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. |