Life Cycle of the Sun 1) Before nuclear fusion, when the sun was still being created in the interstellar nebula, it was just a protostar. A protostar forms when gravity and pressure begin to condense matter in the centre of the nebula, but no nuclear fusion is yet taking place. When nuclear fusion begins, a star is "born". Our sun started burning hydrogen at 4.5 billion years ago. When the sun started it was a little smaller, less bright and cooler than it is today. 2) The sun is in the middle of its lifespan, and is currently in the main sequence
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