Alpha Centauri, a system of three stars, one of whose membersProxima Centauriis the star nearest the sun. Proxima can be seen only with a telescope; the other two stars appear as a single, very bright star to the unaided eye. Alpha Centauri is slightly more than four light-years from the sun. It is in the constellation Centaurus.
It's tough to wrap your mind around a time when the Earth wasn't here. So how do Earth and the rest of the planets out there get their start in the universe?
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.