How is energy liberated in the sun? What would happen when all the hydrogen in the sun gets consumed?
The source of energy of the sun is nuclear fusion reactions in its interior, of hydrogen gas into helium nuclei. 21H + 21H —–> 42 He + Energy (i) Red giant phase: When all of the hydrogen gas in the interior of the sun is utilized, then the nuclear fusion reaction will stop in the core but will be continue in its outer envelope. After this, the color of the light emitted will become red and the core will begin to contract due to its own gravitation. As a result, the temperature of the core would increase and fusion of helium can start to yield heavier elements like carbon. Due to large amount of energy released the size of the star increases tremendously and its outer layers start glowing red. This stage is known as the red giant phase. (ii) White dwarf: The outer envelope will expand continuously and if the mass of core is equal to or less than sun, the core cannot hold onto the outer envelope which will eventually get lost in space. These lost gases constitute raw materials for new stars. But the core will continue to contract under gravitational forces and the temperature continues to increase. However the temperature rise may not be high enough to initiate further fusion reactions. The core because of high temperature glows white. This stage is called white dwarf.