... The Sun is powered by nuclear fusion: hydrogen `burns' to form helium, releasing energy . ... At an age of ~10 billion years (slightly over twice its present age), the Sun will exhaust the fuel available in its core. ... Pressure overwhelms gravity, and over a period of a few hundred thousand years, the Sun evolves into a `red giant' . ... Red giants have lifetimes of less than 100 million years. The temperature in the red giant core increases to 100 million degrees . ... The Ring Nebula . ...