dipper@utastro.UUCP (Debbie Byrd) (09/27/85)
A comet's nucleus is hidden by a giant dust cloud called a coma. More on the coma of Comet Halley -- after this. September 27 Comet Halley's Coma Comets are frozen balls of ice and dust. A comet heats up as it comes near the sun -- and releases gases and dust particles from its icy nucleus, or core. These particles surround the nucleus. They create an enormous cloud around the comet called its coma. A comet's coma can stretch out for hundreds of thousands of miles. Meanwhile, the comet's inner nucleus may be only a few miles across. Since the nucleus of a comet can't be observed directly, astronomers look to the coma for clues to what the comet is made of. The light from the comet is separated by a spectrograph into a rainbow array of colors -- striped with a distinctive pattern of bands. The location of the bands on the array of colors indicate the existence of specific chemical elements and molecules. The molecules in the coma are not the original material of the comet's nucleus. They are daughter products -- formed as the ices interact with sunlight and boil off the nucleus. Astronomers plot the distribution of the daughter products within the coma, then check out the many possible chemical reactions that would have produced the daughter products. Last February astronomers found in the spectra of Comet Halley traces of the chemical compound cyanogen -- typically the first element noted in a developing coma. As Halley continues traveling sunward, its coma of dust and gases will grow -- effectively hiding the heart of the comet from our view. Script by Diana Hadley. (c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin