Countless stars, dust and other heavenly bodies together form a galaxy. Our universe may contain billions of galaxies. Each galaxy may in turn consist of billions of stars arranged in particular manner. Galaxies vary in size and composition. They may be classified as- spiral galaxy, elliptical galaxy and irregular galaxy. Spiral galaxy is the most common type of galaxy. Our own Milky Way galaxy is a spiral galaxy with the sun being one among the billions of stars. The spiral galaxy consists of a spiral arm made up young blue and white stars. Moreover it is at the arm where new stars are formed. Hence the arm appears very bright. But the central bulge of the galaxy appears red due to the presence of old stars with very rare star formation. The entire mass of this galaxy rotate in a particular manner centered about its galactic center. Some of the galaxies appear to be ellipsoid forming the elliptical galaxy. This galaxy usually consists of old and red stars. The motion of this galaxy is usually radial. These galaxies form only a small part of the universe, dominated by spiral galaxies. Sometimes intergalactic collision may occur within a galaxy due to some gravitational effect. This results in galaxies with irregular and peculiar shapes forming the irregular galaxy.