What form is made when 100 billion stars come together all in one place and start orbiting each other like bees about a hive? One might imagine that such a large collection of stars can take on any random shape it wants.
Astronomers were a bit surprised to find out that there are only a small handful of galaxy molds, with nearly all of the galaxies falling into only three different three-dimensional shapes: elliptical, spiral, and irregular. The elliptical galaxies look like big bright footballs in the sky. They tend to be red in color and can come in a variety of different sizes. In the largest examples there can sometimes be found two or more dense clumps of stars near to their centers, from which we infer that at least some ellipticals form by colliding with one another in events called galaxy mergers. The spiral galaxy cousins are bluer in color, flatter and disk-like with new stars being born in the spiral arms. The spiral arms can be extended straight out, or even tightly wound about the center like an ice skater executing the final pirouette. Some of the time there are two prominent arms (the grand spiral), and at other times there can be many arms that appear to spring from one another more like willow branches (flocculent spiral). The Milky Way is the most famous example of a large spiral galaxy with many arms. The Earth and Sun live in one of the spiral arms situated about two-thirds of the way out from the center. Finally, at all distances from us, but especially as we look deeper into space, a great many irregular galaxies are found. The irregulars are generally small and tend to look bluish like the spirals. Unlike the ellipticals and spirals, the irregulars can take on any shape. We think these more amorphous shapes come about as a result of galaxy mergers. It is thought that irregulars might all have started out very small, and that they grow through a steady process of mergers to become the massive examples of galaxies we see today such as the Milky Way.