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Lyman alpha (Ly-α or Lyα) is a hydrogen spectral line, the first line in the Lyman series and of interest for astronomical observation. It has a wavelength of 121.6 nm, in the extreme ultraviolet (EUV) range (as is the whole Lyman series) and cannot be observed from the ground unless considerably redshifted. Ly-α emission lines in galaxies (Lyman-alpha emitters) are a sign of star formation and at cosmological distances, it a suitable line for determining the galaxy's redshift. It is also used to detect clouds at such long distances by noting the absorption lines in the EMR of quasars at the wavelengths indicating Ly-α various redshifts: the collection of such absorption lines is termed the Lyman-alpha forest. Similarly, the Gunn-Peterson trough is the effects of Ly-α absorption before recombination.
Lyα is also of interest regarding extra-solar planets: detection of absorption lines in the atmosphere through differential spectroscopy allows the spectral line shape to be analyzed, which can reveal whether the velocity is consistent with atmospheric escape.
The term Lyman alpha also applies to corresponding lines of any ion with just one electron. For 4He, the wavelength is less than 1 nm different than that of 1H. (Deuterium's Lyman alpha is also less than 1 nm different.)
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