Hydrogen-Alpha line

The Hydrogen Alpha (Hα) line is a characteristic emission line in the red part of the visible spectrum, and is created when the electron of a hydrogen atom drops from the third to the second orbit (N=3 and N=2) and the energy difference between the orbits is emitted as electromagnetic radiation. The wavelength at which this occurs is 656 nm. Because hydrogen is the most abundant element in the universe this is often the brightest emission line in interstellar space. Because the N=3 to N=2 transition often occurs where hydrogen is being ionised, such as in the corona of a star, the Hα line is often used by astronomers to measure redshift. [1]