Molecular hydrogen is the most abundant constituent in interstellar space, but is difficult to observe in its most common form. Because H2 has no dipole moment, it does not have allowed rotational or vibrational transitions and therefore in its cold, unexcited state it has no radio or infrared spectral features. Therefore the most sensitive method for detecting cold H2 is through its allowed electronic bands, which lie in the far-ultraviolet portion of the spectrum. Previous UV instruments have provided some information on far-UV spectra of cold H2, but all were limited in either throughput or spectral resolving power, or both. The current FUSE mission has a combination of high throughput and moderately high spectral resolution, and is providing information on molecular hydrogen in interstellar regimes that have been previously unexplored. This review summarizes previous UV observations of H2 and then gives an overview of early FUSE results, with an emphasis on H2 in translucent clouds.

Introduction

The mass in the galactic interstellar medium is dominated by molecular hydrogen, which begins to outweigh atomic hydrogen in diffuse clouds and is expected to become completely dominant for translucent and dense clouds; i.e. clouds having visual extinctions greater than about Av = 2 magnitudes. Even in diffuse clouds, H2 is important, playing a crucial role in cloud chemical and physical processes.

Despite the importance of understanding H2 and its distribution, physics, and chemistry, relatively little direct information is available because of the obtuse spectroscopic properties of the molecule.