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 <sect1 id="ai-ellipgal">
 <sect1info>
 <author>
 <firstname>Jasem</firstname>
 <surname>Mutlaq</surname>
 <affiliation><address>
 </address></affiliation>
 </author>
 </sect1info>
 
 <title>Elliptical Galaxies</title>
 <indexterm><primary>Elliptical Galaxies</primary>
 </indexterm>
 
 <para>Elliptical galaxies are spheroidal concentrations of billions
 of stars that resemble Globular Clusters on a grand scale.  They have
 very little internal structure; the density of stars declines smoothly
 from the concentrated center to the diffuse edge, and they can have a
 broad range of ellipticities (or aspect ratios).  They typically
 contain very little interstellar gas and dust, and no young stellar
 populations (although there are exceptions to these rules).  Edwin
 Hubble referred to Elliptical galaxies as <quote>early-type</quote>
 galaxies, because he thought that they evolved to become Spiral
 Galaxies (which he called <quote>late-type</quote> galaxies).
 Astronomers actually now believe the opposite is the case (&ie;, that
 Spiral galaxies can turn into Elliptical galaxies), but Hubble's
 early- and late-type labels are still used.</para>
 
 <para>
 Once thought to be a simple galaxy type, ellipticals are now known to
 be quite complex objects.  Part of this complexity is due
 to their amazing history: ellipticals are thought to be the end
 product of the merger of two Spiral galaxies.  You can
 view a computer simulation MPEG movie of such a merger at <ulink
 url="http://oposite.stsci.edu/pubinfo/pr/2002/11/vid/v0211d3.mpg">
 this NASA HST webpage</ulink> (warning: the file is 3.4 MB).
 </para>
 
 <para>
 Elliptical galaxies span a very wide range of sizes and
 luminosities, from giant Ellipticals hundreds of thousands of light
 years across and nearly a trillion times brighter than the sun, to
 dwarf Ellipticals just a bit brighter than the average globular
 cluster.  They are divided to several morphological classes:
 </para>
 
 <variablelist>
 <varlistentry>
 <term>cD galaxies:</term>
 <listitem><para>
 Immense and bright objects that can
 measure nearly 1 Megaparsec (3 million light years) across.  These
 titans are only found near the centers of large, dense clusters of
 galaxies, and are likely the result of many galaxy
 mergers.</para></listitem>
 </varlistentry>
 
 <varlistentry>
 <term>Normal Elliptical galaxies</term>
 <listitem><para>Condensed Object with
 relatively high central surface brightness. They include the giant
 ellipticals (gE'e), intermediate-luminosity ellipticals (E's), and
 compact ellipticals.</para></listitem>
 </varlistentry>
 
 <varlistentry>
 <term>Dwarf elliptical galaxies (dE's)</term>
 <listitem><para>This class of
 galaxies is fundamentally different from normal ellipticals. Their
 diameters on the order of 1 to 10 kiloparsec with surface brightness
 that is much lower than normal ellipticals, giving them a much more
 diffuse appearance. They display the same characteristic gradual
 decline of star density from a relatively dense core out to a diffuse
 periphery.</para></listitem>
 </varlistentry>
 
 <varlistentry>
 <term>Dwarf spheroidal galaxies (dSph's)</term>
 <listitem><para>Extreme low-luminosity, low
 surface-brightness and have only been observed in the vicinity of the
 Milky Way, and possibly other very nearby galaxy groups, such as the
 Leo group.  Their absolute magnitudes are only -8 to -15 mag.
 The Draco dwarf spheroidal galaxy has an absolute magnitude of -8.6,
 making it fainter than the average globular cluster in the Milky Way!
 </para></listitem>
 </varlistentry>
 
 <varlistentry>
 <term>Blue compact dwarf galaxies (BCD's)</term>
 <listitem>
 <para>Small galaxies that are unusually
 blue. Thehave photometric colors of B-V = 0.0 to 0.30 mag, which is
 typical for relatively young stars of <firstterm>spectral type</firstterm> A.
 This suggests that BCDs
 are currently actively forming stars.  These systems also have
 abundant interstellar gas (unlike other Elliptical galaxies).
 </para></listitem>
 </varlistentry>
 </variablelist>
 
 <tip>
 <para>
 You can see examples of Elliptical galaxies in &kstars;, using the
 <guilabel>Find Object</guilabel> window
 (<keycombo action="simul">&Ctrl;<keycap>F</keycap></keycombo>).
 Search for NGC 4881, which is the Giant cD galaxy in the Coma
 cluster of galaxies.  M 86 is a normal Elliptical galaxy in the Virgo
 cluster of galaxies.  M 32 is a dwarf Elliptical that is a satellite
 of our neighbor, the Andromeda galaxy (M 31).  M 110 is another
 satellite of M 31 that is a borderline dwarf spheroidal galaxy
 (<quote>borderline</quote> because it is somewhat brighter than most other
 dwarf spheroidals).
 </para>
 </tip>
 </sect1>