Astronomy Colloquium

Alignment is one of the most fundamental properties of cosmic dust. Grain alignment is believed usually believed to happen in respect to magnetic field and thus provides a way of tracing magnetic fields with extinction and emission polarimetry. Potentially, this is also a great way to use polarization measurements to test the dust properties, e.g. magnetic properties, and dust environment. This, however, requires a quantitative understanding of grain alignment, which has been missing for nearly 60 years. I shall show that the textbook solutions of the alignment problem are not tenable for most of the interstellar grains. Instead, I shall identify grain helicity, which stems from their irregularity, as the most important component of the alignment theory. I shall demonstrate a simple analytical model of a helical grain that is able to explain the existing observational data (including the cases when the alignment fails) and thus allows reliable studies of magnetic fields. I shall discuss observations that can determine whether aligned grains contain superparamagnetic particles, as well as situations when we expect aligned grains to fail tracing of magnetic fields.