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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> 0010 0011 <title>Elliptical Galaxies</title> 0012 <indexterm><primary>Elliptical Galaxies</primary> 0013 </indexterm> 0014 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> 0028 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> 0038 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> 0046 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> 0057 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> 0065 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> 0076 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> 0087 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> 0100 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>