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 :

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.

M87 in Virgo Cluster
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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.

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.

M100 in Virgo Cluster
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Magellanic Galaxy: Irregular Galaxy
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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.