Pedagoguery
One of the classic sights of astronomy is the spiral galaxy. Most galaxies that we observe have this spiral structure; most with a central bar, some without. Our own galaxy is a barred spiral, although that wasn't confirmed until recently. How does the spiral structure form, and how does it persist?
Spiral galaxies are rotating structures, but they do not rotate rigidly like a solid disc. Instead, they are made up of myriad stars and gas clouds, each in its own orbit around the center of the galaxy. In addition, the orbits are not like the orbits of the planets around the sun. In that case, the bulk of the matter in the solar system is the sun, whereas the central black hole of a galaxy is tiny compared to the mass of the galaxy itself. As a result, stellar orbits are messy things. They are elliptical, but the ellipse does not close in on itself. Instead, the trace of a star's orbit around the center of the galaxy more resembles a Spirograph trace; an ellipse with the semimajor axis twisting around. As an example, the Sun orbits the galaxy about once every 230 million years, but in a single orbit, the orbit gets twisted around by 105 degrees. As a result, the Sun's orbit makes a full rotation every 790 million years. It is this fact about stellar orbits that is central to spiral formation.
Spirals in galaxies are density waves. In some places, there are more stars than average, and in some places there are fewer. The density wave arises when stellar orbits synchronize with each other. If the orbits align with each other, a bar wave results, with the area of greatest density along the semimajor axis of the ellipse. If the orbits are progressively offset from each other, a spiral results, with the area of greatest density where the ellipses crowd together. The density wave tends to be self-perpetuating. Since there are more stars in the area of highest density, stars are attracted to the area. The wave itself travels around the galaxy at a set speed, and there are three circles that are important in determining the structure and evolution of the spiral. The first is the corotation circle. This is the point at which a star orbiting the galaxy would travel at the same speed as the spiral wave. Stars inside the corotation circle will catch up with the spiral, slow down in the spiral, and finally exit the spiral, in much the same way as a motorist on a freeway approaching an area of major congestion. Stars outside the corotation circle will get “rear ended” by the spiral wave, which will linger around the star, and eventually pass it. The other two are inside and outside of this circle and are called Lindblad resonances. They are the circles at which the rotation rate of a star's orbit matches the speed of the spiral wave. These circles tend to demarcate the boundaries of the spiral itself.
All of this was good in theory, but closer examination of the dynamics indicated that the spiral wave should eventually lose energy and dissipate. A number of alternatives were suggested, most involving reflection and amplification of the wave at the corotation circle and the Lindblad resonances, but they tended to make the spiral structure dissipate even sooner. Finally, the crucial link was discovered: gas. Gas clouds behave differently within the spiral structure than stars. First of all, they are much bigger. Secondly, they tend to collide, while stars do not. As a result, they will tend to lose energy due to shock and radiation, and slowly spiral inward. If the galaxy has a bar, then an additional dynamic takes hold. The bar exerts a torque on the gas clouds. Clouds outside the corotation circle rotate slower than the bar, so the gravity of the bar drags them forward, adding orbital energy and causing them to spiral outwards. Gas clouds inside the corotation circle rotates faster than the stars in the bar. As a result, the rotation of the clouds tends to get held back by the gravity of the bar, and the cloud spirals inward. Once such a cloud reaches the outer edge of the bar, it will pile up there in a ring where a great deal of star formation happens.
The combined dynamics of the bar, stars, and gas clouds has an interesting result. During the period where the bar exists, some gas clouds are drawn toward the center, but most are kept away. However the star formation just outside the bar leads to the bar's undoing as the additional stars cause the orbits to become desynchronized. With the bar gone, the standard collision process of the gas clouds take over, and the galaxy starts to accrete additional gas from its surroundings. Eventually, orbits in the center of the galaxy will resynchronize and the bar will reform, starting the whole process over again. Based on observations, galaxies spend about ¾ of their life with a bar.
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