re temperature of the dark sky in article apr princeton edu richmond spiff princeton edu stupendous man writes if that s the case let me point out that interstellar dust and molecules provide many instances of things that are well not too far from being blackbodies many different observations including iras and cobe have determined that interstellar dust grain temperatures can range from k to k interstellar grains are not at all close to blackbodies the large grains have sizes of order micron and absorb visible light with fair efficiency however at temperatures below k of the thermal emission will be beyond 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 microns grains colder than k will radiate primarily at longer wavelengths and iras would be relatively insensitive to them in the extreme limit grains as cold as 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 atoms and when they are hit by a single photon they heat up to temperatures of several hundred or 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 k thermal emission from zodiacal dust tc k and thermal emission from galactic dust tc k at the ecliptic poles the emissivities or dilution factors are respectively e e and e 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 zero mag stars with b v color of no doubt careful work could do much better the person who suggested starlight had a dilution factor of e 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 microwave background galactic dust e zodiacal dust emission e zodiacal dust scattering e starlight e the fourth root of is k outside the solar system the result would be k i find these results surprising especially the importance of zodiacal dust but i don t see any serious mistakes steve willner phone bitnet willner cfa cambridge ma usa internet willner cfa harvard edu member league for programming freedom contact lpf uunet uu net
