The hydroxyl radical (OH) was the first species detected in the interstellar medium
by means of its radio (rotational) spectrum, by
Weinreb et al. in 1963.
It was first observed toward the supernova remnant
Many additional detections followed, including the first
detection in 1971. It was observed in
comet Cunningham in 1940 by
Swings et al. in the ultraviolet.
It was also detected in comets
Biraud et al. and
Stacey et al., respectively.
A large part of the
Wiki article on OH discusses its astronomical
significance (note however that the image of the orbital with the unpaired electron is incorrect).
OH is a very important species. It is very reactive and plays critical roles in both