Introductory Astronomy: The Milky Way Galaxy

The Milky Way Galaxy (MW)is our home galaxy. To quote a popular super hero: "It's where we keep all our Stuff."(The Tick, Fox Network, 1995). The MW is about 30,000 parsecs in diameter (over 100,000 light years) and contains over 100 billion stars. It has a mass of about 10^9 solar masses. It is a spiral galaxy, classed as an Sbcor SBbc, which means that it is intermediate between an Sc and an Sb galaxy (and that we aren't completely certain whether it has a central bar or not).

Like all spiral galaxies, the MW has three main components: a disk, a central bulge, and a large halo. In the picture below, the disk is seen edge on, the bulge is the spherical white blob in the center, and the halo is made of the Globular Clusters (seen as white dots) filling the rest of the picture.

The Sun is located about 8,500 parsecs from the center, roughly in the center of the disk. On a clear night, we can see the bright central regions of the galaxy as a bright streak across the sky. This is what we normally call the Milky Way. It is important to remember that we are a part of that bright streak! In fact, almost every object we can see with the naked eye is a part of the MW. (The only exceptions, the Magellanic clouds and the Andromeda galaxy, are members of the Local Group. The Local Group is the cluster of galaxies to which we belong.)

To Learn more about the separate components of the MW, click on the appropriate topic:

Formation of the MW Galaxy

There are many theories on how the MW galaxy formed. Some astronomers believe that the halo formed first. In this theory, as gravity pulled the spherical halo material inward, the material formed a disk to conserve its angular momentum. During the collapse, stars in the halo continued to evolve, producing metals (elements other than hydrogen and helium) through the process of fusion. These metals were spewed into the galactic medium through stellar winds and supernova explosions and became part of the disk. This means that the stars in the disk formed out of metal rich material. This process is known as the "Outside-In" theory.

Other scientists believe in the "Inside-Out" theory. In this version, the disk and bulge formed first from a small cloud of material. The halo formed later as other small clouds were attracted to the galaxy by gravity and became bound to it.

It is important to know how the galaxy formed, what it is made of, and where we fit into it. It is, after all, our home.

After you review these sections, try a few sample questions to test your understanding. These questions are typical of questions given in introductory astronomy course exams. They are meant only to give you an idea of what kinds of questions MIGHT be on your exam. Just because these questions are here does NOT mean that you will have questions like them on your exam, NOR does it even mean that you will have questions on these topics on your exam. They are just PRACTICE questions!