IRAS map of M3I.[NASA]
It is actually quite surprising that we see any spiral structure in galaxies. The differential rotation of a galaxy should smear out any pattern on a time scale comparable to the orbital period. For the Sun, the orbit period is about 200 million years. We think that the age of the galaxy is about 10 billion years. This means that any initial spiral
Scenario for temporary spiral structure. (a) The dark area is the center of the galaxy, and the arrows indicate the orbital speeds of material at different distances from the center. (b) A large-scale burst of star formation takes place. (c) The differential rotation stretches the stars out, producing part of a spiral arm. (d) After a few rotations the arm is stretched out so much that we can no longer detect its presence.
pattern would have had ample time to smear out. Therefore, spiral arms must be temporary, or there must be a way of perpetuating them.
A scenario for temporary arm formation is shown in Fig. 17.15. For one reason or another, star formation starts in one region. It may even spread via some of the triggering mechanisms discussed in Chapter 15. The region with new stars is then stretched out by the differential rotation into a piece of a spiral pattern. This may explain why we see pieces of spiral features but no complete arms in our galaxy. In this scenario each 'arm' lives for a short time, and new ones are always forming.
If spiral patterns persist for many revolutions of a galaxy, then it may be that the pattern and the matter itself are moving at different speeds. At first this may seem strange, but we can use an analogy to see how it might work (Fig. 17.16). Suppose we have cars moving along a two-lane highway, and we are looking from above, in a traffic helicopter. A truck breaks down in the right lane, causing a traffic jam. If we look from above, we see cars backed up for some distance behind the truck. The density of cars is higher for this region. Far behind the truck, the cars are still moving at their normal speed, and after the cars squeeze past the truck, they will resume their normal speed. If we come back a few minutes later, we will see the same pattern of cars. However, the specific cars involved in the buildup will be different. The cars that we saw originally will be far down the road. In this case, the cars are moving along, but the pattern stays in the same place because the truck stays in the same place.
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