A galaxy contains stars, gas and dust. In a spiral galaxy like the Milky Way, the stars, gas, and dust are organized into a "bulge," a "disk" containing "spiral arms," and a "halo." Elliptical galaxies have a bulge-like central region and a halo, but do not have a disk.
- The bulge is a round structure made primarily of old stars, gas, and dust. The outer parts of the bulge are difficult to distinguish from the halo. The bulge of the Milky Way is roughly 10,000 light years across.
- The disk is a flattened region that surrounds the bulge in a spiral galaxy. The disk is shaped like a pancake. The Milky Way's disk is 100,000 light years across and 1,000 light years thick. It contains mostly young stars, gas and dust, which are concentrated in spiral arms. Some old stars are also present.
- Spiral Arms
- The spiral arms are curved extensions that begin at the bulge of a spiral galaxy, giving it a "pinwheel" appearance. Spiral arms contain a lot of gas and dust as well as young blue stars. Spiral arms are found only in spiral galaxies.
- The halo primarily contains individual old stars and clusters of old stars ("globular clusters"). The halo also contains "dark matter," which is material that we cannot see but whose gravitational force can be measured. The Milky Way's halo may be over 130,000 light years across.
- Stars, gas, and dust
- Stars come in a variety of types. Blue stars, which are very hot, tend to have shorter lifetimes than red stars, which are cooler. Regions of galaxies where stars are currently forming are therefore bluer than regions where there has been no recent star formation. Spiral galaxies seem to have a lot of gas and dust, while elliptical galaxies have very little gas or dust.
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