Formation of Stars and Galaxies & Gravity> Gravity boosts some irregularities in the thickness of primordial gas and more gas becomes thicker and thicker, even as the universe continuously expands at a certain speed. Under their own gravity the small clouds of cosmic gas starts to collapse, becoming hot enough to trigger Nuclear Fusion Reactions between Hydrogen Atom, creating the very first stars. This happened around 150 million years after the Big Bang.
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