Elements of astronomy ..with explanatory notes, and questions for examination . s belonging to this group. NEBULA 787. Scattered throughout the sky are seen, either bythe naked eye or by the aid of the telescope, dim mistyobjects of various shapes and sizes, stationary to all ap-pearance like the stars themselves. These objects arenamed nebula?1, and are arranged under the following 1. For the meaning of this word, see page 13 note 3. What observations and computations have been made which show that it contains a vastnumber of stars ? What is supposed of our sun 1 What are nebulae 7 How classi
Elements of astronomy ..with explanatory notes, and questions for examination . s belonging to this group. NEBULA 787. Scattered throughout the sky are seen, either bythe naked eye or by the aid of the telescope, dim mistyobjects of various shapes and sizes, stationary to all ap-pearance like the stars themselves. These objects arenamed nebula?1, and are arranged under the following 1. For the meaning of this word, see page 13 note 3. What observations and computations have been made which show that it contains a vastnumber of stars ? What is supposed of our sun 1 What are nebulae 7 How classified ? 312 STARRY HEAVENS. classes; viz., Elliptical, Annular, Planetary,Double, Spiral, and Irregular Nebulje. 788. Elliptical Nebulae. One of the finest speci-mens of this class is situated in the girdle of is visible to the naked eye, and was noticed and de-scribed by Simon Marius in 1612 ; and there is reasonfor believing that it was seen even as early as nebula? is of vast size extending over an area 157 indiameter. It is delineated in Fig. 87. FIG. NEBULA OF ANDROMEDA. 789. Annular Nebulae. A remarkable annularnebula easily detected with a telescope of ordinary poweris found in the constellation of the Lyre. It has the ap-pearance of a flat oval ring, the central space not beingquite dark but appearing, says Herschel, to be filledwith faint nebula?, like a gauze stretched over a hoop. Ne-bula? of this class are very rare. Nine comprise theentire number. 790. Planetary Nebulae. Planetary nebidce are socalled from their similarity in form to planets, being eitherround or somewhat oval. Only about 25 of this class rtive examples of elliptical and annular nebula-7 How many of the latter kind tirenow known 1 IRREGULAR NEBULA. 313 have yet been discovered, and nearly three quarters ofthis number are in the southern hemisphere. One of the most beautiful is situated in the constella-tion of the Cross. It is a well denned circular figure,12 in diameter, looki
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