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ALGOL

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Originally appearing in Volume V01, Page 656 of the 1911 Encyclopedia Britannica.
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ALGOL , the Arabic name (signifying " the Demon ") of 6 Persei, a See also:

star of the second magnitude, noticed by G. Montanan in 1669 to fluctuate in brightness. See also:John Goodricke established in 1782 the periodicity of its See also:change in about 2d 21h, and suggested their cause in recurring eclipses by a large dark See also:satellite. Their intermittent See also:character prompted the supposition. The See also:light of Algol remains See also:constant during See also:close upon 56 See also:hours; then declines in 62 hours (approximately) to nearly one-See also:fourth its normal amount, and is restored by sensibly the same gradations. The See also:amplitude of the phase is 1.1 magnitude; and the See also:absence of any stationary See also:interval at minimum proves the See also:eclipse to be partial, not See also:annular. Its conditions were investigated from photometric data, by See also:Professor E. C. See also:Pickering in 188o;' and their realization was finally demonstrated by Dr H. C. See also:Vogel's spectroscopic See also:measures in 1889.2 Previously to each obscuration, the star was found to be moving rapidly away from the See also:earth; its velocity then diminished to zero pari passu the loss of light, and reversed its.direction during the See also:process of recovery. Algol, in fact, travels at the See also:rate of 26.3 See also:miles a second See also:round the centre of gravity of the See also:system which it forms with an invisible See also:companion, while the two together approach the See also:sun with an unvarying See also:speed of 2.3 miles per second.

The elements of this disparate pair, calculated by Dr Vogel on the somewhat See also:

precarious See also:assumption that its dark and See also:bright members-are of equal mean See also:density, are as follows: See also:Diameter of Algol . . . . r,o6i,000 See also:English miles. Satellite . . . 834,300 Distance from centre to centre . 3,230,000 „ „ See also:Mass of Algol s See also:solar mass. Satellite . . 9 Mean density . about 1 solar. The See also:plane of the See also:joint See also:orbit, in which no deviation from circularity has yet been detected, nearly coincides with the See also:line of sight. The See also: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 See also:short of its See also:original value.

By an exhaustive discussion, Dr S. See also:

Chandler ascertained in 1888 the compensatory nature of these disturbances;3 and he afterwards found the most important among several which probably conspire to produce the observed effects, to be comprised in a period of 15,000 light-cycles, See also:equivalent to 118 years.¢ An explanatory See also:hypothesis, propounded by him in 1892,5 is still on its trial. The system of Algol, according to this view, is triple; it includes a large, obscure See also:primary, round which the eclipsing pair revolves in an orbit somewhat smaller than that of See also:Uranus, very slightly elliptical, and inclined 200 to the line of sight, the periodic See also:time being 118 years. The alternate delay and See also:acceleration of the eclipses are then merely apparent; they represent the changes in the length of the light-See also:journey as the stars perform their wide See also:circuit. If these suppositions have a basis of reality, the proper See also:motion of Algol should be disturbed by a small, but measurable undulation, corresponding to the See also:projection of its orbit upon the See also:sky; and although certainty on the point cannot be attained for some years to come, See also:Lewis See also:Boss regarded the See also:evidence available in 1895 as tending to confirm Dr Chandler's theory.° Proceedings Amer. Acad. vol. xvi. p. 27. z Astr. Nach. No. 2947. See also:Asir.

See also:

Journal, No. 165. a Ibid. No. 509. 5 Ibid. Nos. 255-256. 6 Ibid. No. 343 A See also:rival See also:interpretation of the phenomena it dealt with was put forward by F. See also:Tisserand in 18951 It involved the See also: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 See also:

moot question whether changes of the latter See also:kind actually occur. When they are proved to do so, Tisserand's hypothesis will hold the See also:field. Algol gives a See also: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 See also:limb can be little, if at all, darkened by the " See also:smoke-See also:veil " absorption conspicuous in the sun. The rays of this star spend close upon a See also:century in travelling hither. Dr See also:Chase's measures with the Yale See also:heliometer indicated for it, in 1894, a See also:parallax of about 0" .035 ; 2 and it must, accordingly, be of nearly four times the See also:total brightness of Sirius, while its aerial lustre exceeds seventy-See also:fold that of the solar photosphere. Variables of the Algol class are rendered difficult to discover by the incidental character of their fluctuations. At the end of 1905, however, about 37 had been certainly recognized, besides some outlying cases of in-determinate type, in which continuous occultations by two bright stars, revolving in virtual contact, are doubtfully supposed to be in progress. (A. M.

End of Article: ALGOL

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