  Conditions within the Homunculus nebula around Eta Car are determined by many
of the same physical processes that occur in molecular clouds in the
interstellar medium. But there is one major exception -- we know when the
ejection occurred and something about its composition and initial state. The
gas was warm, ionized, and dust-free when it was located within the star's
atmosphere and it is currently cold, molecular, and dusty. It undertook this
transformation in a bit over 150 years. It offers a laboratory for the study of
physical processes in a well-constrained environment. We derive a
photoionization model of the Homunculus nebula that reproduces many of its
observed properties. We conclude by outlining how observations of the
Homunculus could address basic problems in the physics of the interstellar
medium.
