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From: dipper@utastro.UUCP (Debbie Byrd)
Newsgroups: net.astro
Subject: StarDate: October 12 The Halo of the Milky Way
Message-ID: <38@utastro.UUCP>
Date: Sat, 12-Oct-85 02:00:31 EDT
Article-I.D.: utastro.38
Posted: Sat Oct 12 02:00:31 1985
Date-Received: Mon, 14-Oct-85 05:38:27 EDT
Organization: U. Texas, Astronomy, Austin, TX
Lines: 36

Our Milky Way galaxy is surrounded by a halo.  More on what's in the
halo -- when we come back.

October 12  The Halo of the Milky Way

The Milky Way galaxy contains our sun -- and some hundred billion other
stars.  The galaxy is generally shaped like a pancake -- round and flat
-- with a great bulge of stars in its center.

The Milky Way also has a halo -- a thin scattering of stars around the
flat disk of the galaxy.  The halo of the Milky Way contains the
galaxy's oldest stars -- some traveling alone -- some in round, compact
globular star clusters.  Unlike the disk of the galaxy, the halo lacks
the gas and dust that are the raw materials for new stars.

It's thought that the halo of the Milky Way is a fossil remnant
leftover from the formation of the galaxy.  When the Milky Way formed
-- more than 10 billion years ago -- it probably started out as a more
or less round ball of gas, consisting mostly of hydrogen and helium.
The ball began to contract under the influence of its own gravity.  As
it shrank in volume, it fragmented into individual clouds from which
the first stars were born.

The stars that now make up the halo of the galaxy still lie in that
more or less round volume of space surrounding the flat disk of the
galaxy.  Meanwhile, the rest of the galaxy continued to contract -- to
collapse finally into a rotating disk.  Today, most of the galaxy --
including our sun -- lies within this rotating disk.  But the halo of
stars remains outside the disk -- a skeleton left behind from the early
evolution of the galaxy.


Script by Deborah Byrd.


(c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin