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Characteristics of Spiral Galaxies

Astronomers divide galaxies into three major classes: irregular, elliptical and spiral. Our own galaxy, the Milky Way, falls into the latter category. Spiral galaxies can be subdivided into barred spirals and pure spirals. According to a 2005 article in "New Scientist," the Milky Way is actually a barred spiral galaxy, rather than a pure spiral as was once believed. Regardless of their types, all spirals have certain features in common.

Shape
- A spiral galaxy's most obvious distinguishing characteristic is its shape. Spiral galaxies feature a pinwheel-shaped disk centered around a bright thick bulge in the middle. The bright spiral arms coil outwards from this inner nucleus. Spiral galaxies also sport a thin halo of stars and globular clusters. The central regions in spiral galaxies are believed to house supermassive black holes--the very heart and core of each galaxy.
Rotation
- Spiral galaxies all rotate, albeit very slowly. According to NASA, the Milky Way will make one rotation every 250 million years. As slow as this pace may seem, however, it's much faster than the rate of rotation exhibited by elliptical galaxies, many of which do not rotate at all.
Formation
- According to NASA's Worldbook, there's still some uncertainty over how spiral galaxies form and why they maintain their shape. The current hypothesis posits that the spiral arms are actually regions of high density, a little like sound waves traveling through air. As the compression wave travels through dust and gas, it creates the appearance of spiral arms of dust and gas, where increased rates of star formation occur.

Nature