What is Interstellar Space

The detection of interstellar gas again goes back to observations of stars. 

In 1904, lines of singly ionized Ca were discovered in absorption in the spectrum of the binary star 6 Ori and it became clear that these lines were not associated with the stellar atmosphere but rather with the interstellar space between this star and the sun. However, three decades elapsed before the conception of interstellar matter as a gas, mostly hydrogen and helium intimately mixed with dust particles, emerged and became generally accepted. 

About 1940, three interstellar radicals were detected through their UV absorption lines in stellar spectra. This made it clear that interstellar matter had another component Interstellar molecules. 

A major step forward in our knowledge of interstellar matter was made in 1951 with the discovery of the first interstellar spectral line in the radio range, the famous X21 cm hyperfine structure line of atomic hydrogen. Most of our present knowledge of the distribution and physical state of interstellar matter in our Galaxy is based on observations of this line. At first it looked as 

To summarize the distances between stars are larger by many orders of magnitude than their diameters. The interstellar space, however, is not empty it is filled with gas, dust particles and radiation consisting of both photons and high-energy particles. The density and temperature of the gas vary over a wide range. In the vicinity of very hot stars a large volume of interstellar gas is almost fully ionized. 

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