dipper@utastro.UUCP (Debbie Byrd) (07/15/85)
Our Milky Way galaxy has the shape of a pinwheel. We talk more about why -- in just a minute. July 15 The Illusion of the Pinwheel The galaxy we live in is called the Milky Way. It's a collection of billions of stars, the closest one of which is our sun. You've probably seen artists' drawings of our galaxy. From the outside, the Milky Way is thought to look like a gigantic pinwheel, with a bright center and long, winding spiral arms. The center and the arms of the galaxy are made of stars. But these winding spiral arms are in part an illusion. The "gaps" between the arms aren't really empty. The spiral arms are thought to outline a shock wave that travels around the center of the galaxy. This shock wave is thought to set off the formation of new stars. As it moves around the galaxy, it encounters huge clouds of interstellar gas and dust -- the raw materials for new stars. The shock wave pushes on the clouds and helps them start collapsing to form stars. The clouds give birth to many stars -- some very massive and bright. These bright stars let us see the outline of the shock wave in the form of a spiral arm. The bright stars eventually die, with the brightest ones burning their fuel in a rush and dying first. Where they've died, we see the "gaps" between the spiral arms. But these gaps really contain lots of fainter stars, as well as debris leftover from the stars that once lived and then died. The debris gradually comes together into new clouds. These clouds are there when the shock wave passes through again -- to spawn the next generation of bright young stars. Script by Deborah Byrd. (c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin