Spiral arms and density waves

The spiral arms are regions where stars are being formed. Here we find the hottest, youngest and brightest stars, and it is for this reason that the arms can be so visible. Along with fully formed stars, we find sites of stellar formation, with hot glowing clouds of gas and dust forming the "stellar nurseries" which we see as nebulae in our own galaxy.

If spiral arms are the areas where stars are formed, what mechanism forms the spiral arms ? A quick "thought experiment" shows that it isn't simply due to the rotation of the galaxy. If this was the case, the arms would wind up as the galaxy rotated, becoming tighter and tighter with time. Galaxy rotation periods are far shorter than the age of the universe (for example, our own Sun completes one "orbit" of the galatic centre every 240 million years), so we would expect to see the great majority of galaxies in this "wound-up" state. Yet, when we look out into the universe, we see galaxies with a whole range of spiral arm designs - some tight, and some loose. So it can't just be a simple case of a galaxy's rotation which causes the spiral shape. There must be some other cause. But what is it ?

This is a question which has occupied astronomers for many years. The most favoured theory is called the density wave theory. A good example of a density wave is very frequently observed on motorway journeys, where a traffic jam forms. We slow down and join the queue, moving forwards painfully slowly, and expecting to pass the obstruction on the road which has caused the hold-up. But instead, we see nothing unusual, and in a (hopefully !) short time, we've moved through the queue of vehicles and begin to speed up again. In reality, these hold-ups can be caused by very minor events - for instance, a car slowing down to turn off the motorway.

Now imagine what this same scene must look like from someone in a helicopter high above the road. Briefly looking down the length of the motorway, the observer would see cars travelling at similar speeds, and with some space inbetween each other. But at one point, the observer notices that the cars are moving very slowly and are close together, spending some time in this group before leaving it, speeding up and continuing on. So, whilst this "bunch" of cars - the queue - exists for some time, at any particular time it is made up of a different set of cars. This is a density wave.

In a spiral galaxy, it is believed that the density wave rotates slower than the material in the galactic disc, so that stars and gas are able to "overtake" the wave. In effect, the spiral pattern is not "frozen into" the stars, but instead it moves through them. As gas in the interstellar medium passes into the density wave, it becomes more dense, and (as discussed in the section on stellar evolution), this can lead to the formation of new stars. Now, the very hottest, (and brightest) stars have short lifetimes, so that they are born, live their lives, and die very close to the density wave. This is why the spiral arms are traced by the brightest stars. There are many faint stars, but few very bright ones, in the gaps between the arms, because these are areas which the density wave passed through long ago, and the bright "beacons" have had time to die. But in time to come, the wave will again revisit these regions, and star formation will occur there again.

Credit: University of Leicester, Department of Physics and Astronomy

http://www.star.le.ac.uk/edu/Spiral.shtml