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<sect1 id="ai-ellipgal">
<sect1info>
<author>
<firstname>Jasem</firstname>
<surname>Mutlaq</surname>
<affiliation><address>
</address></affiliation>
</author>
</sect1info>
<title>Elliptical Galaxies</title>
<indexterm><primary>Elliptical Galaxies</primary>
</indexterm>
<para> 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 center 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>early-type</quote>
galaxies, because he thought that they evolved to become Spiral
Galaxies (which he called <quote>late-type</quote> 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>
<para>
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">
this NASA HST webpage</ulink> (warning: the file is 3.4 MB).
</para>
<para>
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>
<variablelist>
<varlistentry>
<term>cD galaxies:</term>
<listitem><para>
Immense and bright objects that can
measure nearly 1 Megaparsec (3 million light years) across. These
titans are only found near the centers of large, dense clusters of
galaxies, and are likely the result of many galaxy
mergers.</para></listitem>
</varlistentry>
<varlistentry>
<term>Normal Elliptical galaxies</term>
<listitem><para>Condensed Object with
relatively high central surface brightness. They include the giant
ellipticals (gE'e), intermediate-luminosity ellipticals (E's), and
compact ellipticals.</para></listitem>
</varlistentry>
<varlistentry>
<term>Dwarf elliptical galaxies (dE's)</term>
<listitem><para> 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></listitem>
</varlistentry>
<varlistentry>
<term>Dwarf spheroidal galaxies (dSph's)</term>
<listitem><para>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></listitem>
</varlistentry>
<varlistentry>
<term>Blue compact dwarf galaxies (BCD's)</term>
<listitem>
<para> 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>spectral type</firstterm> A.
This suggests that BCDs
are currently actively forming stars. These systems also have
abundant interstellar gas (unlike other Elliptical galaxies).
</para></listitem>
</varlistentry>
</variablelist>
<tip>
<para>
You can see examples of Elliptical galaxies in &kstars;, using the Find
Object window
(<keycombo action="simul">&Ctrl;<keycap>F</keycap></keycombo>).
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 neighbor, the Andromeda galaxy (M 31). M 110 is another
satellite of M 31 that is a borderline dwarf spheroidal galaxy
(<quote>borderline</quote> because it is somewhat brighter than most other
dwarf spheroidals).
</para>
</tip>
</sect1>
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