  The use of ultraviolet (UV) emission as a tracer of galaxy star-formation
rate (SFR) is hampered by dust obscuration. The empirical relationship between
UV slope, $\beta$, and the ratio between far-infrared and UV luminosity, IRX,
is commonly employed to account for obscured UV emission. We present a simple
model that explores the physical origin of variations in the IRX - $\beta$ dust
attenuation relation. A relative increase in FUV attenuation compared to NUV
attenuation and an increasing stellar population age cause variations towards
red UV slopes for a fixed IRX. Dust geometry effects (turbulence, dust screen
with holes, mixing of stars within the dust screen, two-component dust model)
cause variations towards blue UV slopes. Poor photometric sampling of the UV
spectrum causes additional observational variations. We provide an analytic
approximation for the IRX - $\beta$ relation invoking a subset of the explored
physical processes (dust type, stellar population age, turbulence). We discuss
observed variations in the IRX - $\beta$ relation for local (sub-galactic
scales) and high-redshift (normal and dusty star-forming galaxies, galaxies
during the epoch of reionization) galaxies in the context of the physical
processes explored in our model. High spatial resolution imaging of the UV and
sub-mm emission of galaxies can constrain the IRX - $\beta$ dust attenuation
relation for different galaxy types at different epochs, where different
processes causing variations may dominate. These constraints will allow the use
of the IRX - $\beta$ relation to estimate intrinsic SFRs of galaxies, despite
the lack of a universal relation.
