Editing Hydrogen line (section)

==Discovery==
[[File:Green Banks - Ewen-Purcell Horn Antenna.jpg|right|thumb|[[Horn antenna]] used by Ewen and Purcell for the first detection of hydrogen line emission from the [[Milky Way]]]]
During the 1930s, it was noticed that there was a radio "hiss" that varied on a daily cycle and appeared to be extraterrestrial in origin. After initial suggestions that this was due to the Sun, it was observed that the radio waves seemed to propagate from the [[Galactic Center|centre of the Galaxy]]. These discoveries were published in 1940 and were noted by [[Jan Oort]] who knew that significant advances could be made in astronomy if there were [[emission line]]s in the radio part of the spectrum. He referred this to [[Hendrik van de Hulst]] who, in 1944, predicted that [[electric charge|neutral]] [[hydrogen]] could produce radiation at a [[frequency]] of {{val|1420.4058|u=MHz}} due to two closely spaced energy levels in the [[ground state]] of the [[hydrogen atom]].<ref>{{cite journal
 | title=Line Spectra in Radio Astronomy
 | first=E. M. | last=Purcell | author-link=Edward Mills Purcell
 | journal=Proceedings of the American Academy of Arts and Sciences
 | year=1953 | volume=82 | issue=7 | pages=347–349
 | doi=10.2307/20023736 | jstor=20023736 }}</ref>

The 21&nbsp;cm line (1420.4&nbsp;MHz) was first detected in 1951 by [[Harold Irving Ewen|Ewen]] and [[Edward Mills Purcell|Purcell]] at [[Harvard University]],<ref>{{cite journal |last1=Ewen |first1=H.I. |first2=E.M. |last2=Purcell |author2-link=Edward Mills Purcell |date=September 1951 |title=Observation of a line in the galactic radio spectrum: Radiation from galactic hydrogen at 1,420&nbsp;Mc./sec. |journal=[[Nature (journal)|Nature]]|volume=168 |issue=4270 |page=356 |doi=10.1038/168356a0 |bibcode = 1951Natur.168..356E |s2cid=27595927 }}</ref> and published after their data was corroborated by Dutch astronomers Muller and Oort,<ref>{{cite journal |last1=Muller |first1=C.A. |first2=J.H. |last2=Oort |date=September 1951 |title=The interstellar hydrogen line at 1,420&nbsp;Mc./sec., and an estimate of galactic rotation |journal=[[Nature (journal)|Nature]] |volume=168 |issue=4270 |pages=357–358 |doi=10.1038/168357a0 |bibcode = 1951Natur.168..357M |s2cid=32329393 }}</ref> and by [[Wilbur Norman Christiansen|Christiansen]] and Hindman in Australia. After 1952 the first maps of the neutral hydrogen in the Galaxy were made, and revealed for the first time the spiral structure of the [[Milky Way]].<ref>{{cite journal
 | title=The spiral structure of the outer part of the Galactic System derived from the hydrogen emission at 21 cm wavelength
 | display-authors=1 | last1=van de Hulst | first1=H. C.
 | last2=Muller | first2=C. A. | last3=Oort | first3=J. H.
 | journal=Bulletin of the Astronomical Institutes of the Netherlands
 | volume=12 | page=117 | date=May 1954
 | bibcode=1954BAN....12..117V
}}</ref><ref>{{cite journal
 | title=The distribution of atomic hydrogen in the outer parts of the Galactic System
 | last=Westerhout | first=G. | author-link=Gart Westerhout
 | journal=Bulletin of the Astronomical Institutes of the Netherlands
 | volume=13 | page=201 | date=May 1957
 | bibcode=1957BAN....13..201W
}}</ref>