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Encyclopedia Britannica - Main :: AJA-ALL |
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ALGOL , the Arabic name (signifying " the Demon ") of 6 Persei, a star
change in about 2d 21h, and suggested their cause in recurring eclipses by a large dark satellite. Their intermittent character prompted the supposition. The light of Algol remains constant during close upon 56 hours; then declines in 62 hours (approximately) to nearly one-fourth its normal amount, and is restored by sensibly the same gradations. The amplitude of the phase is 1.1 magnitude; and the absence of any stationary interval
measures
star
the loss of light, and reversed its.direction during the process of recovery. Algol, in fact, travels at the rate of 26.3 miles a second round the centre of gravity of the system which it forms with an invisible companion, while the two together approach the sun with an unvarying speed of 2.3 miles per second. The elements of this disparate pair, calculated by Dr Vogel on the somewhat precarious
bright members-are of equal mean density, are as follows:Diameter of Algol . . . . r,o6i,000 English miles.Satellite . . . 834,300 Distance from centre to centre . 3,230,000 Mass of Algol s solar mass. Satellite . . 9 Mean density . about 1 solar. The plane of the joint orbit, in which no deviation from circularity has yet been detected, nearly coincides with the line of sight. The period of Algol, as measured by its eclipses, is subject to complex irregularities. It shortened fitfully by eight seconds between 1790 and 1879; soon afterwards, restoration set in, and its exact length in 1903 was 2d 2oh 48'n 56, being only two seconds short of its original
Proceedings Amer. Acad. vol. xvi. p. 27. z Astr. Nach. No. 2947. Asir. Journal, No. 165. a Ibid. No. 509. 5 Ibid. Nos. 255-256. 6 Ibid. No. 343 A rival interpretation of the phenomena it dealt with was put forward by F. Tisserand in 18951 It involved the action of no third mass, but depended solely upon the progression of the line of apsides in a moderately elliptical orbit due to the spheroidal shape of the globes traversing it. Inequalities of the required sort in the returns of the eclipses would ensue; moreover, their duration should concomitantly vary with the varying distance from periastron at the times of their occurrence. It is a moot question whether changes of the latter kind actually occur. When they are proved to do so, Tisserand's hypothesis will hold the field. Algol gives a helium-spectrum which undergoes no alteration at minimum. Hence the light from the marginal and central portions of the disc is identical in quality, and the limb can be little, if at all, darkened by the " smoke-veil " absorption conspicuous in the sun. The rays of this star spend close upon a century in travelling hither. Dr Chase's measures
parallax
bright stars, revolving in virtual contact, are doubtfully supposed to be in progress. (A. M. C.)End of Article: ALGOL If you wish, you can link directly to this article.
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