Astronomy Colloquium

Dust formation is generally viewed as an act of magic or voodoo, to be viewed with the same level of disbelief as invocations of rotation or magnetic fields.  But the formation of dust is actually a fairly simple problem that has been obscured by invocations of the problems in classical nucleation theory. While classical nucleation theory has many problems, they are not generally relevant to observational problems in astrophysics.                                           
When a transient source ejects mass, dust formation becomes inevitable if the density is sufficiently high and the radiation environment is benign.
High density means that the condensible species forming the dust have a significant collision rate.  A benign radiation environment means that
UV photons cannot destroy fragile, small grains before they can grow.  All the problems in nucleation theory boil down to modest and relatively
uninteresting shifts in when and where the dust forms.  Once dust forms, the future evolution is well-specified and provides strong,
quantitative constraints on the ejecta.  When the dust optical depth is high, the mid-IR emission constrains the luminosity while the evolution of
the optical emission constrains the ejecta mass.   As the optical depth drops, the roles of the wavelengths reverses. I will discuss these
issues in the context of a range of astrophysical sources.