Introductory Astronomy: Spiral Structure

The most obvious identifier of spiral galaxies (except the S0 class!) is the presence of spiral arms which extend outward from the nucleus (or central bar) in a spiral pattern, like a pinwheel. These arms are the sites of active star formation in galaxies. They are associated mostly with gas (particularly large clouds of ionized hydrogen), dust, and young stars such as O and B stars. (All types of stars form in the spiral arms, but stars generally start to move away from the site of their formation as soon as they come into existence. O and B stars live such short lives that they do not have time to go very far and therefore must still be quite close to the spiral arms.)

What are the spiral arms? What causes them? Astronomers used to believe that the arms were material arms. (In other words, that the arms were patterns of dense regions of material that rotated around the galaxy together.) However, our galaxy (and other spiral galaxies) rotate differentially. This means that material closer to the center of the galaxy rotates faster than material farther away. If the spiral arms were matter arms, they would have wound up very tightly around the nucleus of our galaxy long ago. The fact that we see spiral arms in ours and so many other galaxies would be too big of a coincidence if the arms were matter arms that formed and disappeared (because they wound up) very quickly.

Astronomers now believe that the spiral arms are actually density waves. Density waves move through the stars and gas in the galaxy, compressing them briefly, and then moving on to compress the stars and gas in the next region of the galaxy. The stars and gas themselves do not move with the wave; they are simply compressed by it as it passes through their region. The compression causes the stars and gas to heat up, and triggers star formation. This is why we see ionized gas (hot gas) and young (newly formed) stars in the spiral arms.