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.
Try Another Word