Astronomy and the radio spectrum. This part of the electromagnetic spectrum can largely pass through the atmosphere. Yellow lines are the main emission wavelengths of carbon monoxide (CO), hydrogen cyanide (HCN), ammonia (NH3), water (H2O), formaldehyde (H2CO) and hydrogen (H). Blue lines are wavelengths that are kept free of human radio traffic to prevent such "noise" drowning out astronomical sources. Short radio waves are absorbed in the lower atmosphere, so instruments to detect them must be on mountains or in space. Longer wavelengths can be detected at ground level. Radio wave sources include the Milky Way (upper center) and a variety of other bodies.