Galaxies come in a variety of sizes and shapes. They can have as few as 10 million stars or as many as 10 trillion (the Milky Way has about 200 billion stars). In 1936, Edwin Hubble classified galaxy shapes in the Hubble Sequence.
- Elliptical: These have a faint, rounded shape, but they're devoid of gas and dust, with no visible bright stars or spiral patterns. They also don't have galactic disks, which we'll learn about below. Their classification varies from E0 (circular) to E7 (most elliptical). Elliptical galaxies probably comprise about 60 percent of the galaxies in the universe. They show wide variation in size -- most are small (about 1 percent the diameter of the Milky Way), but some are about five times larger than the diameter of the Milky Way.
- Spiral: The Milky Way is one of the larger spiral galaxies. They're bright and distinctly disk-shaped, with hot gas, dust and bright stars in the spiral arms. Because spiral galaxies are bright, they make up most of the visible galaxies, but they're thought to make up only about 20 percent of the galaxies in the universe. Spiral galaxies are subdivided into these categories: S0: Little gas and dust, with no bright spiral arms and few bright stars Normal Spiral: Obvious disk shape with bright centers and well-defined spiral arms. Sa galaxies have large nuclear bulges and tightly wound spiral arms, while Sc galaxies have small bulges and loosely wound arms. Barred Spiral: Obvious disk shape with elongated, bright centers and well-defined spiral arms. SBa galaxies have large nuclear bulges and tightly wound spiral arms, while SBc galaxies have small bulges and loosely wound arms (recent evidence suggests that the Milky Way is a SBc galaxy).
- Irregular: These are small, faint galaxies with large clouds of gas and dust, but no spiral arms or bright centers. Irregular galaxies contain a mixture of old and new stars and tend to be small, about 1 percent to 25 percent of the Milky Way's diameter.
What are the parts of a galaxy?
Spiral galaxies have the most complex structures. Here's a view of the Milky Way as it would appear from the outside.
- Galactic disk: Most of the Milky Way's more than 200 billion stars are located here. The disk itself is broken up into these parts: Nucleus: The center of the disk Bulge: The area around the nucleus, including the immediate areas above and below the plane of the disk Spiral arms: These extend outward from the center. Our solar system is located in one of the spiral arms of the Milky Way.
- Globular clusters: A few hundred of these are scattered above and below the disk. The stars here are much older than those in the galactic disk.
- Halo: A large, dim, region that surrounds the entire galaxy. It's made of hot gas and possibly dark matter.
All of these components orbit the nucleus and are held together by gravity. Because gravity depends upon mass, you might think that most of a galaxy's mass would lie in the galactic disk or near the center of the disk. However, by studying the rotation curves of the Milky Way and other galaxies, astronomers have concluded that most of the mass lies in the outer portions of the galaxy (like the halo), where there is little light given off from stars or gases.
On the next page, we'll take a walk through the history of galaxies.