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The Sun has another 500 million years of normal life left. For this time, it will carry on creating energy by combining atoms of hydrogen to create the next heaviest element, helium. The helium will stay at the core, and gradually all the hydrogen there will be used up. In order to find new fuel, the hydrogen-burning process will move out into a shell around the core, gradually eating up fuel. The move to 'shell burning' makes the Sun's outer layers become unstable, and the Sun will swell up to become a red giant, probably with a diameter engulfing the Earth's orbit.
Eventually, the temperature at the heart of the Sun will become so high that helium burning can begin - fusion of helium to create even heavier elements, and release energy. The Sun will then quietly continue for a few million years, burning hydrogen in a shell, and helium in the core. But the helium will be used up much more quickly than the hydrogen, and eventually the Sun's core will be full of heavier elements, and helium burning will move out into a shell as well. More massive stars can create enough energy to begin burning the heavier elements, but not stars like our Sun. The shell burning will swell the Sun to a red giant again, but this time it will continue to grow and the atmosphere will be blown off into space, leaving the slowly cooling core exposed. The Sun will end its life as an expanding sphere of gas, called a 'planetary nebula', surrounding a dead 'white dwarf' star.
