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 +
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.
copyright 1997 STScI
a Red Giant star, as is it's companion star pictured in
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.
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.