351. _Colors of Double and Multiple Stars._--The components of double and multiple stars are often highly colored, and frequently the components of the same system are of different colors. Sometimes one star of a binary system is _white_, and the other _red_; and sometimes a _white_ star is combined with a _blue_ one. Other colors found in combination in these systems are _red_ and _blue_, _orange_ and _green_, _blue_ and _green_, _yellow_ and _blue_, _yellow_ and _red_, etc. [Illustration: Fig. 415.] If these double and multiple stars are accompanied by planets, these planets will sometimes have two or more suns in the sky at once. On alternate days they may have suns of different colors, and perhaps on the same day two suns of different colors. The effect of these changing colored lights on the landscape must be very remarkable. New and Variable Stars. 352. _Variable Stars._--There are many stars which undergo changes of brilliancy, sometimes slight, but occasionally very marked. These changes are in some cases apparently irregular, and in others _periodic_. All such stars are said to be _variable_, though the term is applied especially to those stars whose variability is _periodic_. [Illustration: Fig. 416.] 353. _Algol._--_Algol_, a star of _Perseus_, whose position is shown in Fig. 416, is a remarkable variable star of a short period. Usually it shines as a faint second-magnitude star; but at intervals of a little less than three days it fades to the fourth magnitude for a few hours, and then regains its former brightness. These changes were first noticed some two centuries ago, but it was not till 1782 that they were accurately observed. The period is now known to be two days, twenty hours, forty-nine minutes. It takes about four hours and a half to fade away, and four hours more to recover its brilliancy. Near the beginning and end of the variations, the change is very slow, so that there are not more than five or six hours during which an ordinary observer would see that the star was less bright than usual. This variation of light was at first explained by supposing that a large dark planet was revolving round Algol, and passed over its face at every revolution, thus cutting off a portion of its light; but there are small irregularities in the variation, which this theory does not account for.