differs depending on the material composition of the neutron star envelope. For a hydrogen layer with cosmic abundances ($X=0.73$), the predicted luminosity is $\approx2.0{\ensuremath{\times 10^{38}} }$[[erg[s$^{-1}$]{}]{}]{}, whereas the luminosity of a pure helium layer ($X=0$) is larger by a factor of 1.73. In [SAX J1808]{}, the ratio in bolometric flux between the peak and pause/dip is $1.68\pm0.13$.

The following scenario now emerges for the X-ray burst evolution. As the critical ignition point is reached in the helium layer, the flame front quickly spreads across the stellar surface and an intense radiation field starts to diffuse outward. After about 0.5[[s]{}]{}, the radiation pressure reaches the local Eddington limit of the hydrogen layer, causing that layer to expand. Meanwhile, the intensity of the radiation field continues to increase, either expelling or diluting the hydrogen layer, so that the observed spectrum becomes dominated the PRE of the helium layer. Over the following 10[[s]{}]{}, we observe the full helium PRE cycle, causing the photosphere to cool and then heat, as the envelope expands and then contracts. Once the envelope touches back down on the stellar surface, the burst flux is still comparable to hydrogen Eddington limit, although at the time this touchdown occurs, there 