1<sect1 id="ai-ellipgal"> 2<sect1info> 3<author> 4<firstname>Jasem</firstname> 5<surname>Mutlaq</surname> 6<affiliation><address> 7</address></affiliation> 8</author> 9</sect1info> 10 11<title>Elliptical Galaxies</title> 12<indexterm><primary>Elliptical Galaxies</primary> 13</indexterm> 14 15<para>Elliptical galaxies are spheroidal concentrations of billions 16of stars that resemble Globular Clusters on a grand scale. They have 17very little internal structure; the density of stars declines smoothly 18from the concentrated center to the diffuse edge, and they can have a 19broad range of ellipticities (or aspect ratios). They typically 20contain very little interstellar gas and dust, and no young stellar 21populations (although there are exceptions to these rules). Edwin 22Hubble referred to Elliptical galaxies as <quote>early-type</quote> 23galaxies, because he thought that they evolved to become Spiral 24Galaxies (which he called <quote>late-type</quote> galaxies). 25Astronomers actually now believe the opposite is the case (&ie;, that 26Spiral galaxies can turn into Elliptical galaxies), but Hubble's 27early- and late-type labels are still used.</para> 28 29<para> 30Once thought to be a simple galaxy type, ellipticals are now known to 31be quite complex objects. Part of this complexity is due 32to their amazing history: ellipticals are thought to be the end 33product of the merger of two Spiral galaxies. You can 34view a computer simulation MPEG movie of such a merger at <ulink 35url="http://oposite.stsci.edu/pubinfo/pr/2002/11/vid/v0211d3.mpg"> 36this NASA HST webpage</ulink> (warning: the file is 3.4 MB). 37</para> 38 39<para> 40Elliptical galaxies span a very wide range of sizes and 41luminosities, from giant Ellipticals hundreds of thousands of light 42years across and nearly a trillion times brighter than the sun, to 43dwarf Ellipticals just a bit brighter than the average globular 44cluster. They are divided to several morphological classes: 45</para> 46 47<variablelist> 48<varlistentry> 49<term>cD galaxies:</term> 50<listitem><para> 51Immense and bright objects that can 52measure nearly 1 Megaparsec (3 million light years) across. These 53titans are only found near the centers of large, dense clusters of 54galaxies, and are likely the result of many galaxy 55mergers.</para></listitem> 56</varlistentry> 57 58<varlistentry> 59<term>Normal Elliptical galaxies</term> 60<listitem><para>Condensed Object with 61relatively high central surface brightness. They include the giant 62ellipticals (gE'e), intermediate-luminosity ellipticals (E's), and 63compact ellipticals.</para></listitem> 64</varlistentry> 65 66<varlistentry> 67<term>Dwarf elliptical galaxies (dE's)</term> 68<listitem><para>This class of 69galaxies is fundamentally different from normal ellipticals. Their 70diameters on the order of 1 to 10 kiloparsec with surface brightness 71that is much lower than normal ellipticals, giving them a much more 72diffuse appearance. They display the same characteristic gradual 73decline of star density from a relatively dense core out to a diffuse 74periphery.</para></listitem> 75</varlistentry> 76 77<varlistentry> 78<term>Dwarf spheroidal galaxies (dSph's)</term> 79<listitem><para>Extreme low-luminosity, low 80surface-brightness and have only been observed in the vicinity of the 81Milky Way, and possibly other very nearby galaxy groups, such as the 82Leo group. Their absolute magnitudes are only -8 to -15 mag. 83The Draco dwarf spheroidal galaxy has an absolute magnitude of -8.6, 84making it fainter than the average globular cluster in the Milky Way! 85</para></listitem> 86</varlistentry> 87 88<varlistentry> 89<term>Blue compact dwarf galaxies (BCD's)</term> 90<listitem> 91<para>Small galaxies that are unusually 92blue. Thehave photometric colors of B-V = 0.0 to 0.30 mag, which is 93typical for relatively young stars of <firstterm>spectral type</firstterm> A. 94This suggests that BCDs 95are currently actively forming stars. These systems also have 96abundant interstellar gas (unlike other Elliptical galaxies). 97</para></listitem> 98</varlistentry> 99</variablelist> 100 101<tip> 102<para> 103You can see examples of Elliptical galaxies in &kstars;, using the 104<guilabel>Find Object</guilabel> window 105(<keycombo action="simul">&Ctrl;<keycap>F</keycap></keycombo>). 106Search for NGC 4881, which is the Giant cD galaxy in the Coma 107cluster of galaxies. M 86 is a normal Elliptical galaxy in the Virgo 108cluster of galaxies. M 32 is a dwarf Elliptical that is a satellite 109of our neighbor, the Andromeda galaxy (M 31). M 110 is another 110satellite of M 31 that is a borderline dwarf spheroidal galaxy 111(<quote>borderline</quote> because it is somewhat brighter than most other 112dwarf spheroidals). 113</para> 114</tip> 115</sect1> 116