Galaxy Origin2 - will form But perhaps the situation is...

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Galaxy Origins All galaxies began forming at about the same time approximately 13 billion years ago. The origin of galaxies and how they changed over billions of years is an active field of research in astronomy today. Models for galaxy formation have been of two basic types: "top-down" and "bottom-up". The "top-down" model on the origin of the galaxies says that they formed from huge gas clouds larger than the resulting galaxy. The clouds began collapsing because their internal gravity was strong enough to overcome the pressure in the cloud. If the gas cloud was slowly rotating, then the collapsing gas cloud formed most of its stars before the cloud could flatten into a disk. The result was an elliptical galaxy. If the gas cloud was rotating faster, then the collapsing gas cloud formed a disk before most of the stars were made. The result was a spiral galaxy. The rate of star formation may be the determining factor in what type of galaxy
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Unformatted text preview: will form. But, perhaps the situation is reversed: the type of galaxy determines the rate of star formation. Which is the "cause" and which is the "effect"? A more recent variation of the "top-down" model says that there were extremely large gas clouds that fragmented into smaller clouds. Each of the smaller clouds then formed a galaxy. This explains why galaxies are grouped in clusters and even clusters of galaxy clusters (superclusters). However, the model predicts a very long time for the collapse of the super-large clouds and fragmentation into individual galaxy clouds. There should still be galaxies forming today. One galaxy called "I Zwicky 18" was recently discovered to be a very young galaxy that began forming stars only 500 million years ago. However, it is a dwarf galaxy whose formation might better be explained by the "bottom-up" model called "hierarchical clustering"....
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This note was uploaded on 12/15/2011 for the course AST AST1002 taught by Professor Emilyhoward during the Fall '10 term at Broward College.

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Galaxy Origin2 - will form But perhaps the situation is...

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