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The extended emission in the infrared celestial background maybe divided into three main components: the zodiacal background, the large discrete sources in the galaxy and the interstellar dust. The zodiacal background is due to the thermal re-radiation of sunlight absorbed by the dust in the solar system. An earth orbiting infrared telescope will detect the diffuse emission from this dust in all directions with maximum intensity lying roughly along the ecliptic plane where the density of dust is highest. Structure with scale lengths of 10° have been measured in both the visual and infrared; finer structure has been detected in the infrared by the Infrared Astronomy Satellite (IRAS). H II regions, areas of ionized gas mixed with and surrounded by dust, are the brightest discrete objects in the galaxy in the long wavelength infrared (LWIR, 7-30km). The visible radiation from the hot star(s) embedded in these regions is absorbed by the dust and re-emitted in the infrared with a range of temperatures characteristic of the thermal equilibrium for the surroundings of the dust. These regions are relatively large and, if close to the sun, can subtend a significant angular area of sky. The emission from the interstellar dust produces a filimentary structured background, the infrared "cirrus". The observed far infrared color temperature of ~20-35K for the cirrus is consistent with emission from graphite and silicate grains which absorb the interstellar radiantion field. The much larger LWIR color temperature is likely due to a greater abundance of submicron particles in the interstellar medium and, perhaps, from band emission due to polycyclic aromatic hydrocarbons. These galactic sources combine along the line of sight to produce an intense band of emission centered on the galactic plane which has full width at half maxima of about 2°.
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