In about six billion years, the Sun will run out of hydrogen fuel. This will be a very gradual process over billions of years. As the Sun burns through its hydrogen fuel, its core temperature will increase and its outer layers will expand - causing the Sun to grow into a red giant star. During its development into a red giant star, the Sun's diameter is predicted to become approximately 200 times its present diameter and its brightness maybe more than 1,000 times its present brightness.

The Sun's energy production in its central core will be much higher than at present and it will lose mass through an intensified solar wind. Its surface temperature should drop due to the expansion and cooling of its outer layers but its overall luminosity will be greater than at present. When it become a red giant, the Sun will extend well beyond Earth's current orbit and may engulf Earth.

After this, the Sun's unstable outer layer will drift off into space, forming a planetary nebula, and the core of the Sun will be exposed. The core, called a white dwarf, is very hot but faint as it continues to cool off and dim over trillions of years.