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When star like our Sun reaches the end of its life, it enters one last phase, ballooning up to many times its original size. Astronomers call these objects red giant star, and you’ll want to learn more about them, since this is the future fate for the Sun. Don’t panic, we’ve got another 7 billion years or so before the Sun becomes a red giant star.
As you probably know, stars shine because they’re converting hydrogen into helium in their cores through a process called nuclear fusion. Our own Sun has been performing fusion at its core for 4.5 billion years, and will continue to do so for another 7 billions years, at least. The helium byproduct from this fusion reaction slowly builds up in the core of a star, and they have no way to get rid of it. Eventually, billions of year down the road, a star uses up the last of its hydrogen fuel.
Once a star exhausts this fuel source, it no longer has the outward light pressure to counteract the gravity pulling in on itself. And so, the star begins to collapse. Before the star can collapse too far, though, this contraction heats up a shell of hydrogen around the core of the star to the point that it can support nuclear fusion. The higher temperatures lead to increasing reaction rates, and the star’s energy output increases by a factor of 1000 to 1000x. This new extreme light pressure pushes out the star’s outer layers beginning its life as a red giant star.
A red giant will expand outward many times its original size. Our own Sun, for example will grow so large that it engulfs the orbits of Mercury, Venus and even Earth; although, it’s not certain if Earth will actually be destroyed when this happens.
The core of the star will become so hot and dense that the leftover helium fuel will no able to star fusing into heavier elements. Stars with the mass of our Sun will stop with helium, but more massive stars will keep going, fusing carbon and even heavier elements together.
Without any more fuel to burn, these stars will expel their outer layers and then contract down to become white dwarfs.
We have written many articles about stars here on Universe Today. Here’s an article about a planet surviving when its star became a red giant. And were you wondering if the Earth will survive when the Sun becomes a red giant?