ALGOL, the Arabic name (signifying "the Demon") of, 6 Persei, a star of the second magnitude, noticed by G.
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.
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 period of Algol, as measured by its eclipses, is subject to complex irregularities.
The system of Algol, according to this view, is triple; it includes a large, obscure primary, round which the eclipsing pair revolves in an orbit somewhat smaller than that of Uranus, very slightly elliptical, and inclined 20° to the line of sight, the periodic time being 118 years.
Algol gives a helium-spectrum which undergoes no alteration at minimum.
Variables of the Algol class are rendered difficult to discover by the incidental character of their fluctuations.
There are several wellmarked varieties of short-period variables; the most important are typified by the stars Algol, # Lyrae, Geminorum and S Cephei.
In the Algol variables one of the component stars is dark (that is to say, dark in comparison with the other), and once in each revolution, passing between us and the bright component, partially hides it.
The variability of Algol ((3 Persei) was discovered in 1783 by John Goodricke (1764-1786), but, judging from its name, which signifies ' ` the demon," it seems possible that its peculiarity may have been known to the ancient astronomers.
Algol is ordinarily of magnitude 2.3, but once in a period of 2d.
Ever since the variability of Algol was observed it was suspected to be due to a partial eclipse of the star by a dark body nearly as large as itself revolving round it; but the explanation remained merely a surmise until K.
Vogel of Potsdam, by repeated measurements of the motion of Algol in the line of sight, showed that the star is always receding from us before the loss of light and approaching us afterwards.
This leaves no room for doubt that an invisible companion passes between us and Algol about the time the diminution of light takes place, and so proves the correctness of the explanation.
The dimensions of the Algol system have been calculated, with the result that Algol appears to have'a diameter of 1,000,000 m.
About 56 Algol variables were known in 1907; the variables of this class are the most difficult to detect, for the short period of obscuration may easily escape notice unless the star is watched continuously.
It differs from the Algol type in having two unequal minima separated by two equal maxima.
No hard and fast physical distinction can be drawn between the various classes of short-period variables; as the distance between the components diminishes the Algol variable merges insensibly into the (3 Lyrae type.
Stars of the class to which the Algol type of variables belongs will appear to us to vary only in the exceptional case when the plane of the orbit passes so near our sun that one body appears to pass over the other and so causes an eclipse.
A continuous gradation can be traced from the most widely separated visual binaries, whose periods are many thousand years, to spectroscopic binaries, Algol and # Lyrae variables, whose periods are a few hours and whose components may even be in contact, and from these to dumb-bell shaped stars and finally to ordinary single stars.
For the Algol variables it is possible to form even more direct calculations of the density, for from the duration of the eclipse an approximate estimate of the size of the star may be made.
Roberts concluded in this way that the average density of the Algol variables and their eclipsing companions is about one-eighth that of the sun.