A galaxy is a massive, gravitationally bound system that consists
stars, an interstellar medium of gas and dust, and dark matter. Typical galaxies range
dwarfs with as few as ten million stars up
giants with one trillion stars, all orbiting a common center
gravity. Galaxies can also contain a large number of multiple star systems and star clusters as well as various types of interstellar clouds. Historically, galaxies have been categorized according
their apparent shape. Interactions between nearby galaxies, which may ultimately result
a galaxy merger, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. There are probably more than a hundred billion galaxies
the observable universe. Most galaxies are a thousand
a hundred thousand parsecs
diameter and are usually separated
one another by distances
the order of millions of parsecs. Intergalactic space, the space between galaxies, is filled
a tenuous gas with an average density less than one atom
cubic meter. There is some evidence that supermassive black holes may exist at the center of many, if not all, galaxies. These massive objects are believed to be the primary cause of active galactic nuclei found
the core of some galaxies. The Milky Way galaxy appears to harbor
least one such object within its nucleus.
Source: http://en.wikipedia.org/wiki/Galaxy
