Hydrogen shell burning stage

  • Hydrogen depleted in core ~ 10 billion years
      • Core fills with helium, not hot enough to fuse it
  • Helium core contracts and releases gravitational energy
  • Overlying layer infalls and heats up
  • Hydrogen fusion begins in the shell
      • Fusion rate is faster than before
        • Luminosity increases
      • Star swells to subgiant
      • Helium produced in the shell falls into the core
        • Core gets more massive, contracts and heats up
      • Electron degeneracy pressure dominates core
          • Not affected by temperature

Helium flash

  • Electron degeneracy pressure dominates the core
      • Does not increase with temperature
  • Helium fusion begins in the core
      • Fusion heats core rapidly
      • No rise in pressure
        • No expansion
          • No drop in temperature
      • No stabilization
    • Runaway rise in temperature
  • Helium fusion begins, explosively

Helium burning stage

  • Starts with Helium flash
  • Core expands rapidly
  • Pushes H burning shell outward
  • Stellar wind blows off 20 -30% star mass
  • Shell temperature lowers
  • H fusion rate decreases
  • Total energy production falls
  • Luminosity lowers
  • Star settles into a new equilibrium state
      • Helium fusing in the core
      • Hydrogen fusing in a shell

Helium shell stage

  • Core fills with helium, core fusion shuts down
      • Still have hydrogen shell fusion
  • Core contracts, density increases
  • Electron degeneracy pressure again dominates the core
      • Not affected by temperature
      • Core gets hotter
  • Begin helium shell fusion
  • Star expands to a red giant
  • Core continues to heat up
  • Envelope expands
      • Helium shell flashes, star may pulsate
  • Shell expands into a planetary nebula
  • Core is left as a white dwarf

The above HR diagram shows the evolutionary path of a sunlike star after it leaves the main sequence. From stage 7 (main sequence star) to stage 8 (subgiant), the surface temperature of the star declines as the core fills and hydrogen fusion begins to take place in a shell around the core. The luminosity increases as the star expands into a subgiant.


The nearly vertical ascent between the subgiant and red giant region happens because while the core is shrinking and heating up, the outer layers are expanding and cooling. Thus, the surface temperature remains relatively constant, but the increasing size increases the luminosity of the star.


The onset of helium fusion in the core, the helium flash, blows away the outer third of the star. The helium fusion increases the temperature, while the size reduction and lowered rate of fusion diminish the luminosity.


The return to the asymptotic giant branch takes place as the core fills with carbon. The loss of fusion decreases the temperature, but the surface temperature remains relatively constant as the star swells up in size. The increase in size produces an increase in luminosity.

The envelope of the star wafts off into space as a planetary nebula. The core of the star becomes a white dwarf. It moves to the lower left on the HR diagram; it is very hot, but low luminosity because it is very small. Gradually, with no fusion left to produce heat, the white dwarf cools off and dims, eventually becoming a black dwarf star.

The above diagram tracks the post-main-sequence evolutionary paths of two stars that are more massive than the sun, a four-solar-mass star and a ten-solar-mass star. Note that these stars move much more horizontally after leaving the main sequence. This means that their luminosities stay much more constant than sunlike stars.


The cores of  massive stars are less dense than sunlike stars when helium fusion begins, so electron degeneracy pressure is not as prevalent. This means that the gas pressure responds to changes in temperature. The gravitational force is also stronger, due to the larger mass, which holds on more tightly to the outer layers.


The onset of helium fusion is less dramatic. There is no helium flash in massive stars. Stars above eight solar masses get hot enough for fusion of carbon into oxygen to take place in their cores. much more massive stars continue to fuse heavier and heavier elements up to iron in their cores.

Globular clusters make good "laboratories" for studying star evolution because they are intact populations of stars that formed at relatively the same time, out of the same material. This sequence of diagrams illustrates how the evolution of a globular cluster appears on a Herzsprung-Russell diagram. Notice that the More massive, more luminous stars leave the main sequence stars first, followed by the lower mass stars. The point at which the stars are presently leaving the main sequence is known as the "main sequence turnoff" and gives a measure of the age of the golbular cluster.