Galaxies are massive systems of gas and dust and innumerable stars. It is impossible for us to try to count all the galaxies in the universe. Our observable universe may contain as many as 100 billion of them, the farthest of them almost impossible to see with our largest telescopes.

Galaxies can be grouped into two major classifications: ellipticals and spirals. But even within these groups, there are different types. American astronomer Edwin Hubble (1889–1953) created a galaxy classification system, often referred to as “Hubble’s tuning fork.” Here, he divided the galaxies into the two groups. The elliptical galaxies were given numbers from 0 to 7, characterizing the ellipticity of the galaxy: E0 being almost round, and E7 being very elliptical. For spirals, however, he divided those into two groups: normal spirals and barred spirals. In the spiral galaxies, Hubble assigned letters to them — a to c — with the Sa spirals being tightly wound and the Sc being loosely wound. For the barred spirals, they have “B” in their classification. S0 galaxies, which are also called lenticular galaxies, lie in the transition between ellipticals and spirals.