D. Formation of Galaxies

When a clump becomes massive enough, it starts to collapse under its own gravity. At this point, the clump becomes a protogalaxy. Astronomers hypothesize that protogalaxies consist of both dark matter and normal hydrogen gas. Due to collisions within the gas, the hydrogen loses energy and falls to the central region of the protogalaxy. Because of the collisions of the gas, protogalaxies should emit infrared light. The dark matter remains as a halo surrounding the protogalaxy.

Astronomers think that the difference in appearance between elliptical and spiral galaxies is related to how quickly stars were made. Stars form when gas clouds in the protogalaxy collide. If the stars are formed over a long period of time, while some stars are forming, the remaining gas between the stars continues to collapse. Due to the overall motion of matter in the protogalaxy, this gas settles into a disk. Further variations in the density of the gas result in the establishment of "arms" in the disk. The result is a spiral galaxy. If, on the other hand, stars are made all at once, then the stars remain in the initial spherical distribution that the gas had in the protogalaxy. These form an elliptical galaxy.

Astronomers also think that collisions between galaxies play a role in establishing the different types of galaxies. When two galaxies come close to each other, they may merge, throw out matter and stars from one galaxy, and/or induce new star formation. Astronomers now think that many ellipticals result from the collision of galaxies. We now know that giant ellipticals found in the center of galaxy clusters are due to multiple galaxy collisions.

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