Although all galaxies are composed of the same things, each one is unique.
Some galaxies are mostly made up of clouds of dust and gases.
Others contain more stars than clouds. Galaxies can appear flat or spherical.
Some are bright and clear; others are dim and obscure. Galaxies can stand alone or interact with other galaxies in clusters.
Perhaps most important of all, galaxies are not fixed. They not only travel as the universe expands, but they rotate on their axes and orbit other galaxies, as well.
Neighboring galaxies sometimes join each other; larger galaxies cannibalize, or consume, smaller ones.
Stars within galaxies are constantly changing as they are born, reach maturity, and die out.
In spite of all their differences, however, most galaxies share common characteristics and can be categorized by shape as one of three types: spiral, elliptical, or irregular.
Astronomer Edwin Hubble classified galaxies according to shape: elliptical, irregular, or spiral, spiral is further divided into normal or barred spirals.
The Milky Way is a spiral galaxy.