Part IV

The Milky Way

Most of the light we can see from our galaxy appears as a narrow band around the sky. From its appearance, we think that we are in the plane of a disk, and that this disk looks something like the Andromeda galaxy. However, our location within our own galaxy makes its structure very difficult to study. In this part we will see both how we learn about our galaxy and what we have learned about it so far.

Most of the light that we see comes directly from stars. Among all the objects we can see, the stars provide most of the mass.Averaged over the whole galaxy, the gas and dust between the stars - the interstellar medium - contains only about 1% as much mass as the stars themselves. Of the interstellar medium, 99% of the mass is in the form of gas, and 1% of the mass is in the form of dust. However, this small amount of dust is very efficient at blocking light, making optical observations of distant objects difficult.

We expect that stars form out of interstellar material. Since most of the mass of the interstellar material is in the form of gas, it is the gas that will provide the gravitational attraction for the star formation process. In this part, we will first look at the contents of the interstellar medium.We will then look at how stars are born. Finally, we will see how the stars and interstellar medium are arranged in the galaxy as a whole.

Chapter 14

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