Galaxies are perhaps one of the most attractive deep-sky objects to observe and photograph. They represent massive collections of dust, gas and countless billions of stars and are best described as islands of space matter across the universe. They are characterized as being either spiral or elliptical in nature and usually have a bright central core. Equally stunning to observe (with large-aperture telescopes) and image are galaxy clusters which represent collections of gravitationally-bound groups of galaxies whose members number from tens to thousands and which are classified on the basis of their richness (ie. number of galaxy members), shape (spherical, flat or irregular) and content (spiral or elliptical). Charles Messier's list of deep-sky celestial gems includes approximately forty bright and large galaxies.

Our own galaxy, the Milky Way, represents a galaxy within the Local Group (of Galaxies) numbering 30 galaxies in total, five million light years in diameter and which is moving in unison towards the constellation of Virgo. Other prominent galaxies within the Local Group include Andromeda (150,000 light-years in diameter), Triangulum (40,000 light-years in diameter) and the Large and Small Magellanic Clouds (30,000 and 20,000 light-years in diameter respectively). The center of the Local Group is believed to lie between the Milky Way and the Andromeda Galaxy. The richest member of our Local Group is the Virgo Cluster which is believed to contain approximately 2500 galaxies mostly elliptical in nature.

Note: One of the most puzzling and interesting galaxy formations involve a central galaxy core which is surrounded by seemingly empty space and all enclosed by a perfect circular ring and very similar to planetary nebulae. Although there is no firm concensus on how these galaxies are formed, it is believed that they are the end result of a collision between a compact galaxy and a larger spiral galaxy. A slightly different version of ring galaxies are the PRG's (polar-ring galaxies), studied and described in detail by Whitmore et al (AJ (100): 1489, 1990, New observations and a photographic atlas of polar-ring galaxies), which are three-dimensional photographic objects and in contrast to ring galaxies which are effectively two-dimensional photographic objects.

Note: For an interesting article on ring galaxies, see Astronomy Now (Jan/2006).