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Moon and Regulus
Every star in the night sky is zipping through the galaxy at hundreds of thousands of miles an hour. They're all so incredibly far away, though, that it would take many lifetimes for even a tiny change to become visible to the unaided eye. Yet with telescopes and sensitive detectors, astronomers can easily plot this motion.
An example is Regulus, the brightest star of Leo, the lion, which is to the lower left of the Moon at nightfall.
Astronomers measure two types of motion for Regulus and all the other stars.
One is proper motion -- the star's side-to-side motion across the sky. Regulus is fairly close as stars go, so its proper motion is pretty fast. It moves a distance equal to the diameter of the Moon in about 7,000 years. But it'll take tens of thousands of years for Regulus and Leo's other stars to move enough to make any noticeable change in the lion's outline.
The other motion is known as radial velocity, and it's the star's motion toward or away from us. Regulus is moving away at about 13,000 miles an hour.
Radial velocity can reveal other details about a star, too, such as whether it has unseen companion stars or planets. The companions exert a gravitational pull on the star, causing tiny changes in its radial velocity.
This technique shows that a small stellar corpse orbits close to Regulus. It's too faint to see through the glare of Regulus, but we know it's there -- thanks in part to Regulus's motion through space.
Script by Damond Benningfield, Copyright 2010