
Interstellar grains are not at all close to blackbodies.  The "large"
grains have sizes of order 0.1 micron and absorb visible light with
fair efficiency.  However, at temperatures below 100 K, 90% of the
thermal emission will be beyond 22 microns, where radiating
efficiency is poor.  (A small antenna cannot easily radiate at long
wavelengths.)  Thus the grains must heat up more in order to radiate
the energy they have absorbed.

Moreover, the IRAS observations had a maximum wavelength of 100
microns.  Grains colder than 30 K will radiate primarily at longer
wavelengths, and IRAS would be relatively insensitive to them.  In
the extreme limit, grains as cold as 5 K will be almost undetectable
by any conceivable observation.

Worse still, IRAS color temperatures are heavily contaminated by a
population of "small" grains.  These grains have only perhaps 50
atoms, and when they are hit by a single photon they heat up to
temperatures of several hundred or 1000 K.  Of course they cool
quickly and then stay cold for a while, but _when they are radiating_
the characteristic temperature is several hundred K.  Even a small
population of these grains can dramatically raise the observed
"average" temperature.

A model for local infrared emission consistent with COBE data has
three components.  These represent scattered radiation from Zodiacal
dust (color temperature 5500 K), thermal emission from Zodiacal dust
(Tc = 280 K), and thermal emission from Galactic dust (Tc=25 K).  At
the ecliptic poles, the emissivities or dilution factors are
respectively 1.9E-13, 4E-8, and 2E-5.  The first two are roughly
doubled in the ecliptic plane.

To find the thermal equilibrium temperature, we add up the dilution
factor times the fourth power of temperature for all components, then
take the fourth root.  In the table below, starlight comes from
Allen's number that stellar emission from the whole sky is equivalent
to 460 zero mag stars with B-V color of 0.75.  No doubt careful work
could do much better.  (The person who suggested starlight had a
dilution factor of E-4 must have been remembering wrong.  We would be
cooked if that were the case.  In any event, the energy density of
starlight comes out about the same as that of the microwave
background, and I believe that to be correct.)

                         Dilution   Temp.   DT^4
Microwave background         1        2.7     53
Galactic dust              2E-5      25        8
Zodiacal dust (emission)   6E-8     280      369
Zodiacal dust (scattering) 3E-13   5500      275
Starlight                  1E-13   5500       92
                                           -----
                                             797

The fourth root of 797 is 5.3 K.  Outside the Solar system, the
result would be 3.5 K.  

I find these results surprising, especially the importance of
Zodiacal dust, but I don't see any serious mistakes.
