By definition, galaxies are gravitationally bound aggregates of many (one million [106 ] to one trillion [1012 ]) stars, gas, dust, and dark matter. Astronomers estimate that there are at least 100 billion galaxies in the known universe. What is truly fascinating about galaxies, however, is the huge range of mass, size, composition, and brightness that they span. For example, the mass of a galaxy may be 10 million to a trillion times the mass of the Sun. In addition, with diameters ranging from 1,500-300,000 light-years, a galaxy may comprise a range of stars and interstellar material; old stars and little interstellar material to young stars and copious gas and dust. Finally, galaxies may have luminosity a million to a trillion times that of the Sun.

Every galaxy is associated with one of three distinct classes. These categories are: spiral (S), elliptical (E), and irregular (Irr). Even though...