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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phase. At this stage in its life the sun is fusing hydrogen into helium. 3) As the Sun becomes older, it grows slightly larger, brighter and hotter. At about 1.1 billion years from now the Sun will be 10% bigger than it is now. As a result, the extra solar energy will dry up Earth's atmosphere and many kinds of life. In about 3.5 billion years, the sun will be 40% bigger and will probably mean that all life on Earth will cease to exist. The next stage will be for the sun to become a Red Giant. 4) Then the sun is 12.2 billion years old it will enter the Red Giant phase. The sun will become big enough to completely consume the planet Mercury. As a red giant, the sun will be cooler in temperature, but brighter. When the Sun reaches its maximum size as a red giant, helium fusion will occur in the core, and the star will begin to release more energy. As a result, the sun will shrink in size, but become hotter. Once the Sun is 12.3 billion years old, it will run out of helium. Since the sun doesn't have any more fuel to burn, this stage marks the beginning of the end. 5) As the sun nears the end of its Red Giant phase, most of its outer layers (which contain the lightest elements) will be vented off into space. This forms a planetary nebula. This is the shortest phase of the sun's life cycle. The result is that the sun loses mass at a high rate and becomes cooler in temperature, but very bright. Eventually, all that will be left of our star is its dense core.6) By the end of its planetary nebula phase, the sun will have lost most of its mass, and only its dense core will remain. At this point, the sun will become a white dwarf. A white dwarf is very dense because it contains only the heaviest elements fused over its lifetime, but no more nuclear fusion will be taking place. Because there is no more nuclear fusion occurring in its core, the sun will become much cooler in temperature and only give off a fraction of the light it once did. Over time the Sun will stop generating any heat or light. When no more nuclear fusion is taking place, the Sun will end its life as a black dwarf. Scientists aren't sure what this will look like because the process can take trillions of years, and our universe is only 13.7 billion years old.