At the turn of the century, astronomers wanted to know whether matter existed between the stars, and if so, whether it affected their readings of starlight. Otto Struve, a Russian-American astronomer, discovered that interstellar matter, particularly hydrogen, pervades the galaxy. Today would have been his 100th birthday.
By 1904, interstellar calcium had been detected near some stars. Struve and his co-worker G.P. Gerasimov expanded on that work, finding in 1929 that interstellar calcium existed much more widely. And in 1937, Struve's studies of the wavelengths of radiation revealed the presence of interstellar hydrogen, in ionized form, which was more prevalent than calcium but more difficult to detect.
Struve also helped found and plan the second largest reflecting telescope in the world, at the McDonald Observatory on Mount Locke in Texas, which he directed from 1939 to 1950.
[Source: John Dainteth, et al., Eds. Biographical Encyclopedia of Scientists, Second Edition. Institute of Physics Publishing. Bristol and Philadelphia. 1994.]