21 cm radiation

What is 21 cm radiation? What element creates this type of radiation? Why is the 21 cm line useful to astronomers? 21-centimeter line or Hydrogen line is ground state hydrogen atom changing from a higher energy state to a lower energy state. It is emitted by neutral hydrogen having a frequency of 1420 megahertz and wavelength of 21 Centimeters. The element that creates this type of radiation is Hydrogen. It is used to study the amount and velocity of hydrogen in the galaxy.