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