Final answer:
Hydrogen is the most abundant element in the universe, but it is hard to detect in the visible spectrum due to its lack of strong spectral lines at low energies. The 21-cm hydrogen line detected via radio-astronomical studies allows for the mapping of cold interstellar hydrogen, which is essential for understanding cosmic structures and the process of stellar fusion.
Step-by-step explanation:
Radio-astronomical studies of hydrogen are particularly vital due to hydrogen's status as the most abundant element in the universe. Visible spectroscopy often fails to detect hydrogen, especially in the cold interstellar medium, where hydrogen atoms are mostly in the ground state. However, the 21-cm radio wavelength known as the hydrogen line can be detected, which arises from the hyperfine splitting in the ground state of neutral hydrogen (H I). This 21-cm line allows astronomers to map the distribution of interstellar hydrogen which cannot be achieved using visible light telescopes.
Since hydrogen is the primary fuel for stellar fusion reactions and plays a key role in the structure and evolution of stars, understanding its distribution gives us insight into the formation and structure of galaxies as well as the makeup of the universe. The study of radio emission and absorption by interstellar hydrogen is thus a central method for exploring vast amounts of cold hydrogen and obtaining a detailed picture of the cosmos.