Several investigations have been devoted to understand the physics, maintenance and evolution of extended filamentary structures in quiescent molecular clouds with no on-going star formation. Individual filaments are often composed of clumps or subfilaments. The linear polarization and extinction of light from distant stars passing through nearby molecular clouds was studied. Polarimetric observations at the NOT have been obtained and numerical calculations of the polarization produced by elongated dust grains aligned in magnetic fields have been made. Due to a higher gas-grain collision frequency within the cloud, the polarization caused by the cloud may well be dominated by background/foreground polarization, as is also demonstrated by the observations.
Special attention is given to small-scale, helical, interstellar filaments, like the ``elephant trunk'' structures in the Rosette nebula, which were studied with the NOT. The observed sinusoidal filaments are suggested to be helices lined up by magnetic fields. We propose that the Rosette elephant trunks form an interconnected system of rope-like structures which are relics from filamentary skeletons of magnetic fields in the primordial cloud. An interesting consequence of helical magnetic fields is the implication that electric currents are driven through the filaments.
Stochastic mass fractionation of a molecular cloud was simulated numerically. It was found that geometry alone may constrain the resulting mass spectrum of molecular cloud clumps. Further fragmentation of the cloud clumps, under the assumption of a lower limit of the self-similar regime, produces a mass spectrum that has qualitative and quantitative similarities with the empirically determined stellar initial mass function.