- 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
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