Editing Stellar evolution (section)

====Red-giant-branch phase====
[[Image:The life of Sun-like stars.jpg|thumb|upright=1.35|Artist's depiction of the life cycle of a Sun-like star, starting as a main-sequence star at lower left then expanding through the [[subgiant]] and [[giant star|giant]] phases, until its outer envelope is expelled to form a [[planetary nebula]] at upper right]]

{{Main|Red-giant branch}}
The expanding outer layers of the star are [[convection|convective]], with the material being mixed by turbulence from near the fusing regions up to the surface of the star.  For all but the lowest-mass stars, the fused material has remained deep in the stellar interior prior to this point, so the convecting envelope makes fusion products visible at the star's surface for the first time. At this stage of evolution, the results are subtle, with the largest effects, alterations to the [[isotopes]] of hydrogen and helium, being unobservable. The effects of the [[CNO cycle]] appear at the surface during the first [[dredge-up]], with lower <sup>12</sup>C/<sup>13</sup>C ratios and altered proportions of carbon and nitrogen. These are detectable with [[spectroscopy]] and have been measured for many evolved stars.

The helium core continues to grow on the red-giant branch.  It is no longer in thermal equilibrium, either degenerate or above the [[Schönberg–Chandrasekhar limit]], so it increases in temperature which causes the rate of fusion in the hydrogen shell to increase.  The star increases in luminosity towards the [[tip of the red-giant branch]].  Red-giant-branch stars with a degenerate helium core all reach the tip with very similar core masses and very similar luminosities, although the more massive of the red giants become hot enough to ignite helium fusion before that point.