Diagram of the energy levels (red horizontal bars) and radiative transitions for the different model atoms of silicon. The number of bound-bound transitions in each model is indicated in the lower right corner of the panels. Top panel: the full, comprehensive model of Si i + Si ii atom. Note that the diagrams for the Si i and Si ii atoms are shown separately. Bottom panels: the Si i working model with 605 bb transitions (left) and the simplest one (right) with six bb transitions between the fine structure levels of the Si i lower 4s ³P^o and upper 4p ³P terms. These bb transitions are marked by a thick red line. One of these transitions ( – 4p ³P₂) gives rise to the Si i 10 827 Å line. Solid and dashed-dotted lines indicate, respectively, bb and bf radiative transitions. The bf transitions are shown only for the simplest model. See the main text for further information.