View source for Timeline of knowledge about the interstellar and intergalactic medium

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{{More citations needed|date=December 2009}}
[[Timeline]] of knowledge about the [[interstellar medium]] and [[Outer space#Intergalactic space|intergalactic medium]]:

* 1848 – [[William Parsons, 3rd Earl of Rosse|Lord Rosse]] studies [[Messier object|M1]] and names it the [[Crab Nebula]]. The telescope is much larger than the small refactors typical of this period and it also reveals the spiral nature of M51.
* 1864 – [[William Huggins]] studies the [[optical spectrum|spectrum]] of the [[Orion Nebula]] and shows that it is a cloud of gas.
* 1904 – Interstellar calcium detected on spectrograph at Potsdam.<ref name=":5">{{Cite book|url=https://books.google.com/books?id=VYvQ_8I_kTwC&q=hartmann+interstellar+matter+potsdam&pg=PA154|title=The Cosmic Connection: How Astronomical Events Impact Life on Earth|last=Kanipe|first=Jeff|date=2011-01-27|publisher=Prometheus Books|isbn=9781591028826|language=en}}</ref>
* 1909 – Slipher confirms Kapteyn's theory of interstellar gas.<ref name=":0">{{Cite web|url=http://www.azarchivesonline.org/xtf/view?docId=ead/lowell/VMSlipher.xml|title=V. M. Slipher Papers, 1899-1965.|website=www.azarchivesonline.org|access-date=2019-11-29}}</ref>
* 1912 – Slipher confirms interstellar dust.<ref name=":0" />
* 1927 – [[Ira Sprague Bowen|Ira Bowen]] explains unidentified [[spectral line]]s from space as [[forbidden line|forbidden transition line]]s.
* 1930 – [[Robert Julius Trumpler|Robert Trumpler]] discovers absorption by [[interstellar dust]] by comparing the angular sizes and brightnesses of [[globular cluster]]s.
* 1944 – [[Hendrik C. van de Hulst|Hendrik van de Hulst]] predicts the [[21 cm line|21 cm hyperfine line]] of neutral interstellar [[hydrogen]].
* 1951 – [[Harold I. Ewen]] and [[Edward Mills Purcell|Edward Purcell]] observe the 21&nbsp;cm [[hyperfine]] line of neutral interstellar hydrogen.<ref name=nature168_4270/>
* 1956 – [[Lyman Spitzer]] predicts [[Coronal cloud|coronal gas]] around the [[Milky Way]].
* 1965 – [[James Gunn (astronomer)|James Gunn]] and [[Bruce Peterson]] use observations of the relatively low absorption of the blue component of the [[Lyman-alpha line]] from [[3C9]] to strongly constrain the density and ionization state of the intergalactic medium.<ref name=apj142_1633/>
* 1969 – [[Lewis Snyder]], [[David Buhl]], [[Benjamin Zuckerman|Ben Zuckerman]], and [[Patrick Palmer (astronomer)|Patrick Palmer]] find [[interstellar formaldehyde]].
* 1970
** [[Arno Allan Penzias|Arno Penzias]] and [[Robert Woodrow Wilson|Robert Wilson]] find interstellar [[carbon monoxide]].<ref name=apj161_L43/>
** [[George Robert Carruthers|George Carruthers]] observes [[molecular hydrogen]] in space.
* 1977 – [[Christopher McKee]] and [[Jeremiah P. Ostriker|Jeremiah Ostriker]] propose a three component theory of the interstellar medium.
* 1990 – Foreground "contamination" data from the [[Cosmic Background Explorer|COBE]] spacecraft provides the first all-sky map of the ISM in [[microwave]] bands.

==References==
<references>

<ref name=nature168_4270>{{citation | last1=Ewen | first1=H. I. | last2=Purcell | first2=E. M. | title=Observation of a Line in the Galactic Radio Spectrum: Radiation from Galactic Hydrogen at 1,420 Mc./sec. | journal=Nature | volume=168 | issue=4270 | page=356 |date=September 1951 | doi=10.1038/168356a0 | bibcode=1951Natur.168..356E | s2cid=27595927 }}</ref>

<ref name=apj142_1633>{{citation | last1=Gunn | first1=James E. | last2=Peterson | first2=Bruce A. | title=On the Density of Neutral Hydrogen in Intergalactic Space | journal=Astrophysical Journal | volume=142 | pages=1633–1641 |date=November 1965 | doi=10.1086/148444 | bibcode=1965ApJ...142.1633G | doi-access=free }}</ref>

<ref name=apj161_L43>{{citation | last1=Wilson | first1=R. W. | last2=Jefferts | first2=K. B. | last3=Penzias | first3=A. A. | title=Carbon Monoxide in the Orion Nebula | journal=Astrophysical Journal | volume=161 | pages=L43 |date=July 1970 | doi=10.1086/180567 | bibcode=1970ApJ...161L..43W | doi-access=free }}</ref>

</references>

{{DEFAULTSORT:Timeline of knowledge about the interstellar and intergalactic medium}}
[[Category:Astronomy timelines|Interstellar and intergalactic medium]]
[[Category:Astrochemistry]]


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