Editing Lyman-alpha (section)

{{Short description|Spectral line of hydrogen in the Lyman series}}
[[File:All-sky New Horizons Alice Lyα maps.jpg|thumb|upright|The All-sky [[New Horizons]] Alice Lyα maps in ecliptic coordinates centered on the anti-Sun direction, and the lower map is in Galactic coordinates. The c.&nbsp;90,000 stars in the M. A. Velez et al. (2024) catalog are overlaid as black dots, where the size of the dot is proportional to the logarithm of the expected Alice count rate from each star. The map in Galactic coordinates indicates the outlines of four of the important LISM clouds ("LIC", in red; "Aql" in green; "Blue", in blue; and "G", in tan).<ref name=lyman>{{Cite journal |last1=Gladstone |first1=G. Randall |last2=Shull |first2=J. Michael |last3=Pryor |first3=Wayne R. |last4=Slavin |first4=Jonathan |last5=Kammer |first5=Joshua A. |last6=Becker |first6=Tracy M. |last7=Lauer |first7=Tod R. |last8=Postman |first8=Marc |last9=Spencer |first9=John R. |last10=Parker |first10=Joel Wm. |last11=Retherford |first11=Kurt D. |last12=Velez |first12=Michael A. |last13=Versteeg |first13=Maarten H. |last14=Davis |first14=Michael W. |last15=Froning |first15=Cynthia S. |date=April 2025 |title=The Lyα Sky as Observed by New Horizons at 57 au |journal=The Astronomical Journal |language=en |volume=169 |issue=5 |pages=275 |doi=10.3847/1538-3881/adc000 |doi-access=free |issn=1538-3881}}</ref>]]
'''Lyman-alpha''', typically denoted by '''Ly-α''' or '''Lyα''', is a [[spectral line]] of [[hydrogen]] (or, more generally, of any [[Hydrogen-like atom|one-electron atom]]) in the [[Lyman series]]. It is emitted when the atomic [[electron]] transitions from an ''n''&nbsp;=&nbsp;2 [[atomic orbital|orbital]] to the [[ground state]] (''n''&nbsp;=&nbsp;1), where ''n'' is the [[principal quantum number]]. In hydrogen, its [[wavelength]] of 1215.67 [[angstrom]]s ({{val|121.567|u=nm}} or {{val|1.21567e-7|u=m}}), corresponding to a [[frequency]] of about {{val|2.47e15|u=Hz}}, places Lyman-alpha in the [[ultraviolet]] (UV) part of the electromagnetic spectrum. More specifically, Ly-α lies in [[vacuum ultraviolet|vacuum UV]] (VUV), characterized by a strong absorption in the [[air]].