dipper@utastro.UUCP (Debbie Byrd) (06/14/85)
Ever wonder how astronomers find the distances to stars? We'll talk about some of the ways -- when we come back. June 14 Cosmic Distances Hold a single finger out in front of you. Blink first one eye, then the other -- and you'll see your finger appear to shift with respect to things in the background. Astronomers use a variation of this technique to measure the DISTANCES of nearby stars. They sight angles on stars from the longest baseline we can get -- on opposite sides of the Earth's orbit around the sun. This technique, called stellar parallax, works for the ten thousand or so stars within a few hundred light-years of Earth. For more distant stars, there are other ways to find distances. For example, variable stars called Cepheids change in brightness at a rate that depends on each star's true intrinsic brightness. If you figure out how bright a star really is, then you can see how bright it looks -- to find its distance. Individual cepheid variables can be seen out to a few million light-years from Earth. Beyond that, astronomers look to the BRIGHTEST stars in each distant galaxy -- the red and blue supergiants. They assume the brightest stars in distant galaxies are about as bright intrinsically as the brightest stars nearby. They again see how dim these faraway stars look -- to find their distances. This method works out to about 30 million light-years. And beyond that, astronomers can measure how bright a GALAXY looks -- and make educated guesses about its intrinsic brightness -- to estimate distances as far away as 100 million light-years! But the scale of the universe is vast. Tomorrow we'll talk about another way to measure cosmic distances. Script by David Slavsky. (c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin