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0001 <sect1 id="ai-spiralgal">
0003 <sect1info>
0004 <author>
0005 <firstname>Mike</firstname>
0006 <surname>Choatie</surname>
0007 </author>
0008 </sect1info>
0010 <title>Spiral Galaxies</title>
0011 <indexterm><primary>Spiral Galaxies</primary>
0012 </indexterm>
0014 <para>
0015 Spiral galaxies are huge collections of billions of stars, most of
0016 which are flattened into a disk shape, with a bright, spherical bulge
0017 of stars at its center.  Within the disk, there are
0018 typically bright arms where the youngest, brightest stars are
0019 found.  These arms wind out from the center in a spiral pattern, giving
0020 the galaxies their name.  Spiral galaxies look a bit like hurricanes,
0021 or like water flowing down a drain.  They are some of the most beautiful
0022 objects in the sky.
0023 </para>
0024 <para>
0025 Galaxies are classified using a <quote>tuning fork diagram</quote>.
0026 The end of the fork classifies <link linkend="ai-ellipgal">elliptical
0027 galaxies</link> on a scale from the roundest, which is an E0, to
0028 those that appear most flattened, which is rated as E7.  The
0029 <quote>tines</quote> of the tuning fork are where the two types of
0030 spiral galaxies are classified: normal spirals, and
0031 <quote>barred</quote> spirals.  A barred spiral is one whose nuclear
0032 bulge is stretched out into a line, so it literally looks like it has
0033 a <quote>bar</quote> of stars in its center.
0034 </para><para>
0035 Both types of spiral galaxies are sub-classified according to the
0036 prominence of their central <quote>bulge</quote> of stars, their overall
0037 surface brightness, and how tightly their spiral arms are wound.  These
0038 characteristics are related, so that an Sa galaxy has a large central bulge,
0039 a high surface brightness, and tightly-wound spiral arms.  An Sb galaxy
0040 has a smaller bulge, a dimmer disk, and looser arms than an Sa, and so on
0041 through Sc and Sd.  Barred galaxies use the same classification scheme,
0042 indicated by types SBa, SBb, SBc, and SBd.
0043 </para><para>
0044 There is another class of galaxy called S0, which is morphologically a
0045 transitional type between true spirals and ellipticals.  Its spiral arms are
0046 so tightly wound as to be indistinguishable; S0 galaxies have disks with a
0047 uniform brightness.  They also have an extremely dominant bulge.
0048 </para><para>
0049 The Milky Way galaxy, which is home to earth and all of the stars in our
0050 sky, is a Spiral Galaxy, and is believed to be a barred spiral.  The name
0051 <quote>Milky Way</quote> refers to a band of very faint stars in the sky.
0052 This band is the result of looking in the plane of our galaxy's disk from
0053 our perspective inside it.
0054 </para><para>
0055 Spiral galaxies are very dynamic entities.  They are hotbeds of star
0056 formation, and contain many young stars in their disks.  Their central
0057 bulges tend to be made of older stars, and their diffuse halos are
0058 made of the very oldest stars in the Universe.  Star formation is active
0059 in the disks because that is where the gas and dust are most concentrated;
0060 gas and dust are the building blocks of star formation.
0061 </para><para>
0062 Modern telescopes have revealed that many Spiral galaxies harbor
0063 supermassive black holes at their centers, with masses that can exceed
0064 that of a billion Suns.  Both elliptical and spiral galaxies are known
0065 to contain these exotic objects; in fact many astronomers now believe
0066 that <emphasis>all</emphasis> large galaxies contain a supermassive
0067 black hole in their nucleus.  Our own Milky Way is known to harbor
0068 a black hole in its core with a mass millions of times bigger than a
0069 star's mass.
0070 </para>
0072 <tip>
0073 <para>
0074 There are many fine examples of spiral galaxies to be found in
0075 &kstars;, and many have beautiful images available in their
0076 <link linkend="popup-menu">popup menu</link>.  You can find them
0077 by using the <link linkend="findobjects">Find Object</link> window.
0078 Here is a list of some spiral galaxies with nice images available:
0079 <itemizedlist>
0080 <listitem><para>M 64, the Black-Eye Galaxy (type Sa)</para></listitem>
0081 <listitem><para>M 31, the Andromeda Galaxy (type Sb)</para></listitem>
0082 <listitem><para>M 81, Bode's Galaxy (type Sb)</para></listitem>
0083 <listitem><para>M 51, the Whirlpool Galaxy (type Sc)</para></listitem>
0084 <listitem><para>NGC 300 (type Sd) [use DSS image link]</para></listitem>
0085 <listitem><para>M 83 (type SBa)</para></listitem>
0086 <listitem><para>NGC 1530 (type SBb)</para></listitem>
0087 <listitem><para>NGC 1073 (type SBc)</para></listitem>
0088 </itemizedlist>
0089 </para>
0090 </tip>
0091 </sect1>