  Star patterns, reminiscent of a wide range of diffusively controlled growth
forms from snowflakes to Saffman-Taylor fingers, are ubiquitous features of ice
covered lakes. Despite the commonality and beauty of these ``lake stars'' the
underlying physical processes that produce them have not been explained in a
coherent theoretical framework. Here we describe a simple mathematical model
that captures the principal features of lake-star formation; radial fingers of
(relatively warm) water-rich regions grow from a central source and evolve
through a competition between thermal and porous media flow effects in a
saturated snow layer covering the lake. The number of star arms emerges from a
stability analysis of this competition and the qualitative features of this
meter-scale natural phenomena are captured in laboratory experiments.
