A state of stellar evolution beyond the main-sequence life of a
star. A red giant core is degenerate ionized helium, surrounded
by a shell of hydrogen fusion, that expands the outer atmosphere
in response to higher core temperatures. The hydrogen fusing
shell eats through the surrounding atmosphere and deposits
helium onto the shrinking core. The ballooning atmosphere cools
and glows red; hence red giant. The Sun will become a red giant
the size of Earth's orbit in five to six billion years. Once the
helium core reaches 100 million degrees, it explosively begins
fusing helium. The birth of the active helium core is called the
helium flash. The Sun as a red giant will fuse helium for about
2 billion years after the helium flash.