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The word galaxy is derived from the Greek word galaktos, which means “milk”. During the summer months our own galaxy, the Milky Way, can be seen as a milky band that extends across the night sky.  The Milky Way galaxy consists of approximately 380 billion stars and is 100,000 light years in diameter.  It is part of a cluster of twenty galaxies known as the Local Group, and is a classified as a spiral galaxy.

Besides spiral, galaxies can be classified as elliptical or irregular. Like the name implies, a spiral galaxy consists of several arms that swirl around a dense center. Star formation takes place in the arms of the galaxy, and the younger stars are found in these arms. A galaxy picture of M74 illustrates this classic spiral configuration.  Spiral galaxies are labeled “S” and are further divided as “0”, “a”, “b” or “c” according to the arrangement of their spiral arms. Some spiral galaxies are further labeled “B” for barred galaxy. The center of these galaxies appears to have a bar running through it. Since spiral galaxies are composed of the youngest stars and are therefore the brightest of the galaxies, they are more often identified and studied than elliptical and irregular galaxies even thought there are far more elliptical galaxies in the universe.

Unlike spiral galaxies, elliptical galaxies consist of mainly older stars, contain much less gas and dust, and have no true structure. These galaxies are labeled E0, which are the most spherical, to E7, which are the flattest.  Irregular galaxies do not fit the two previous categories and vary in size and shape. Type I irregular galaxies are single galaxies that do not contain many complex elements and are very primitive. A galaxy picture of the Large Magellanic Cloud or a galaxy picture of the Small Magellanic Cloud exemplifies this model. Both the Magellanic Clouds are satellites of our own Milky Way and can be seen with the naked eye. Type II irregular galaxies are formed from two or more galaxies interacting, colliding or combining. A galaxy picture of the Whirlpool galaxy depicts a larger galaxy collecting a stream of gas from a much smaller galaxy. The Milky Way will eventually interact with the Small and Large Magellanic Clouds just in the same way that the Whirlpool galaxy is interacting with its satellite. The Milky Way is currently colliding with and ripping apart the satellite Sagittarius Dwarf.

Before these distinctions were possible, however, a “galaxy” had to be rightly defined as well as identified. Immanuel Kant in 1775 first correctly speculated that a galaxy is a large rotating body that contains billions of stars, which are held together by gravitational force. Prior to the 1920’s, galaxies, which appeared to be fuzzy patches in the night sky, were wrongly named nebulae, which are clouds of dust and particles that are interspersed throughout galaxies.  In 1920 Edwin Hubble, while studying the Andromeda “nebula”, discovered Cepheid variables, which led to the revelation that Andromeda was in fact a galaxy and not a nebula. A galaxy picture of Andromeda clearly illustrates its spiral structure. Hubble further advanced the field by formulating a diagram known as the Hubble Tuning Fork that was able to classify galaxies according to their appearance.

Besides Hubble’s tuning fork, there are two other systems for categorizing galaxies. While searching for comets in 1774, Charles Messier routinely came across fuzzy objects in the sky that seemed to be related to comets, but were intrinsically dissimilar. These Messier Objects, or galaxies, formed a database of galaxies labeled M1 to M110. Another catalogue is the New General Category or NGC. This naming system contains not only galaxies, but also other deep sky objects. It contains over 8,000 objects and was begun by Dreyer in 1880.

Digital-picture-printing-frames offers a variety of galaxy pictures to chose from. A galaxy picture of the Pinwheel galaxy, a galaxy picture of NGC 6822, or a galaxy picture of Stephan’s Quintet would be a grateful addition for any space enthusiast or future space explorer.

Charlsie Medellin, 2004