Most interstellar matter is too cold to be observed optically. Its radiation is predominantly emitted at longer wavelengths, such as infrared and radio. This behavior is an example of

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Wien's law relates the temperature of an object to the peak wavelength of its emitted radiation. Since interstellar matter is cold, it emits radiation at longer wavelengths, such as infrared and radio, which is consistent with Wien's law. This law states that the peak wavelength of the emitted radiation from a black body is inversely proportional to its temperature.

Most Interstellar Matter Is Too Cold to Be Observed Optically