Stars do not last forever. After around 10 billion years, a star like the Sun will start to run low on the supply of hydrogen in its core, which it uses to form helium and keep shining. Once that happens, the core - losing its outward pressure source - will shrink under its own gravity, while the outer layers are pushed out. As its surface expands, its becomes cooler and redder but much more luminous, forming what astronomers call a red giant. This image depicts a red giant in a hypothetical planetary system. It has reached out almost to the orbit of its innermost planets, and caused perturbations in their orbits which have led to two of them colliding.
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