our sun is instead a second- or third-generation star, formed about 5 billion years ago by a local recoalescence. As that was occurring, a small proportion of heavier elements in the vicinity escaped incorporation into the new star, and instead collected into the planets that now rotate around our sun. This includes our own planet, which was far from hospitable in its early days. Initially very hot, and bombarded with continual massive collisions, Earth gradually cooled, developed an atmosphere, and became potentially hospitable to living things by about 4 billion years ago. A mere 150 million
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