Catch the Wave

The existence of spiral arms presents a puzzle. We know that the stars and other matter in the galactic disk orbit differentially—faster toward the center, slower toward the periphery. This rotational pattern will stretch any large clump of stars or gas in the disk of a spiral galaxy into a spiral structure relatively quickly.

So quickly, in fact, that if differential rotation were to account for spiral arms, the arms would soon get totally wrapped up around a galaxy's bulge and disappear. Somehow, the Milky Way and other spiral galaxies retain their spiral arm structure for long periods of time, long enough that they are plentiful in the universe.

Most astronomers are convinced that the arms of spiral galaxies are the result of compressions, called spiral density waves, moving through a galaxy's disk. These ripples in the pond of a galaxy's disk move around the galaxy center, compressing clouds of gas until they collapse and form stars. It is mostly the ionized gas surrounding hot, young stars (formed by the passage of a density wave) that we see as spiral arms.

The theory of spiral density waves neatly resolves the problem of the effect of differential rotation. As you may recall from our discussion of waves in Chapter 7, "Over the Rainbow," a wave is a disturbance that moves through matter, a ripple in the pond. Thus, spiral density waves move through the matter of a galactic disk and are not caught up in differential rotation. The stars that they form may then get stretched out by this differential rotation, but the wave keeps on moving.

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