Like some people, some galaxies grow "bulges" around the middle as they age. In the case of a galaxy, it appears to be a healthy step in the aging process.
Many of the big galaxies in the universe are shaped like disks -- including our own galaxy, the Milky Way. But there's a lot of structure within the disks. There are fat "bulges" of stars in the middle, often surrounded by "bars" of stars that are shaped like loaves of bread. Spiral arms spin off the bars. And rings of stars can encircle the bar and the outside of the disk itself.
Over the last three decades, Texas astronomer John Kormendy has studied how the bars are responsible for much of this structure. They act like the blades of a blender, stirring up the stars and gas around them.
The stirring process dumps a lot of gas in the center of the galaxy. The gas condenses to give birth to new stars. The new stars build up the galaxy's central bulge.
The process also builds the galaxy's spiral arms, which wrap around the bulge. The arms are "waves" that ripple through the galaxy, squeezing gas clouds to give birth to new stars. And finally, the stirring creates "rings" of new stars. In some galaxies, the rings are just outside the bar. In others, they're outside the disk.
All of these steps are a natural result of how a galaxy ages. And similar processes appear to be at work in stars, star clusters, and the planet-forming disks around stars. More about that tomorrow.
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