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Timeline of knowledge about theinterstellar medium andintergalactic medium:

• 1848 –Lord Rosse studiesM1 and names it theCrab Nebula. The telescope is much larger than the small refactors typical of this period and it also reveals the spiral nature of M51.
• 1864 –William Huggins studies thespectrum of theOrion Nebula and shows that it is a cloud of gas.
• 1904 – Interstellar calcium detected on spectrograph at Potsdam.^[1]
• 1909 – Slipher confirms Kapteyn's theory of interstellar gas.^[2]
• 1912 – Slipher confirms interstellar dust.^[2]
• 1927 –Ira Bowen explains unidentifiedspectral lines from space asforbidden transition lines.
• 1930 –Robert Trumpler discovers absorption byinterstellar dust by comparing the angular sizes and brightnesses ofglobular clusters.
• 1944 –Hendrik van de Hulst predicts the21 cm hyperfine line of neutral interstellarhydrogen.
• 1951 –Harold I. Ewen andEdward Purcell observe the 21 cmhyperfine line of neutral interstellar hydrogen.^[3]
• 1956 –Lyman Spitzer predictscoronal gas around theMilky Way.
• 1965 –James Gunn andBruce Peterson use observations of the relatively low absorption of the blue component of theLyman-alpha line from3C9 to strongly constrain the density and ionization state of the intergalactic medium.^[4]
• 1969 –Lewis Snyder,David Buhl,Ben Zuckerman, andPatrick Palmer findinterstellar formaldehyde.
• 1970
  • Arno Penzias andRobert Wilson find interstellarcarbon monoxide.^[5]
  • George Carruthers observesmolecular hydrogen in space.
• 1977 –Christopher McKee andJeremiah Ostriker propose a three component theory of the interstellar medium.
• 1990 – Foreground "contamination" data from theCOBE spacecraft provides the first all-sky map of the ISM inmicrowave bands.

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