8 - Interstellar Atoms, Molecules, and Cosmic Masers

Summary

In a talk given during the German occupation of the Netherlands, Henk van der Hulst discussed possible 21 cm radio emission from interstellar hydrogen atoms but pessimistically concluded that “the existence of the line remains speculative.” Nearly 20 years later, Harvard University PhD student Harold (Doc) Ewen surprisingly detected the 21 cm hydrogen line using a simple horn antenna sticking out the window of his laboratory and a novel frequency switching radiometer. van de Hulst had also calculated the possibility of detecting radio recombination lines from highly excited galactic hydrogen, but overestimated the effect of line broadening. Although he concluded that radio recombination lines are “unobservable,” they were subsequently detected in the USSR and the US. Observations of surprisingly strong radio emission from hydroxyl and water vapor were understood to be due to interstellar masers, which could have been detected much earlier if anyone had thought to look in the right place. Later discoveries of interstellar formaldehyde and carbon monoxide opened the door to a new and highly competitive field of astrophysics – molecular radio spectroscopy.