A brief history of observations of H2 and HD molecules is presented. The properties of H2 and HD that make observations of them a uniquely powerful diagnostic probe are pointed out. The interpretations of the observations in the ultraviolet, near infrared and infrared made of a diverse range of astrophysical objects are discussed and the influence of H2 and HD on their evolution is described.

Introduction

The hydrogen molecule occupies a central place in astrophysics. There are many reasons. The hydrogen molecule was the first neutral molecule to be formed in the Universe and it played a crucial role in the collapse of the first cosmological objects. It is the most abundant molecular species in the Universe, able to survive in hostile environments and found to exist in diverse astronomical objects ranging from planets to quasars. Hydrogen molecules have been detected by the absorptions and emissions at ultraviolet and infrared wavelengths to which they give rise. Molecular hydrogen has specific radiative and collisional properties that make it a diagnostic probe of unique capability.

Ultraviolet absorption

The first detection of H2 beyond the solar system was accomplished by Carruthers (1970) who employed a rocket-borne ultraviolet spectrometer and detected absorption bands of the Lyman system of H2 looking towards the star ξ Persei. More extensive data at much higher spectral resolution were obtained with a spectrometer aboard the Copernicus satellite (Spitzer and Jenkins 1975).