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Hydrogen-alpha
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<p>Hydrogen-alpha, typically shortened to H-alpha or Hα, is a deep-red visible <a href="/facts/Spectral_line/HuI4Bj5f">spectral line</a> of the <a href="/facts/Hydrogen_atom/ncVpPbuE">hydrogen atom</a> with a wavelength of 656.28 <a href="/facts/Nanometre/a98sjHyu">nm</a> in air and 656.46 nm in vacuum. It is the first spectral line in the <a href="/facts/Balmer_series/Vc4s4SIt">Balmer series</a> and is emitted when an electron falls from a hydrogen atom's third- to second-lowest energy level. H-alpha has applications in <a href="/facts/Astronomy/0ArJVM1p">astronomy</a> where its emission can be observed from <a href="/facts/Emission_nebula/UzUTgCp8">emission nebulae</a> and from features in the <a href="/facts/Sun/CadPCVHh">Sun</a>'s <a href="/facts/Sun/CadPCVHh">atmosphere</a>, including <a href="/facts/Solar_prominence/ShLDJjCn">solar prominences</a> and the <a href="/facts/Chromosphere/TurLRnLk">chromosphere</a>. </p>