How does the energy absorbed by dust when blocking UV light manifest as a measurable output?
It is re-radiated primarily in the infrared at longer wavelengths.
When energetic ultraviolet light emitted by young, massive stars encounters dust particles mixed within the interstellar medium, this energy is not lost from the system; rather, it is absorbed by the dust grains. This absorption process heats the dust. As the dust cools down, it thermalizes the absorbed energy and re-emits it at much longer wavelengths, predominantly in the infrared portion of the spectrum, especially the Far-Infrared (FIR). This process of absorption and subsequent re-radiation is precisely what makes FIR luminosity an excellent tracer of the *total* radiation field powered by young stars, as it accounts for activity hidden in the dusty regions that block the UV.
