1<sect1 id="ai-ellipgal">
2<sect1info>
3<author
4><firstname
5>Jasem</firstname
6> <surname
7>Mutlaq</surname
8> <affiliation
9><address>
10</address
11></affiliation>
12</author>
13</sect1info>
14
15<title
16>Elliptical Galaxies</title>
17<indexterm
18><primary
19>Elliptical Galaxies</primary>
20</indexterm>
21
22<para
23>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 centre 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
24>early-type</quote
25> galaxies, because he thought that they evolved to become Spiral Galaxies (which he called <quote
26>late-type</quote
27> 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>
28
29<para
30>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"
31> this NASA HST webpage</ulink
32> (warning: the file is 3.4 MB). </para>
33
34<para
35>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>
36
37<variablelist>
38<varlistentry>
39<term
40>cD galaxies:</term>
41<listitem
42><para
43>Immense and bright objects that can measure nearly 1 Megaparsec (3 million light years) across. These titans are only found near the centres of large, dense clusters of galaxies, and are likely the result of many galaxy mergers.</para
44></listitem>
45</varlistentry>
46
47<varlistentry>
48<term
49>Normal Elliptical galaxies</term>
50<listitem
51><para
52>Condensed Object with relatively high central surface brightness. They include the giant ellipticals (gE'e), intermediate-luminosity ellipticals (E's), and compact ellipticals.</para
53></listitem>
54</varlistentry>
55
56<varlistentry>
57<term
58>Dwarf elliptical galaxies (dE's)</term>
59<listitem
60><para
61>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
62></listitem>
63</varlistentry>
64
65<varlistentry>
66<term
67>Dwarf spheroidal galaxies (dSph's)</term>
68<listitem
69><para
70>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
71></listitem>
72</varlistentry>
73
74<varlistentry>
75<term
76>Blue compact dwarf galaxies (BCD's)</term>
77<listitem>
78<para
79>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
80>spectral type</firstterm
81> A. This suggests that BCDs are currently actively forming stars. These systems also have abundant interstellar gas (unlike other Elliptical galaxies). </para
82></listitem>
83</varlistentry>
84</variablelist>
85
86<tip>
87<para
88>You can see examples of Elliptical galaxies in &kstars;, using the Find Object window (<keycombo action="simul"
89>&Ctrl;<keycap
90>F</keycap
91></keycombo
92>). 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 neighbour, the Andromeda galaxy (M 31). M 110 is another satellite of M 31 that is a borderline dwarf spheroidal galaxy (<quote
93>borderline</quote
94> because it is somewhat brighter than most other dwarf spheroidals). </para>
95</tip>
96</sect1>
97