A galaxy is a gravitationally bound collection of a large number of stars that appear to exhibit one of a number of basic shapes to an earthbound observer. These basic shapes are generally grouped into one of the following categories (Kaufman and Freedman, 1999; Smith, HREF 1999; Mitton 998):
As implied by the name, Elliptical Galaxies are ellipsoidal in shape. They are generally further classified by the extent of their eccentricity. They do not have spiral arms or a visible disk-like structure associated with the central ellipse. Generally, they appear to have little current active star formation. They are amongst the largest galaxies known in the Universe.
Spiral Galaxies are characterized by a number of spiral arms arranged in a flat disk. In the middle of the disk, is a large central bulge. Spiral galaxies are further classified by the size of the central bulge and the degree of tightness in the spiral arm winding. The spirals are not permanent fixed structures, but are thought to be density waves transmitted through the disk medium. As material in the wave is compressed, spirals appear to form as gas and other material is compressed resulting in areas of active star formation characterized by young hot O and B class stars. Older, cooler red stars populate the central bulge. Doppler measurements indicate that rate at which stars and other matter orbit the galactic centre is influenced by the presence of great quantities of matter that cannot be seen; the so-called dark matter that is believed to comprise some 90% of the known Universe.
Barred Spiral Galaxies are similar to spiral galaxies, except they are characterized by a single bar-like structure connecting spiral arms on opposite sides of the central bulge. As with spiral galaxies, they are further categorized based on the size of their central bulge and the tightness of arm windings. Our own Milky Way galaxy is believed to be a Barred Spiral. Whether a central bar develops in a spiral galaxy is currently thought to be determined by the amount of dark matter enshrouding the galaxy.
Lenticular Galaxies have a disk but no discernible spiral structure. Their name is derived from their lens-like appearance. Their appearance is generally described as being something between an elliptical and a spiral. Edwin Hubble, who attempted to classify galaxies using his now famous tuning fork diagram, once thought that Lenticular Galaxies represented an evolutionary transition from an elliptical to a spiral galaxy. We know now this not to be true, since measurements have shown that elliptical galaxies do not tend to rotate. On the other hand, spirals and barred spiral galaxies rotate. It is unlikely that a non-rotating elliptical would start spinning on its way to becoming a spiral or barred spiral, so this evolutionary theory has been abandoned.
Irregular Galaxies exhibit no visible elliptical or spiral structure, although many of them are known to have rotating disks of hydrogen gas.
Understanding the size, shape, mass and rotational characteristics is important in classifying galaxies, and provides clues regarding the course of galactic evolution and the presence of dark matter. While some basic questions like "what is dark matter" are open research issues, other less puzzling morphologic issues can be examined through basic observational science and mathematics. For example, some spiral galaxies appear to be round disks to an earthbound observer. Other spiral galaxies appear more elliptical in shape. This paper provides evidence that all spiral galaxies are round disks, and the elliptical shape observed in some of them is due only to the orientation of the disk relative to the observer.