two can be compared. Such a comparison can be made here: the drawing of Omega Centauri and that of Ml3 on page 186 are at the same scale. Note that Ml3 appeared only half the size ofOmega Centauri through the 8-inch -even though the latter was very low in the sky, while the A VISUAL ATLAS OF DEEP-SKY OBJECTS former was observed nearly overhead. This is not intended to denigrate the splendor of M13, but rather to put it in perspective for northern observers for whom Omega Centauri is one of those tantalizing, legendary objects on or below the southern horizon. Since M 13 is called the Great Globular Cluster in Hercules, perhaps Omega Centauri should be named the "Super Great Cluster in Centaurus" . 172 173
-- --- -- VISUAL ASTRONOMY OF THE DEEP SKY A VISUAL ATLAS OF DEEP-SKY OBJECTS • • -.' .. ,. ., :.\ .. .. ' .. . : . , ' - . . ... 4 .. -. - , \ " • • " . . - ---5' --- Photograph ofNGC 5139 (Omega Centauri) . South is up . (Courtesy National Optical Astronomy Observatories.) Drawing of NGC 5139 (Omega Centauri) . Scale: 1.2 arc-min/cm 8-inch fill.5 Cassegrain 20mm Erfle (117x) 12.4mm Erfle (188 X, best view) 7mm Erfle (334X) Viewing Distance (cm) 25 X : 1 15 200 X: 14 50X : 57 300x: 10 lOO X : 29 400 X: 7 air mass: 2.78, faintest star: 13 · 8 at zenith " 188X· no tracking 5/15/83 7:40-8:20 UT at Barbers Point ' Hawaii· , R. Clark 174 175
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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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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: -- VISUAL ASTRON0MY OF THE DEEP SKY
Page 97 and 98: -- -- VISUAL ASTRONOMY OF THE DEEP
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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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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