Article Posted on : - Sep 04, 2009

The Andromeda galaxy M31.

A galaxy is a system of stars, dust, and gas held together by gravity. Scientists estimate that there are over 100 billion galaxies scattered throughout the visible Universe. Astronomers have photographed millions of them through telescopes. The most distant galaxies ever photographed are as far as 10 billion to 13 billion light-years away.

Galaxies range in diameter from a few thousand to a half-million light-years. Small galaxies have fewer than a billion stars. Large galaxies have more than a trillion.

Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology). Astronomers classify most galaxies by shape as either spiral or elliptical galaxies.

Spiral galaxies are disk-shaped assemblages with curving, dusty arms and bulge at the centre. They consist of a flat, rotating disk containing stars, gas and dust, and the bulge contains a concentration of stars.

These are surrounded by a much fainter halo of stars, many of which reside in globular clusters. Spiral galaxies are named for the (usually two-armed) spiral structures that extend from the centre into the disk.

An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flattened and in size from hundreds of millions to over one trillion stars.

Elliptical galaxies range in shape from almost perfect spheres to flattened globes. The light from an elliptical galaxy is brightest in the centre and gradually becomes fainter toward its outer regions. As far as astronomers can determine, elliptical galaxies rotate much more slowly than spiral galaxies or not at all.

Most elliptical galaxies are composed of older, low-mass stars, with a sparse interstellar medium and minimal star formation activity. They are surrounded by large numbers of globular clusters.    -- Courtesy: NASA/ Wikipedia