Algol, a variable star in the constellation Perseus. (A variable star is one that changes in brightness.) Algol, which was observed by the ancient Greeks and named by the Arabs, is now known to be a complex system of at least four stars revolving around one another. A pair of the stars forms an eclipsing binarytwo stars that pass in front of each other in turn, causing a periodic (cyclical) dimming of their combined light. Algol dims regularly every 21 days and 21 hours.
A billion years ago, the moon stopped being geologically active. But that's not to say that the moon is doing nothing for us. Let's find out why we owe our moon some thank-yous.
Hundred of meteors fly across the sky every night, but only a few make it to Earth. Meteors are best known for the brilliant streaks of light they make as they burn up in the atmosphere. Learn about 10 memorable meteor crashes that left an impression.