Spiral Galaxies

Spiral galaxies are flattened pinwheels in shape. They can have from one to three spiral arms coming from a central core. The Great Andromeda Spiral Galaxy is a good example of a spiral galaxy. The shape of the Milky Way is not visible from Earth, but astronomers have measured that the Milky Way is also a spiral galaxy. American astronomer Edwin Hubble further classified spiral galaxies by the tightness of their spirals. In order of increasingly open arms, Hubble’s types are Sa, Sb, and Sc.

Some galaxies have a straight, bright, bar-shaped feature across their center, with the spiral arms coming off the bar or off a ring around the bar. With a capital B for the bar, the Hubble types of these galaxies are SBa, SBb, and SBc.