The unfettered access to the infrared spectrum in space gave ISO a tremendous advantage in studying any infrared line in galaxies regardless of redshift, until the lines leave the window at high redshifts. The low thermal backgrounds of a cryogenic telescope in space allowed it to tackle much lower surface brightness sources. As a result, ISO extended the spectroscopic studies of starburst nuclei that the Kuiper Airborne Observatory (KAO) had carried out to the less intense star formation in normal galaxies. The spectrocopic capabilities of all four instruments were especially valuable tools in characterizing the interstellar gas and radiation field, and in constraining the overall energetics and star formation rate.
Many lines in the infrared range carry substantially more luminosity than the lines most studied from the ground and normally used for probing the non-ionized ISM. The HI 21 cm fine-structure line, popular because its flux can be safely assumed to be proportional to the total emitting HI mass, carries about 10-9 LFIR. The CO pure rotational lines in the millimeter and submillimeter range (J = 1 -> 0, 2 -> 1, 3 -> 2, etc. at 2.3 mm, 1.15 mm, etc.) carry a few times 10-6 LFIR. By contrast, the molecular hydrogen rotational lines discussed below carry a few times 10-4 LFIR, and several of the infrared fine-structure lines such as [CII], [OI], [SiII], or [OIII] carry 10-3 to 10-2 LFIR. They are thus by far more significant as a measure of the energetics of interstellar gas, and easy to detect at greater distances, making them valuable tools for probing the star formation process in the most distant, youngest galaxies.