A galaxy undergoes a remarkable process as it ages: Its middle gets thicker, while its outskirts get thinner. A thick "bar" of stars in the center of the galaxy may be responsible. It stirs up the galaxy's stars and gas. That causes some of the gas to fall toward the middle of the galaxy, where it gives birth to new stars. Over time, that builds a big "bulge" of stars in the middle, while thinning out the regions outside the bulge.
This idea is a fairly new one, pioneered by Texas astronomer John Kormendy. But it turns out that the same thing happens in many other objects, from stars to star clusters. Gravity works the same way in all these objects, making their centers denser and their outer regions thinner.
A star like the Sun, for example, consists of a hot, dense core that's surrounded by thinner layers of gas. As the star ages, the core shrinks and gets denser and hotter. The core's intense radiation pushes on the outer layers, so they puff outward. Eventually, they bloat to hundreds of times the star's original size. Then they drift off into space, leaving only the core behind.
And the same thing is seen in globular clusters -- tightly packed balls of hundreds of thousands of stars. Over time, the heavier stars sink to the cluster's center, while the less-massive stars are thrown out to greater distances.
Since galaxies grow central bulges in a closely related way, there's a common mechanism at work throughout the universe.
For more skywatching tips, astronomy news, and much more, read StarDate magazine.