According to Edwin Hubble's classification system, the Milky Way is a spiral galaxy, although more recent mapping evidence indicates that it may be a barred spiral galaxy. The Milky Way has more than 200 billion stars (this was estimated from its mass -- see next page). It's approximately 100,000 light years in diameter, and the sun is located about 28,000 light years from the center. If we look at the structure of the Milky Way as it would appear from the outside, we can see the following parts:
- Galactic disk: This is where most of the Milky Way's stars are located. The disk is made of old and young stars, as well as vast amounts of gas and dust. Stars within the disk orbit the galactic center in roughly circular orbits. (Gravitational interactions between the stars cause the circular motions to have some up-and-down motion, like horses on a merry-go-round). The disk itself is broken up into these parts: Nucleus: The center of the disk Bulge: This is the area around the nucleus, including the immediate areas above and below the plane of the disk. Spiral arms: These areas 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 plane of the disk. Globular clusters orbit the galactic center in elliptical orbits in which the directions are randomly scattered. The stars in the globular clusters are much older stars than those in the galactic disk, and there's little or no gas and dust.
- Halo: This is a large, dim, region that surrounds the entire galaxy. The halo is 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.
The Milky Way's gravity acts on two smaller satellite galaxies called the Large and Small Magellanic Clouds (named after Ferdinand Magellan, the Portuguese explorer). They orbit below the plane of the Milky Way and are visible in the Southern Hemisphere. The Large Magellanic Cloud is about 70,000 light years in diameter and 160,000 light years away from the Milky Way. Astronomers think that the Milky Way is actually siphoning off gas and dust from these satellite galaxies as they orbit.
So, how many stars does the Milky Way actually contain? We'll show you the formula on the next page.