What would happen when the Sun goes out?
Short answer: Before the Sun goes out, Earth will get very hot. People won't be able to live here any more so they will need to find another place to live. They could live on another planet or in huge spaceships. If anything is left of the planet Earth after the end of the Sun, it will eventually get very cold.
Long answer: The Sun will go through several phases before it "goes out", and the whole thing will take a very long time.
First, over the next few billion years, the Sun with slowly get bigger and redder. As this happens the Earth will get hotter. We don't know exactly when Earth will get too hot to live here—it could be as little as a few hundred million years or it could take billions of years. We can be fairly sure that in about 5 billion years the Sun will become a red giant star and will definitely be too big and hot for anything to survive on Earth.
The Sun will get so big that it will consume the closest planets, Mercury and Venus. It will actually grow to approximately the size of Earth's orbit, so it may or may not consume our planet. Not that it makes much difference—Earth will be dead either way.
If any people are still alive they'll need to move somewhere else. The first step would be to move to the outer Solar System, farther from the giant Sun. Eventually even that won't be enough, though, and people will almost certainly have to move out into the rest of the galaxy. They could find other planets to live on or they could create artificial living environments in space.
The Sun will be a red giant for about a billion years, during which it will lose about a third of its mass as it shoots material out into space. Then things get a bit hectic as the Sun shrinks and expands again, pulsing, changing temperature and generally behaving in an unstable manner. It will "puff off" its outer layers, creating a beautiful nebula. By this time we do not expect any of the Sun's planets to survive—they'll either be consumed by the Sun or lost into interstellar space.
Finally, the core of the Sun will shrink to a relatively small size and become a white dwarf star. This will be the longest stage of the Sun's entire life, lasting trillions of years (so the Sun will actually spend most of its life with no planets orbiting it).
At the very end, all the Sun's heat will be gone and it will fade into a black dwarf star.
The good news is that none of this is our problem. It will be at least millions of years before the aging Sun becomes any kind of issue to worry about.
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Author: Dave Owen