Milky Way

The nucleus and halo of our galaxy contain older stars from the first batch to form. The globular clusters in the halo are anywhere from 10 to 17 billion years old and are among the oldest objects in the galaxy. These older stars are called population II stars. The disk and spiral arms consist of younger, second to third generation stars (population I) as well as interstellar gas and dust. This difference in location between the older and younger stars in the Milky Way suggests something about the origin and evolution of the Milky Way. The older population II stars are distributed spherically in the halo, suggesting that when the galaxy first formed it had a spherical shape. The youngest stars are found in the flat disk, suggesting that the Milky Way has gradually flattened into a disk shape. Why? It is spinning. As the galaxy spins around, it flattens out.

But the history of the Milky Way may not be so simple. Detailed studies of the globular clusters in the halo and the different ages of stars in the halo and disk reveal some anomalies. For example, the contents of the halo do not always orbit in the same direction that the disk does. In addition, portions of the halo have very different ages. From this evidence, astronomers have concluded that the Milky Way may have formed as the result of a merger of smaller systems such as globular clusters or dwarf elliptical galaxies.