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+<sect1 id="ai-spiralgal">
+
+<sect1info>
+<author>
+<firstname>Mike</firstname>
+<surname>Choatie</surname>
+</author>
+</sect1info>
+
+<title>Spiral Galaxies</title>
+<indexterm><primary>Spiral Galaxies</primary>
+</indexterm>
+
+<para>
+Spiral galaxies are huge collections of billions of stars, most of
+which are flattened into a disk shape, with a bright, spherical bulge
+of stars at its center.  Within the disk, there are
+typically bright arms where the youngest, brightest stars are
+found.  These arms wind out from the center in a spiral pattern, giving
+the galaxies their name.  Spiral galaxies look a bit like hurricanes,
+or like water flowing down a drain.  They are some of the most beautiful
+objects in the sky.
+</para>
+<para>
+Galaxies are classified using a <quote>tuning fork diagram</quote>.
+The end of the fork classifies <link linkend="ai-ellipgal">elliptical
+galaxies</link> on a scale from the roundest, which is an E0, to
+those that appear most flattened, which is rated as E7.  The
+<quote>tines</quote> of the tuning fork are where the two types of
+spiral galaxies are classified: normal spirals, and
+<quote>barred</quote> spirals.  A barred spiral is one whose nuclear
+bulge is stretched out into a line, so it literally looks like it has
+a <quote>bar</quote> of stars in its center.
+</para><para>
+Both types of spiral galaxies are sub-classified according to the
+prominence of their central <quote>bulge</quote> of stars, their overall
+surface brightness, and how tightly their spiral arms are wound.  These
+characteristics are related, so that an Sa galaxy has a large central bulge,
+a high surface brightness, and tightly-wound spiral arms.  An Sb galaxy
+has a smaller bulge, a dimmer disk, and looser arms than an Sa, and so on
+through Sc and Sd.  Barred galaxies use the same classification scheme,
+indicated by types SBa, SBb, SBc, and SBd.
+</para><para>
+There is another class of galaxy called S0, which is morphologically a
+transitional type between true spirals and ellipticals.  Its spiral arms are
+so tightly wound as to be indistinguishable; S0 galaxies have disks with a
+uniform brightness.  They also have an extremely dominant bulge.
+</para><para>
+The Milky Way galaxy, which is home to earth and all of the stars in our
+sky, is a Spiral Galaxy, and is believed to be a barred spiral.  The name
+<quote>Milky Way</quote> refers to a band of very faint stars in the sky.
+This band is the result of looking in the plane of our galaxy's disk from
+our perspective inside it.
+</para><para>
+Spiral galaxies are very dynamic entities.  They are hotbeds of star
+formation, and contain many young stars in their disks.  Their central
+bulges tend to be made of older stars, and their diffuse halos are
+made of the very oldest stars in the Universe.  Star formation is active
+in the disks because that is where the gas and dust are most concentrated;
+gas and dust are the building blocks of star formation.
+</para><para>
+Modern telescopes have revealed that many Spiral galaxies harbor
+supermassive black holes at their centers, with masses that can exceed
+that of a billion Suns.  Both elliptical and spiral galaxies are known
+to contain these exotic objects; in fact many astronomers now believe
+that <emphasis>all</emphasis> large galaxies contain a supermassive
+black hole in their nucleus.  Our own Milky Way is known to harbor
+a black hole in its core with a mass millions of times bigger than a
+star's mass.
+</para>
+
+<tip>
+<para>
+There are many fine examples of spiral galaxies to be found in
+&kstars;, and many have beautiful images available in their
+<link linkend="popup-menu">popup menu</link>.  You can find them
+by using the <link linkend="findobjects">Find Object</link> window.
+Here is a list of some spiral galaxies with nice images available:
+<itemizedlist>
+<listitem><para>M 64, the Black-Eye Galaxy (type Sa)</para></listitem>
+<listitem><para>M 31, the Andromeda Galaxy (type Sb)</para></listitem>
+<listitem><para>M 81, Bode's Galaxy (type Sb)</para></listitem>
+<listitem><para>M 51, the Whirlpool Galaxy (type Sc)</para></listitem>
+<listitem><para>NGC 300 (type Sd) [use DSS image link]</para></listitem>
+<listitem><para>M 83 (type SBa)</para></listitem>
+<listitem><para>NGC 1530 (type SBb)</para></listitem>
+<listitem><para>NGC 1073 (type SBc)</para></listitem>
+</itemizedlist>
+</para>
+</tip>
+</sect1>