Timeline of the interstellar medium 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