He began to study interstellar matter (ISM) in the late 1930s. Noting that elliptical galaxies by contrast lacked ISM and contained old stars, he suggested that the bright stars seen in the arms of spiral galaxies had recently condensed from interstellar gas and dust. After World War II, he extended a pre-war interest on the effects of magnetic fields on the ISM as a trigger for star formation to techniques for containing a plasma in the laboratory, as a step towards utilizing nuclear fusion as a power source. Spitzer’s work on the dynamics of star clusters led to the concept of relaxation time. His advocacy of an Earth-orbiting telescope led to the development of the Hubble Space Telescope. The Spitzer Space Telescope was named in his memory.