HELIUM BURNING: The Beginning of the End

For stars that live most of their lives in the main sequence, helium burning is the beginning of the end. The overall thermonuclear reaction for helium burning is as follows: 3 He -> 1 C + energy released

Interactive Lab

Helium Burning Process

For the most part, hydrogen in the core is gone. If the star wants to maintain equilibrium between gravity and gas pressure, it needs increased temperatures in the core to re-ignite fusion. The star is forced to burn helium in an effort to maintain stability. It takes a temperature of 10×10⁷ °K to initiate helium burning, whereas it only takes a temperature of 2×10⁷ °K to initiate hydrogen burning.

Remember, to remain stable the star must balance the gas pressure pushing out and the gravitational force pulling in. Gravity will cause the core to contract. Helium burns inside the core, but a rapid hydrogen reaction occurs faster in the shell of the star. As the temperature in the shell of the star increases, the outer layers of the star expand.

Test Your Knowledge!

Star Quiz (part 2)

Helium in the core of the star is still burning hot. Gravity keeps contractingthe core to maintain equilibrium, and as the core contracts the atoms are packed together even tighter than before. The outer shell has expanded in an effort to help heat from the core escape into space. At this point, the star is often termed a red giant. The red giant is the first step in old age.

Fusion is releasing more energy during helium burning than at the main sequence stage, so the star is bigger, but less stable. Eventually, the core will run out of helium fuel, and in order to maintain equilibrium, the core will contract again to initiate the last type of fusion – carbon burning.