Have been reading about cepheid. Can I just check I've got this? 'When at their hottest (when gravity has squashed them) have more doubly ionised He so star becomes more opaque' Now was just wondering what the main mechanism is here? Because He2+ doesn't have any electrons left to absorb photons to promote electrons to higher energy levels! So is the interaction a Compton scattering between the free electrons and or He2+ ions and the photons and this is why photons don't get out or is it particle antiparticle pair productions as the photon gets near the He2+ nucleus or a combination or something else? Regards as always knowledgeable people, Glenn.