Hydrogen line
The hydrogen line refers to the spectral line created by changes in the energy state of neutral hydrogen and occurs at 1420.40575 MHz, or a wavelength of around 21cm. The line is used extensively in astronomy, particularly radio astronomy.
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Cause of the hydrogen line
Neutral hydrogen consists of a single proton orbited by a single electron. The proton and electron are in constant motion, both around the nucleus but also spinning around an axis (in a similar fashion to the Earth's motion). The electron and proton can spin around their axis in either direction, and are either spinning in the same direction, or the opposite direction. A Hydrogen atom that has an electron and proton spinning in the same direction (parallel) has slightly more energy that one where the electron and proton are spinning in opposite directions (anti-parallel). If a Hydrogen atom's spin state flips from parallel (electron and proton spinning in the same direction) to anti-parallel (electron and proton spinning in opposite directions) energy is released in the form a low energy photon. This corresponds to the spectral line that is seen at wavelengths of 21cm (frequency of 1.420 GHz).
Discovery
The hydrogen line was first predicted in 1941, but was not formally discovered untill 1951 when a Harvard graduate student Harold Ewen and his professor Prof. Edward Purcell developed the requisite apparatus and made the observation.
Uses in radio astronomy
Luckily, the spectral line appears within the radio spectrum (in the microwave window to be exact). The line is particularly useful because hydrogen is the most abundant substance in the interstellar medium, making up nearly 80% of the universe's mass.
Assuming that the hydrogen atoms are uniformly distributed throughout the galaxy, each line of sight through the galaxy will reveal a Hydrogen Line. The only difference between each of these lines is the doppler shift that each of these lines have. Hence, one can calculate the relative speed of each arm of our galaxy. The rotation curve of our galaxy has also been calculated using the 21-cm Hydrogen line. It is then possible to use the plot of the rotation curve and the velocity to determine the distance to a certain point within the galaxy.
Hydrogen line observations have also been used indirectly to calculate the mass of galaxies, the universal gravitational constant and dynamics of individual galaxies. It has also been used to explain the structure of quasars.