Both bottom-up and top-down galaxy formation theories revolve around gravity and its collapsing effects on gas in early galaxies. Bottom-up theories propose that this gas collapsed and compressed into clumps the size of a million suns. These smaller clumps (cosmically speaking) then got together to form enormous galaxies, like the Milky Way.
How far has cosmology evolved in the past 50 years?
Answered by Dr. Michio Kaku
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What factors determine a star's luminosity?
Answered by Science Channel