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There are three basic types of galaxies: spirals, ellipticals, and irregulars. The are classified based on the visual appearance of the bulge (round distribution of stars at the center) and the disk (the flat distribution that includes the spiral arms). Ellipticals are all bulge and not disk, irregulars lack symmetry and do not have a clear bulge or disk. Spirals have a clear bulge, disk, and at least two ‘arms’ which are areas of new star formation(usually hot OB stars). This is not to say that there are not further divisions of galaxies, but that these are the three mos encompassing categories.
Elliptical galaxies are somewhat shaped like an American football and can contain trillions of stars. Many scientists think that they are the result of two galaxies colliding. Most elliptical galaxies are composed of old, low mass stars with a sparse interstellar medium and very little new star formation. Ellipticals are surrounded by large numbers of globular clusters. They are thought to make up 10–15% of galaxies in the local universe are commonly found near the center of galaxy clusters.
Irregular galaxies does not have a distinct regular shape. The shape of an irregular galaxy does not fall into any of the regular classes of the Hubble sequence and is often chaotic in appearance, lacking a bulge, disk, or arms. This type is thought to make up 25% of all galaxies. Most irregulars were once spiral or elliptical galaxies that were deformed by distortions in gravitational pull.
Spiral galaxies make up about 35% of the galaxies in the known universe. They consist of a flat, rotating disk that contains stars, gas, and dust along with a bulge full of stars. The bulge and disk are surrounded by a halo. They are named from the spiral appearing arms jutting from the center into the disk. Nearly half of the spiral galaxies have an additional component: a bar-like structure, extending from the central bulge where the spiral arms begin.
You can see hints of further classification within these types of galaxies. There are at least ten further categories of galaxies to look into. Good luck, and enjoy your research.
We’ve also recorded an episode of Astronomy Cast about galaxies. Listen here, Episode 97: Galaxies.