Algol, a remarkable double-star in the constellation Perseus. In the 10th century it was distinctly red, but is now white. It undergoes a cycle of changes in its brightness regularly every two or three days. The light is constant for the greater portion of this period, the star being then of the second magnitude. It then begins to decrease, and has a minimum brightness, of the fourth magnitude, for about twenty minutes, returning to its original condition in ten hours after the variation commenced.