. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. 100 years, and only about 150 whose periods areless than 1000 years. 284. Alpha Centauri.—The star a Centauri, a star of the firstmagnitude in the southern hemisphere, is found to consist oftwo components, one of the first magnitude and the other of thesecond. The relative positions of these two components havebeen carefully noted during recent Fig. 77, A represents one of those compo- cents, and B B! B the apparent path of the 1826other about it. The major axis of the orbitis about 40, and the
. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. 100 years, and only about 150 whose periods areless than 1000 years. 284. Alpha Centauri.—The star a Centauri, a star of the firstmagnitude in the southern hemisphere, is found to consist oftwo components, one of the first magnitude and the other of thesecond. The relative positions of these two components havebeen carefully noted during recent Fig. 77, A represents one of those compo- cents, and B B! B the apparent path of the 1826other about it. The major axis of the orbitis about 40, and the period 75 years. Itseccentricity is Since the distance of a Centauri from theearth is approximately known, we can ob-tain some idea of the dimensions of theorbit. If E in the figure represents the earth,we shall have, in the right-angled triangleCOE, the angle E equal to 20, and the sideE O equal to 21 trillions of miles. Hence, C O = OE x tan 20 = 2,000,000,000 miles:which is about equal to the distance of Uranus from the sun, oito 22 times the radius of the earife» 228 COLORED STARS. Again, knowing the radius of the orbit and the period, wecan obtain an approximate value of the mass of a Centaurifrom the formula in Art. 214. It will be found to be about twice the mass of the sun. COLORED STARS. 285. Many of the stars, both isolated and double, shine witha colored light. The isolated colored stars are usually red oroiange: blue or green stars being very uncommon. The num-ber of red stars now recognized is at least 300, about 40 ofwhich are visible to the naked eye. Among the most con-spicious of the red stars are Aldebaran and Antares; and it isworthy of notice that nearly all the variable stars are of thiscolor. According to Mr. Enniss observations, Capella, Rigel,Procyon, and Spica are blue; Sirius, Vega, and Altair are green;and Arcturus is yellow. The components of the double stars are often of different co-lors; blue and yellow, or green and yellow; and,
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