Our Sun is shining because its core is fusing hydrogen nuclei together to form helium. It's a main sequence star, which means it's in hydrostatic equilibrium and the outward pressure from this nuclear fusion perfectly balances the gravitational forces trying to collapse the Sun in on itself.
But one day (about 5.4 billion years from now) our Sun will run out of hydrogen fuel in its core and only have helium at its centre. The Sun will stop counteracting gravity and contract slightly.
When this contraction happens, the temperature will increase - and hydrogen fusion will start to happen in the shell around the helium core. This will cause the Sun to expand.
The outer layers of hydrogen will decrease in temperature, and this makes them look redder.
The Sun is now a red giant.
All main sequence stars between one-fifth and 10 times the mass of our Sun will become red giants when their hydrogen reserves run out in their core.
Will the Earth survive the Sun's transition to a red giant?
We're not sure. Scientists believe a red giant Sun will grow large enough to encompass the orbits of Mercury, Venus, and maybe even Earth.
Even if the Earth did survive, its going to be pretty close to the intense heat of the red giant Sun. The Earth's surface will, most likely, be scorched and inhospitable to life.
The good news is that the Sun's expansion is predicted to alter the Earth's orbit - but the bad news is that we still won't escape a fiery death.
Why do Red Giants expand? You can find out more here. Here's a pretty big repository of red giant facts from Space.com. Red Giants could also explain the origins of the elements in our universe.
And this is a pretty cool video from Dr. Mark Morris, a professor of astronomy at UCLA, covering the future of our Sun:
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