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From: dipper@utastro.UUCP (Debbie Byrd)
Newsgroups: net.astro
Subject: StarDate: September 27 Comet Halley's Coma
Message-ID: <760@utastro.UUCP>
Date: Fri, 27-Sep-85 02:00:27 EDT
Article-I.D.: utastro.760
Posted: Fri Sep 27 02:00:27 1985
Date-Received: Tue, 1-Oct-85 10:47:31 EDT
Organization: U. Texas, Astronomy, Austin, TX
Lines: 36

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


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