Models of Interstellar Dust:
             Optical Extinction and Infrared Emission

                       B. T. Draine
                   Princeton University

Most of the atoms of elements like Si and Fe in the interstellar
medium are incorporated into dust grains that absorb and scatter
light, much to the annoyance of some astronomers.  This dust plays an
important role in interstellar gas dynamics, heating and cooling of
the gas, chemistry, and even ionization balance in predominantly
neutral regions.  It is therefore very important to try to figure out
what it is!  Interstellar dust modellers have historically focussed
on reproducing the observed wavelength-dependent extinction, and this
by itself provides strong constraints on models.

Observations with Spitzer Space Telescope are allowing new tests for
models for interstellar dust.  In particular, we can now ask whether
our models of interstellar dust, heated by starlight, produce infrared
emission with the spectral energy distribution measured by Spitzer.
The emission features attributed to polycyclic aromatic hydrocarbons
are present in the emission from other galaxies; we can now estimate
the PAH abundances in other galaxies, and compare with what we
estimate for the Milky Way.