Galaxies are giant "cities" of billions of stars. And like cities, they constantly evolve and change.
In the early universe, most of this change came about through interactions between the galaxies. Small galaxies would collide and stick together to form bigger galaxies. And as galaxies sailed past each other, their gravity would pull out streamers of stars, and squish together clouds of gas to give birth to new stars.
But the universe has expanded as a result of the Big Bang, so the galaxies have moved farther apart. So today, the interactions with other galaxies play a smaller role in sculpting a galaxy's appearance.
Instead, Texas astronomer John Kormendy and his colleagues have found that when galaxies are left alone, they evolve on their own. They age by building bulges of stars in their centers, surrounded by long "bars," spiral arms, and rings of bright stars.
Many disk-shaped galaxies -- including our own Milky Way -- have thick bars of stars in their middles. These bars are shaped like loaves of bread. The spiral arms spin off these bars. At the same time, many galaxies have bright blue rings of starlight around the bars -- the light of hot, young stars.
Many other galaxies have similar rings outside their disks, with millions of newborn stars also clustering in their centers -- indications that they've been evolving for a long time, and are continuing to change even now.
More about evolving galaxies tomorrow.
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