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0001 <sect1 id="ai-ellipgal">
0002 <sect1info>
0003 <author>
0004 <firstname>Jasem</firstname>
0005 <surname>Mutlaq</surname>
0006 <affiliation><address>
0007 </address></affiliation>
0008 </author>
0009 </sect1info>
0011 <title>Elliptical Galaxies</title>
0012 <indexterm><primary>Elliptical Galaxies</primary>
0013 </indexterm>
0015 <para>Elliptical galaxies are spheroidal concentrations of billions
0016 of stars that resemble Globular Clusters on a grand scale.  They have
0017 very little internal structure; the density of stars declines smoothly
0018 from the concentrated center to the diffuse edge, and they can have a
0019 broad range of ellipticities (or aspect ratios).  They typically
0020 contain very little interstellar gas and dust, and no young stellar
0021 populations (although there are exceptions to these rules).  Edwin
0022 Hubble referred to Elliptical galaxies as <quote>early-type</quote>
0023 galaxies, because he thought that they evolved to become Spiral
0024 Galaxies (which he called <quote>late-type</quote> galaxies).
0025 Astronomers actually now believe the opposite is the case (&ie;, that
0026 Spiral galaxies can turn into Elliptical galaxies), but Hubble's
0027 early- and late-type labels are still used.</para>
0029 <para>
0030 Once thought to be a simple galaxy type, ellipticals are now known to
0031 be quite complex objects.  Part of this complexity is due
0032 to their amazing history: ellipticals are thought to be the end
0033 product of the merger of two Spiral galaxies.  You can
0034 view a computer simulation MPEG movie of such a merger at <ulink
0035 url="http://oposite.stsci.edu/pubinfo/pr/2002/11/vid/v0211d3.mpg">
0036 this NASA HST webpage</ulink> (warning: the file is 3.4 MB).
0037 </para>
0039 <para>
0040 Elliptical galaxies span a very wide range of sizes and
0041 luminosities, from giant Ellipticals hundreds of thousands of light
0042 years across and nearly a trillion times brighter than the sun, to
0043 dwarf Ellipticals just a bit brighter than the average globular
0044 cluster.  They are divided to several morphological classes:
0045 </para>
0047 <variablelist>
0048 <varlistentry>
0049 <term>cD galaxies:</term>
0050 <listitem><para>
0051 Immense and bright objects that can
0052 measure nearly 1 Megaparsec (3 million light years) across.  These
0053 titans are only found near the centers of large, dense clusters of
0054 galaxies, and are likely the result of many galaxy
0055 mergers.</para></listitem>
0056 </varlistentry>
0058 <varlistentry>
0059 <term>Normal Elliptical galaxies</term>
0060 <listitem><para>Condensed Object with
0061 relatively high central surface brightness. They include the giant
0062 ellipticals (gE'e), intermediate-luminosity ellipticals (E's), and
0063 compact ellipticals.</para></listitem>
0064 </varlistentry>
0066 <varlistentry>
0067 <term>Dwarf elliptical galaxies (dE's)</term>
0068 <listitem><para>This class of
0069 galaxies is fundamentally different from normal ellipticals. Their
0070 diameters on the order of 1 to 10 kiloparsec with surface brightness
0071 that is much lower than normal ellipticals, giving them a much more
0072 diffuse appearance. They display the same characteristic gradual
0073 decline of star density from a relatively dense core out to a diffuse
0074 periphery.</para></listitem>
0075 </varlistentry>
0077 <varlistentry>
0078 <term>Dwarf spheroidal galaxies (dSph's)</term>
0079 <listitem><para>Extreme low-luminosity, low
0080 surface-brightness and have only been observed in the vicinity of the
0081 Milky Way, and possibly other very nearby galaxy groups, such as the
0082 Leo group.  Their absolute magnitudes are only -8 to -15 mag.
0083 The Draco dwarf spheroidal galaxy has an absolute magnitude of -8.6,
0084 making it fainter than the average globular cluster in the Milky Way!
0085 </para></listitem>
0086 </varlistentry>
0088 <varlistentry>
0089 <term>Blue compact dwarf galaxies (BCD's)</term>
0090 <listitem>
0091 <para>Small galaxies that are unusually
0092 blue. Thehave photometric colors of B-V = 0.0 to 0.30 mag, which is
0093 typical for relatively young stars of <firstterm>spectral type</firstterm> A.
0094 This suggests that BCDs
0095 are currently actively forming stars.  These systems also have
0096 abundant interstellar gas (unlike other Elliptical galaxies).
0097 </para></listitem>
0098 </varlistentry>
0099 </variablelist>
0101 <tip>
0102 <para>
0103 You can see examples of Elliptical galaxies in &kstars;, using the
0104 <guilabel>Find Object</guilabel> window
0105 (<keycombo action="simul">&Ctrl;<keycap>F</keycap></keycombo>).
0106 Search for NGC 4881, which is the Giant cD galaxy in the Coma
0107 cluster of galaxies.  M 86 is a normal Elliptical galaxy in the Virgo
0108 cluster of galaxies.  M 32 is a dwarf Elliptical that is a satellite
0109 of our neighbor, the Andromeda galaxy (M 31).  M 110 is another
0110 satellite of M 31 that is a borderline dwarf spheroidal galaxy
0111 (<quote>borderline</quote> because it is somewhat brighter than most other
0112 dwarf spheroidals).
0113 </para>
0114 </tip>
0115 </sect1>