The Sun is expected to turn into a red giant in about 5 billion years. At that time the inside will get very much hotter, and the outer layers will expand and become much cooler than they are now. The size of the sun as a red giant might get as big as to extend out between the orbits of The Earth and Mars, but no one is really sure. Although the temperature of the outer layers of the sun will be much cooler then than they are now, still it will be way too hot for life to exist on Earth. All the oceans will boil away; the atmosphere will be gone, too. But you have nothing to worry about! It is very, very far into the future!
The sun is now in the middle of its life cycle, and will explode in about 4-5 billion years. In the few hundred million years before the explosion, it will swell up and become a red giant as big as the Earth's orbit. It will therefore swallow the Earth. Don't worry, this is a long time from now, and if our civilization survives even a million years we will surely have a way to move the Earth to safety. If we humans kill ourselves off and no other creatures evolve high intelligence, then Earth might be in trouble. The sun is slowly getting hotter, and it might cause a deadly climate shift a few hundred million years from now if nobody is here to move the Earth.The explosion of our sun will not be very impressive by astronomical standards. Massive stars burn very quickly and make much better fireworks, called supernovae. These explosions are visible over the whole universe, can outshine an entire galaxy of 100 billion stars. In their ashes they leave bizarre objects such as neutron stars (a spoonful of neutron star is as heavy as an aircraft carrier) and black holes (a pit of raw gravity from which nothing can escape- not even light). A supernova explosion could fry all civilizations around nearby stars, but they can also create life, since their nuclear reactions are responsible for creating all heavy elements in the universe. By weight, you are mostly made of atoms spewed out by supernovae explosions over 5 billion years ago. There is a supernova explosion about every 100 years in this galaxy, so there is a good chance you will see one. It will look like an extremely bright star, easily visible during the day, and casting shadows at night. Luckily, there are no neighboring stars "due" for a supernova, but a few thousand light-years away (that's a few blocks away, by galactic standards) we see the biggest star in the galaxy, Eta Carinae, getting ready to blow. It is 100 times heavier than the sun and glows with the power of 10 million suns, and it may go supernova within 10,000 years. It might have already exploded, since being a few thousand light-years away, it takes the light a few thousand years to get here. Anyone who sees it explode will have blind spots burned into their eyeballs.
I don't think anyone believes that the sun is going to ever explode. The sun should stay as it is for about another five billion years. After that, the sun will have used up most of its nuclear fuel and enter the red giant stage. When the sun becomes a red giant it will expand to a very large size and the Earth will be destroyed. Since this is quite a ways off, we have a lot of time to consider what to do about this problem.
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