Final answer:
Atomic hydrogen gas is hydrogen in its atomic state, which is rare on Earth but the most common element in the universe. It is mapped through radio observations that detect the specific 21 cm line emitted when hydrogen electrons flip spins.
Step-by-step explanation:
Atomic hydrogen gas refers to hydrogen in its atomic form rather than its molecular form, H2. On Earth, molecular hydrogen is rare as hydrogen bonds readily with other elements, primarily with oxygen to form water. The elemental hydrogen that does exist is in trace amounts, roughly 0.5 parts per million in the atmosphere, because it is incredibly light and can escape Earth's gravity.
Interstellar hydrogen, however, is quite different. It is the most abundant element in the universe and is often used to characterize different regions of space. We've learned that most of the neutral hydrogen in the Milky Way galaxy is found in a layer less than 300 light-years thick, interspersed with temperatures ranging from about 100 K to about 8000 K.
To map the distribution of atomic hydrogen gas in space, radio observations are utilized. These observations look for a specific wavelength known as the 21 cm line, which is radiated when the electron in a hydrogen atom flips its spin. As the atoms are very sparse in space, such transitions are infrequent, making the detection of this wavelength an effective way to locate and map hydrogen.