dipper@utastro.UUCP (Debbie Byrd) (10/29/86)
What the positions of stars reveal -- after this. October 29 Astrometry The stars are sometimes said to be "fixed." On the scale of human lifespans -- and at the level of our perceptual abilities -- they might as well be fixed. They don't appear to move relative to each other. But, in space, nothing ever STOPS moving. The stars in our Milky Way all orbit the center of the galaxy. And the galaxies themselves move -- no one knows exactly how. To get a handle on some of this movement through space, astronomers carefully study the positions of stars on the dome of the night sky. This study is called astrometry. Once you've got an accurate position for a star, then you can come back later to see whether the star has moved. If it has, then the star may be fairly nearby. Though human eyes can't detect it, it may show a "proper motion" among the other stars -- a sideways motion on the dome of night. Astrometry -- the study of stellar positions -- also lets astronomers find the distances to some stars. Some stars have a measurable "parallax." Their positions appear to shift from one season to the next -- caused by Earth's own movement around the sun. It's like holding a finger in front of your face -- closing one eye, then the other -- and seeing the finger shift relative to whatever is in the background. Likewise, nearby stars appear to shift in the course of a year -- and the amount of shift reveals their distance from Earth. Astrometry does still more. It shows that some stars are double -- even when they don't appear to be. Some stars "wiggle" around their positions in space -- because they're orbiting with another, unseen star. Because the companion star is revealed by astrometry, these stars are called "astrometric binaries." Script by Deborah Byrd. (c) Copyright 1985, 1986 McDonald Observatory, University of Texas at Austin