We describe a sub-galactic main sequence (SGMS) relating star formation rate surface density ($Sigma_{textrm{SFR}}$) and stellar mass density ($Sigma_{star}$) for distinct regions within star forming galaxies, including their nuclei. We use a sample of 246 nearby star-forming galaxies from the Star Formation Reference Survey and demonstrate that the SGMS holds down to $ sim $1 kpc scales with a slope of $alpha=0.91$ and a dispersion of 0.31 dex, similar to the well-known main sequence (MS) measured for globally integrated star formation rates (SFRs) and stellar masses. The SGMS slope depends on galaxy morphology, with late-type galaxies (Sc$-$Irr) having $alpha = 0.97$ and early-type spirals (Sa$-$Sbc) having $alpha = 0.81$. The SGMS constructed from sub-regions of individual galaxies has on average the same characteristics as the composite SGMS from all galaxies. The SGMS for galaxy nuclei shows a dispersion similar to that seen for other sub-regions. Sampling a limited range of SFR$-$M$_{star} $ space may produce either sub-linearity or super-linearity of the SGMS slope. For nearly all galaxies, both SFR and stellar mass peak in the nucleus, indicating that circumnuclear clusters are among the most actively star-forming regions in the galaxy and the most massive. The nuclear SFR also correlates with total galaxy mass, forming a distinct sequence from the standard MS of star-formation. The nuclear main sequence will be useful for studying bulge growth and for characterizing feedback processes connecting AGN and star formation.
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