Does the Galaxy have a spiral structure?
Where does such a structure come from?

Other galaxies have a spiral structure. For instance, here is M100 . Here is a closeup view of the center of M100. Here is M83, taken with a blue filter

What is going on?

One model

Doppler shift measurements show that, outside of the core, the stars in a galaxy are orbiting the center with the stars near the center going at about the same speed as the stars far from the center. (This observation has important implications, so we will return to it in some detail later.)

Therefore any structure will be ``wound up'' into a spiral structure. .

Is this where the spiral structure comes from? It could produce short arm segments seen in some rather disorganized looking disk galaxies. But it can't account for big spiral arms that (presumably) stay stable for billions of years.

Why not? Recall that the Sun orbits the Galaxy in about 0.2 x 109 years. So we go from picture 1 to picture 2 in just 0.2 x 109 years. In just a few billion years, the spiral has gotten so wound up that it has been obliterated.

A better model

There are waves going through the galaxy where the stars and gas are compressed compared to their normal density.

There is some support for this idea from dynamical calculations, but this is a pretty difficult subject.

The compression would be pretty much invisible, except that the compressed gas makes formation of big bright stars more likely.

By the time the wave has passed through, the big bright stars have died.

With this model, the spiral arms contain

Look again at galaxies have a spiral structure. For instance, here is M100 . Note the blue color ofthe arms. Here is a closeup view of the center of M100. The nucleus, with a high concentration of older stars, looks yellow, while the arms look blue because they have a high concentration of blue stars. A good way to see the spiral arms clearly in an optical photograph is to use a blue filter. Here is M83, taken with a blue filter

With H-alpha light, you can see HII regions. Here is M33, taken with a blue filter. This shows hot stars. Here is M33, taken with a H-alpha light

How about the Milky Way Galaxy

We can't see far enough in our galaxy with blue light to see O and B stars or HII regions in a spiral structure.

Studies looking at atomic hydrogen gas using 21 cm radiation from H atoms do see spiral structure.

Studies looking for molecular clouds using radio emission from CO molecules also see spiral structure.

However, this is a tricky business. There is no direct way to get the distance to a cloud that is emitting the radio waves that you detect. What astronomers do is to use the doppler shift of the spectral lines they are examining to tell how fast toward us or away from us the cloud is moving. If you know how fast things move as a function of the distance away from the center of the Galaxy, the velocity gives the distance. But you have to learn how fast things move as a function of the distance away from the center of the Galaxy from the same measurements. In my opinion, it's easier to learn about the structure of galaxies by looking at other galaxies.

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@bovine.uoregon.edu