A radio emission line at 1420.4 MHz (21.1 cm wavelength) produced by the hyperfine structure transition in neutral hydrogen atoms. This line is fundamental to radio astronomy as it allows mapping of hydrogen gas throughout the universe.
Named simply for its wavelength of 21.1 centimeters. The line was predicted theoretically by Hendrik van de Hulst in 1944 and first detected in 1951, opening the field of radio astronomy to study the structure of our galaxy.
The 21-cm line is hydrogen's whisper that revolutionized astronomy! This incredibly faint radio signal - so weak that all the 21-cm radiation ever collected by radio telescopes has less energy than a snowflake hitting the ground - revealed the spiral structure of our galaxy and is now used to search for the universe's first stars.
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