Our sun and other stars similar to it, do not stay exactly as they look now. Stellar lifetimes are much longer than ours, but like us they progress through the different stages of their life. The progression of these stages is connected to the formation of planets, their fate and events to come billions of years from now. In this article we will try to learn about the life cycle of our sun and other stars like it.
The sun was born in a dense cold cloud of gas similar to nearby cloud in Perseus Constellation. Within these clouds gravity causes pockets of gas to collapse into individual stars. Collapse lasts about half a million years. Small amounts of gas collapse and there remains a disk around the star. Over several million years the disk may form planets like those in our solar system.
Eventually the center of the star becomes dense enough to fuse hydrogen atoms into helium atoms and star takes its place on the main sequence, where it will spend much of its life. On a main sequence, a star’s luminosity determines its size, mass and future evolution. After hydrogen burning ends the star starts to leave main sequence properties and moves on. At 4.5 billion years old our sun is middle aged star and it is still in the main sequence. Over the next few billion years the rate of hydrogen fusion will gradually increase causing the sun’s temperature to increase and its radius to swell. This will cause earth’s ocean to evaporate and the earth will take an appearance similar to Venus whose surface is molten. 5 billion years from now the supply of the sun’s hydrogen for fusion reaction will run out. The Sun once the hydrogen in its core is gone will leave the main sequence and will become a red giant star. During this stage the outer layers of the sun will expand until it reaches the 200 times larger than its current size. This new sun will be larger than the orbits of Mercury and Venus. They will be swallowed by this expanding star and maybe it will also swallow earth too.
The Red Giant phase will last few million years until all the helium in its core also runs out like hydrogen. Then the sun will go under a cataclysmic process, blasting out its outer layers and will become a white dwarf. The discarded layers will be visible as a planetary nebula. As a white dwarf the sun will be the size of earth and nuclear fusion reaction will no longer occur, but it will still be luminous because its temperature will remain thousands of degrees. Over tens of billions of years, this white dwarf will become much cooler and will become dim and cold as a black dwarf. The time required for a white dwarf to become a black dwarf is more than the current age of the universe. This means that there are no black dwarfs now in the universe for us to observe. So we don’t know what will happen to our sun after trillions of years. So for now we are save and sun in not a bigger threat to us.
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