What Characteristics Are Used to Classify Galaxies?

Elliptic Galaxies

• Elliptic galaxies are classified from E0 through E7 based on their degree of roundness, with E0 galaxies being completely round. Elliptic galaxies become more oval-shaped as their number increases. These are the most massive galaxies in the universe, a consequence of the merging of smaller galaxies. These types of galaxies contain mostly older stars with random orbits and little to no dust.

Lenticular Galaxies

• Lenticular galaxies define the forking point in the Hubble classification diagram. They are galaxies whose shapes fall between those of elliptic and spiral galaxies, and are often called armless-spiral galaxies. They possess a central disk and may have rings of star formation but do not have the arms of a true spiral galaxy. Depending on the observational point of view, they can appear to be spiral or elliptic. They are generally lacking in gas and dust and, like elliptic galaxies, often contain predominately older stars.

Spiral Galaxies

• A spiral galaxy consists of a central nucleus and arms that spiral out from its center. It usually has two or more arms and are further categorized into SB galaxies that contain a central bar and S galaxies for spirals with no bar. Classification also extends to consider the size of the nucleus and how tightly wound the arms are. The spiral arms contain gas and dust that promote star creation. Spiral galaxies have been shown to be rich in dark matter.

Irregular Galaxies

• Irregular galaxies defy classification into the other classes based on their irregular shape. The shape may be the result of interactions with other galaxies, either remnants of a collision between galaxies or a galaxy that is approaching another galaxy and being impacted by its gravitational pull.