What specific wavelength emission is associated with neutral hydrogen's spin-flip transition?

21 centimeters

Neutral hydrogen serves as a vital historical probe for radio astronomy due to a specific, predictable quantum event within its ground state. This event is known as a spin-flip transition, which causes the atom to emit radio waves at a very precise frequency, corresponding to a wavelength of 21 centimeters. This specific emission, sometimes referenced by its frequency of 1420 megahertz, is crucial because these radio waves can penetrate the dense, light-obscuring dust clouds found throughout interstellar space. By measuring the Doppler shift of this 21-cm signal across the galaxy, astronomers can accurately map the velocity and distribution of neutral gas clouds, providing critical data for understanding galactic structure, such as spiral arms and rotation curves.