Hα concentration index, C_Hα, plotted against R-band central light concentration, C₃₀, for field, supercluster and cluster Sa–Sc galaxies. The dashed lines show the values of C_Hα at which an object has three-quarters, half and one-quarter (top to bottom) of its Hα emission within 0.3r₂₄. The solid line has equation C_Hα = 1.269C₃₀ − 0.170 and is the best fit line to the field data. Cluster Sa–Sc spirals tend to have more concentrated Hα emission, and hence star formation, relative to R-band concentration, than their field counterparts. The cluster galaxies lying at C_Hα = 0 are those for which no Hα was detected. Typical error bars are shown in the upper-right corner of each frame.