Page "Stellar evolution" Paragraph 22

When the temperature and pressure in the core become sufficient to ignite helium fusion, a helium flash will occur if the core is largely supported by electron degeneracy pressure ( stars under 1. 4 solar mass ).

In more massive stars, whose core is not overwhelmingly supported by electron degeneracy pressure, the ignition of helium fusion occurs relatively quietly.

Even if a helium flash does occur, the time of very rapid energy release ( on the order of 10 < sup > 8 </ sup > Suns ) is brief, so that the visible outer layers of the star are relatively undisturbed.

The energy released by helium fusion causes the core to expand, so that hydrogen fusion in the overlying layers slows and total energy generation decreases.

The star contracts, although not all the way to the main sequence, and it migrates to the horizontal branch on the HR-diagram, gradually shrinking in radius and increasing its surface temperature.