Cosmology is the study of the origins of the universe. General opinion is that the universe started with a big bang. 100 years from now, who knows what general opinion will be then. As for when it, happened 20 billion years ago, 12 billion years ago or maybe some other time. We really need to be humble here.
Other stars began billions of years before the cosmic gasses in our neighborhood coalesced into our solar system. This period closes with the formation of planet Earth.
MYA = Million Years Ago
BYA = Billion Years Ago
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