Know the four basic types of galaxies

Four main galaxy classifications exist. Elliptical galaxies have shapes that range from round to long and thin. They contain old stars, little gas, and few star-forming regions. Giant ellipticals are the largest galaxies, while dwarf ellipticals rank as the smallest.

Spiral galaxies look like pinwheels, or would if their orientations all were face-on to our point of view. Spirals contain a central bulge surrounded by a disk, and many spirals (such as the Milky Way) feature a central bar from which their arms originate.

Irregular galaxies generally appear featureless. They don’t fall into either elliptical or spiral categories. Many have star-forming regions.

Astronomers classify peculiar galaxies unusual in one or more ways such as brightness, amount of dust, or shape. Peculiar galaxies generally form because of gravitational interactions between galaxies.