Galaxies can age in two different ways. They can age quickly by merging with other galaxies. Or they can age slowly by building "bulges" of stars in their centers.
Texas graduate student David Fisher has used Spitzer Space Telescope to study these processes in hundreds of galaxies. Spitzer measures infrared energy, which is invisible to the human eye, and best studied from outside Earth's atmosphere. In a galaxy, much of this energy comes from clouds of dust that are warmed by newborn stars.
Fisher's survey shows that there's a big difference in the rates of starbirth in disk-shaped galaxies. In about half of these galaxies -- where the bulge of stars is round -- few new stars are taking shape. These bulges probably formed from the mergers of two or more smaller galaxies.
But in the other half of the galaxies, the "bulge" is flat, and it often forms an elongated bar. In these galaxies, the bulge is filled with millions of new stars, and more are being born all the time.
Fisher's observations seem to confirm a new picture of how galaxies evolve. It suggests that a bar funnels gas into a galaxy's center. The gas forms dense clouds that collapse to give birth to new stars.
This process takes billions of years, compared to the few hundred million years it takes to build a bulge through mergers. But the universe has been around for almost 14 billion years, so there's been plenty of time to build the beautiful structures we see today.
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