why will the sun become red giant after 6 billion years and what happens after that?

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​Please find below the solution to the asked query  
   

​The hydrogen in the core continues to fuse to form helium. Thus a helium core is formed at the centre surrounded by hydrogen in the outer shell. The fusion reaction in the core stops and there is rapid fusion reaction of hydrogen in the outer shell only. The radiation expands the outer shell and after sometimes it cools. The colour of the star changes to red and is known as red giant.
Sun will turn into a red giant in the similar way and like any other star, it has to taste death at some point of time. Earth will be unable to support life when this will happen.
 
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Once the Sun changes from burning hydrogen at its core to burning hydrogen around its shell, the core will start to contract and the outer envelope will expand. ... An illustration of what a sunrise may look like from Earth in 6–7 billion years as the Sun becomes a red giant.
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