The relationship between stellar visual surface brightness and colour

Synthetic stellar spectra, computed for model stellar atmospheres for giant stars |$6000 K \geqslant T _{eff} \geqslant 4000 K$| and for a Vega model, are used to study the dependence of stellar visual surface brightness (Fv) on colour. The agreement with observational correlations for Population I stars appears to be quite satisfactory. The models show that whilst the basic relationship is a thermal one, metal abundance effects do occur in the Fv–(B–V), Fv–(V–R) and Fv–(R–I) diagrams and gravity effects occur in the Fv–(B–V) diagram. The combined effects of metal abundance and gravity are smallest in the Fv–(V–R) diagram and the relationship between Fv and (V–R) agrees very well with that found observationally by Barnes & Evans.