As we look around our own and nearby galaxies, we see many regions where stars are being born out of great clouds of gas and dust in space. The Great Nebula in Orion is one example of such a region in our Galaxy. As stars form, the light they emit begins to profoundly affect the remaining gas that cocoons them. This gas consists primarily of molecules of hydrogen and simple hydrocarbons similar to those found in combustion products here on earth. As part of a grand cosmic cycle, after stars begin to form out of a cloud of interstellar gas and dust, the ultraviolet light from the stars begins destroying these molecules in the regions directly adjacent to the forming stars, a process that ultimately will "turn-off" the star formation by removing the raw materials. On the other hand, where these molecular clouds are far from hot stars, the molecules can safely absorb the light without being destroyed. Thus, some stars are destroying the nursery clouds out of which they were born, whereas other more distant clouds are surviving to form stars at a later time. By observing both the stars and the emission from the excited molecules, new insights can be gained about both of these constituents of stellar nurseries.
By looking at hot stars in these nurseries, HUT can detect and study the intervening material by the absorbing effect it has on the star's light. HUT scientists are particularly interested in studying molecular hydrogen (a form of hydrogen where two H atoms are bound together), which has dozens absorption features in the far ultraviolet spectral region. The relative amounts of absorption in the different spectral features allows scientists to determine the temperatures and densities in the absorbing material.
By observing portions of the interstellar cloud directly, HUT scientists will study molecular hydrogen in another way: by analyzing its emitted spectrum. As the molecules in the gas absorb UV light, it excites the electrons into higher energy states. As these electrons "settle down" again, they re-emit the ultraviolet photons with a characteristic spectrum. By studying both the emitted and absorbed spectra of molecular hydrogen, and by observing the stars that are exciting the gas, HUT scientists will test theoretical models of these processes that are so basic to our understanding of the interstellar medium.
In addition, some of the molecules near the stars are absorbing too much ultraviolet light, which causes the molecules to be disrupted (that is the two H atoms become separate). As the molecules are disrupted, they emit a miniature fireworks display of ultraviolet light. HUT will capture this light from several different nebulae to study the efficiency of this process as a function of the temperatures of the exciting stars.
Richard H. Buss, Jr., and the HUT team