Reading: "Spiral Structure in Galaxies" by Chris Impey

When most people think of galaxies, they think of spiral galaxies like our own Milky Way galaxy and the nearby Andromeda galaxy. In reality, only about 30% of all galaxies contain spiral structure. Spiral galaxies come in many different varieties. Broadly they are binned into grand design spirals, which have a set number of clearly defined arms, and flocculent spiral galaxies, which often resemble the hearts of flowers, with many small "pieces" of spiral arms overlaid on top of one another. Trying to understand how these structures arise has been an ongoing problem for theoretical astronomers. Using modern super computers, simulations for two different scenarios have been able to roughly replicate what is seen in the sky.

The more widely accepted theory for spiral arms is the spiral density wave model. In this theory, galaxies form with significant angular momentum in a single direction (and overall bulk rotation in one direction). They also start out with areas that have more mass than others. As the entire system rotates, the volumes with more material are able to gravitationally grow, while the volumes between these over densities empty out. As any one galaxy rotates, differential rotation causes its inner parts to complete one orbit faster than the outer parts of the galaxy. These difference in orbital time causes the material to take on a spiral structure. In spiral galaxies, the arms actually orbit far slower than the material that at any one point in time makes up the arms. This is the same as a traffic jam that may migrate down the high way at a rate slower than any one car that must temporarily get delayed by the traffic jam. Looked at from the perspective a single orbiting star, the star will be accelerated as it nears an arm - a region of extra material with extra gravitational pull. It will then pass through the material but be slowed down as it leaves, as the material in the arm slows it on its orbit. This acceleration into the arm and deceleration out of the arm causes the star to linger longer in the region of the arm than it will linger in other parts of its orbit. By creating an area where material lingers, the spiral density wave can create a standing over density that can be maintained over long periods. Models based on this theory are able to replicate actual galaxy shapes. In addition to replicating galaxies well, this model also explains why star formation primarily takes place in the arms. When clouds of gas enter the spiral arms, they are compressed and this triggers collapse and star formation. To create grand design spirals, systems with perfect pinwheel arms, the spiral density waves have to be enhanced by the effects of an outside force, such as a gravitational interaction with a smaller galaxy. 

While the spiral density wave model is certainly the more widely discussed and accepted model, it is not the only model for creating spiral arms. The competing model, a Stochastic Star Formation model, postulates that spiral arms are driven by waves of star formation (and related waves of supernovae) triggering waves of new star formation that propagate around a galactic disk. Think of it this way, pockets of gas and dust collapse into stars. Over the lifetime of the most massive stars in this system, the cloud of material is twisting into the beginnings of a spiral structure by differential rotation. The star formation leaves behind a region of less dense material behind the region of star formation. As the largest newly formed stars are carried forward and begin to explode, their shock waves trigger a new round of star formation along the leading edge of the spiraling star formation region. While this model can neatly explain small, and particularly irregular systems, such as the Large Magellanic Cloud, it is implausible to believe that this technique can create large grand design spirals.