Editing Hydrogen line (section)

===In radio astronomy===
The 21&nbsp;cm spectral line appears within the [[radio spectrum]] (in the [[L band|L&nbsp;band]] of the [[Ultra high frequency|UHF band]] of the [[microwave window]]). Electromagnetic energy in this range can easily pass through the Earth's atmosphere and be observed from the Earth with little interference.<ref>{{cite book
 | title=The American Practical Navigator: An Epitome of Navigation. 2002 Bicentennial Edition
 | first=Nathaniel | last=Bowditch | author-link=Nathaniel Bowditch
 | publisher=National Imagery and Mapping Agency
 | chapter=10. Radio Waves | page=158 | year=2002
 | chapter-url=https://thenauticalalmanac.com/Bowditch-%20American%20Practical%20Navigator/Chapt-10%20RADIO%20WAVES.pdf
 | access-date=2023-04-28
 | quote="Skywaves are not used in the UHF band because the ionosphere is not sufficiently dense to reflect the waves, which pass through it into space. ... Reception of UHF signals is virtually free from fading and interference from atmospheric noise."
}}</ref> The hydrogen line can readily penetrate clouds of interstellar [[cosmic dust]] that are [[opacity (optics)|opaque]] to [[visible light]].<ref>{{cite book
 | title=The Fullness of Space
 | first=Gareth | last=Wynn-Williams | year=1992
 | page=36 | isbn=9780521426381
 | publisher=Cambridge University Press
 | url=https://books.google.com/books?id=wjxrloC2gyMC&pg=PA36
}}</ref> Assuming that the hydrogen atoms are uniformly distributed throughout the galaxy, each line of sight through the galaxy will reveal a hydrogen line. The only difference between each of these lines is the Doppler shift that each of these lines has. Hence, by assuming [[circular motion]], one can calculate the relative speed of each arm of our galaxy. The [[rotation curve]] of our galaxy has been calculated using the {{Val|21|u=cm}} hydrogen line. It is then possible to use the plot of the rotation curve and the velocity to determine the distance to a certain point within the galaxy. However, a limitation of this method is that departures from circular motion are observed at various scales.<ref>{{cite journal
 | title=The Large-Scale Distribution of Hydrogen in the Galaxy
 | last=Kerr | first=Frank J. | author-link=Frank John Kerr
 | journal=Annual Review of Astronomy and Astrophysics
 | volume=7 | page=39 | year=1969
 | doi=10.1146/annurev.aa.07.090169.000351
 | bibcode=1969ARA&A...7...39K }}</ref>

Hydrogen line observations have been used indirectly to calculate the mass of galaxies,<ref>{{cite journal
 | title=Integral Properties of Spiral and Irregular Galaxies
 | last=Roberts | first=Morton S.
 | journal=Astronomical Journal
 | volume=74 | pages=859–876 | date=September 1969
 | doi=10.1086/110874 | bibcode=1969AJ.....74..859R | doi-access=free }}</ref> to put limits on any changes over time of the [[fine-structure constant]],<ref>{{cite journal
 | title=New limits on the possible variation of physical constants.
 | last1=Drinkwater | first1=M. J. | last2=Webb | first2=J. K–
 | last3=Barrow | first3=J. D. | last4=Flambaum | first4=V. V.
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=295 | pages=457–462
 | date=April 1998 | issue=2 | doi=10.1046/j.1365-8711.1998.2952457.x 
 | doi-access=free | arxiv=astro-ph/9711290 | bibcode=1998MNRAS.295..457D
| s2cid=5938714 }}</ref> and to study the dynamics of individual galaxies. The [[magnetic field]] strength of [[interstellar space]] can be measured by observing the [[Zeeman effect]] on the 21-cm line; a task that was first accomplished by [[Gerrit L. Verschuur|G. L. Verschuur]] in 1968.<ref>{{cite journal
 | title=Positive Determination of an Interstellar Magnetic Field by Measurement of the Zeeman Splitting of the 21-cm Hydrogen Line
 | last=Verschuur | first=G. L. | date=September 1968
 | journal=Physical Review Letters
 | volume=21 | issue=11 | pages=775–778
 | doi=10.1103/PhysRevLett.21.775 | bibcode=1968PhRvL..21..775V
}}</ref> In theory, it may be possible to search for [[antihydrogen]] atoms by measuring the [[Polarization (physics)|polarization]] of the 21-cm line in an external magnetic field.<ref>{{cite journal
 | title=The 21 cm absorption line profile as a tool for the search for antimatter in the universe
 | last1=Solovyev | first1=Dmitry | last2=Labzowsky | first2=Leonti
 | journal=Progress of Theoretical and Experimental Physics
 | volume=2014 | issue=11 | id=111E016 | date=November 2014
 | pages=111E01 | doi=10.1093/ptep/ptu142 | bibcode=2014PTEP.2014k1E01S 
| doi-access=free }}</ref>

Deuterium has a similar hyperfine spectral line at 91.6 cm (327 MHz), and the relative strength of the 21 cm line to the 91.6 cm line can be used to measure the deuterium-to-hydrogen (D/H) ratio. One group in 2007 reported D/H ratio in the [[galactic anticenter]] to be 21 ± 7 parts per million.<ref>{{Cite journal |last1=Rogers |first1=A. E. E. |last2=Dudevoir |first2=K. A. |last3=Bania |first3=T. M. |date=2007-03-09 |title=Observations of the 327 MHz Deuterium Hyperfine Transition |url=https://iopscience.iop.org/article/10.1086/511978/meta |journal=The Astronomical Journal |language=en |volume=133 |issue=4 |pages=1625–1632 |doi=10.1086/511978 |bibcode=2007AJ....133.1625R |s2cid=15541399 |issn=1538-3881|url-access=subscription }}</ref>