Interstellar dust grains do not merely block the light of distant stars, but redirect it in complex ways which, if measurable, give insights into the physical composition of the dust. Reflection nebulae, like the one seen around the Pleiades star cluster, provide natural laboratories for the study of this scattering phenomenon through the measurement of nebular brightness at various wavelengths. However optical and geometric effects are closely intertwined; external constraints are required to unravel the two.
In the Pleiades this is accomplished with the aid of 21cm neutral Hydrogen emission maps and interstellar absorption spectra toward stars in the region, which together constrain the scattering geometry. In so doing, these data reveal a wealth of filamentary structure in the gas clouds on spatial scales smaller than any yet observed. Velocity features in the nebulosity also demonstrate it is not remnant natal material, but the result of an interstellar collision.