Which type of radiation is used to observe dense interstellar clouds and track the motion of cold gas?

• A. Gamma rays
• B. Radio waves and microwaves
• C. Ultraviolet
• D. Visible light

Answer: (B)

Long-wavelength radiation, like radio waves and microwaves, is ideal for studying dense interstellar clouds because dust and cold gas block shorter wavelengths but let radio waves pass through. In these wavelengths, astronomers detect spectral lines from molecules such as hydrogen and carbon monoxide, and the Doppler shift of those lines reveals how the gas is moving. This lets scientists map both the structure of the cloud and its internal motions. Gamma rays come from energetic processes not typical of cold gas, ultraviolet and visible light are largely blocked by the dust in dense clouds, so they don’t reveal the interior or motion well. So, radio waves and microwaves are used to observe dense interstellar clouds and track the motion of cold gas.