Chronology of Interstellar and Intergalactic Medium

1848 Lord Rosse studies M1 and names it the Crab Nebula.
1864 William Huggins studies the spectrum of the Orion Nebula and shows that it is a cloud of gas.
1927 Ira Bowen explains unidentified spectral lines from space as forbidden transition lines.
1930 Robert Trumpler discovers absorption by interstellar dust by comparing the angular sizes and brightnesses of globular clusters.
1944 Hendrik van de Hulst predicts the 21 cm hyperfine line of neutral interstellar hydrogen.
1951 H.I. Ewen and Edward Purcell observe the 21 cm hyperfine line of neutral interstellar hydrogen.
1956 Lyman Spitzer predicts coronal gas around the Milky Way.
1965 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.
1969 Lewis Snyder, David Buhl, Ben Zuckerman, and Patrick Palmer find interstellar formaldehyde.
1970 Arno Penzias and Robert Wilson find interstellar carbon monoxide.
1970 George Carruthers observes molecular hydrogen in space.
1977 Christopher McKee and Jeremiah Ostriker propose a three component theory of the interstellar medium.