Timeline of knowledge about the 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
1990 -- Foreground "contamination" data from the COBE spacecraft provides the first all-sky map of the ISM in microwave bands.

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