In astronomy, which radiation is used to study 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 is ideal for studying dense, dusty regions because dust blocks visible and ultraviolet light but is transparent to radio waves and microwaves. In these wavelengths, astronomers observe spectral lines from molecules like carbon monoxide and other cold-gas tracers, which come from rotational and vibrational transitions in the gas. By analyzing these lines, you can determine what the gas is made of and its temperature. The Doppler shifts of the lines also reveal how the gas is moving along the line of sight, so you can track the motion within the cloud. So radio waves and microwaves are used to study dense interstellar clouds and the kinematics of cold gas. Gamma rays, ultraviolet, and visible light don’t provide the same penetrating view or tracers for cold, dense gas, and infrared, while also useful, isn’t the focus here.