Final answer:
Atomic hydrogen in the Milky Way is primarily observed with radio telescopes using the 21-cm line, enabling astronomers to study and map the galaxy's hydrogen distribution despite dust that obscures other wavelengths.
Step-by-step explanation:
The primary way that we observe the atomic hydrogen that makes up most of the interstellar gas in the Milky Way (MW) is with radio telescopes.
This is because neutral hydrogen atoms at the temperatures typical of the interstellar space do not emit or absorb light in the visible part of the spectrum. However, these hydrogen atoms are detectable in the radio part of the spectrum, particularly at a wavelength of 21-cm.
The 21-cm line is a specific radio frequency that comes from the hyperfine transition of neutral hydrogen. These observations are critical because radio waves can pass through interstellar dust that obscures other wavelengths, allowing us to map the hydrogen distribution in the galaxy, providing insights into the MW's spiral structure.
The lack of spectral features in the visible light for atomic hydrogen and the discovery of the 21-cm line has played a significant role in the study of astronomy and understanding the distribution of interstellar matter in the Milky Way.