Formation and Evolution
Here we will talk about the formation and evolution of two of the three main types of galaxies, spirals and ellipticals. There is still much astronomers don't know about the process, but we'll give you their best guesses.
Galaxies are thought to have begun from large irregular clouds of hydrogen and helium. This gas was created in the first few mintues of the universe. Certain sections of the clouds were probably slightly more dense than others. Because of this higher density, gravity caused them to collapse. As the large cloud collapsed, it cooled. On an even smaller scale, pieces of the collapsing cloud, also collapsed into even smaller pieces. These smaller denser regions created the first stars. When the first stars reached the end of their life cycle, they exploded, heating the surrounding gas and slowing the collapse of the galaxy cloud. These explosions also introduced heavier metals, such as carboon and nitrogen, into the galactic cloud. Eventually, this process of collapse, star formation, and slowing, balanced, giving us stable galaxies.
How this process created elliptical and spiral galaxies, is yet another question. There are two main theories. The first theory is: as the cloud collapsed to form a galaxy, it's spin is what determined what type of galaxy it became. Some theorists believe that spiral galaxies were formed from clouds that had a significant spin. As the cloud collapsed the spin got even faster still, this is a feature of the principle of angular momentum. As this the spin increased, it flattened the material in the cloud along the spin axis, forming the characteristic disk of spiral galaxies. Elliptical galaxies were simply formed from clouds that didn't have this spin. They therefore formed a more round structure, which has no particular axis of rotation.
The second theory is that elliptical galaxies were formed from collisions of spiral galaxies. This theory is supported by a couple of interesting facts. First, in the early universe galaxies were much closer together then they are now. Since they were closer together, especially in galaxy clusters, collisions were probably very common. So if collisions of spirals made ellipticals, the process of elliptical galaxy creation was definitely present. Second, large elliptical galaxies typically occur in rich galaxy clusters, where collisions most likely happen. Third, ellipticals don't have much interstellar gas, when compared to spirals. Why? In the context of this theory, the collision of spirals would have ignited much of the gas, turning them into stars. This process can be seen today in galaxy collisions. Elliptical galaxies do show evidence of this "new" population of star formation, even though they currently have very low formation rates. Newer stars have a different metal composition than older stars, since they were created later in the galaxy's evolution (i.e. after several star life cycles). Astronomers can measure the amount of "heavy" metals in a star through a process called spectrophotometry. In some elliptical galaxies, there are two distinct populations of globular clusters an "old" and a "new."
Stephan's Quartet showing galaxy collsion
Here are some cool links:
Theories behind the large scale structure of the universe.
Here's a movie showing individual galaxy formation.
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... Last modified 18Apr00