Galaxies
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TYPES OF GALAXIES:
Astronomers had no knowledge about the shape
and size of Galaxies. Most of the work of classification and identification
of galaxies was done by Edwin Hubble - an American Astronomer in
the year 1920. Hubble classified the Galaxies
into four general categories which in turn are further subdivided
into classes.
The four major classes of Galaxies are as
follows :
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ELLIPTICAL GALAXIES:
Elliptical Galaxies are
colossal collections of stars arranges in a spheroid shape. Many
of the galaxies found until now are of this type. Elliptical Galaxies
have noticeably less dust, gas and interstellar material. This indicates
that they are older than the Spiral Galaxies. These galaxies have
a wider range in size and can be as small as 5000 light years in
diameter with their brightness-hardly one million times as bright
as the Sun.
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![M87 : Elliptical Galaxy](images/Galaxies/m87-elliptical.gif)
M87
in Virgo Cluster
Source: Goto
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LENTICULAR GALAXIES:
Lenticular Galaxies are disc-like structures
without arms but with a pronounced center and extended area around
the center.
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SPIRAL GALAXIES:
Spiral Galaxies are considerably different
from the Elliptical Galaxies in structure and in their stellar content.
The nucleus of these galaxies resemble an Elliptical galaxy but
the Spiral Galaxies possess spiral arms. A spiral arm is a concentration
of stars, dust and gas. The spiral arms rotate above the galactic
nucleus in a pin wheel fashion taking hundreds of millions of years
for one rotation. Stars near the central region go around the center
more rapidly than those in the outer regions. Spiral Galaxies range
in size from about 20,000 to more than 100,000 light years in diameter.
Milky Way is a Spiral Galaxy.
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![M100 : Spiral Galaxy](images/Galaxies/m100-spiral.gif)
M100
in Virgo Cluster
Source: Goto
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![Magellanic Galaxy: Irregular Galaxy](images/Galaxies/Lmagellanic-irregular.gif)
Magellanic
Galaxy: Irregular Galaxy
Source: Goto Bottom of the Page
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IRREGULAR GALAXIES:
Irregular galaxies are least common. Irregular
Galaxies are quite different from both the
Elliptical and Spiral Galaxies. These show no trace of either circular
or rotational symmetry but instead have an irregular or chaotic
appearance. The best known are the Large and the Small Magellanic
Clouds which are the satellites of Milky Way Galaxy.
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