Hydrogen and helium and their special significance

The lightest isotope, hydrogen, with A = 1, is the prime building block for the elements, and spectroscopic measurements show that H is the most abundant element in stars and interstellar clouds in our Galaxy and in the Universe as a whole: nine out of ten atoms are hydrogen. The second stable hydrogen isotope, deuterium (D), with A = 2, is much less abundant: it has a low binding energy per nucleon (Fig. 1.2), and upon collision with baryons and heavier particles it is readily fused to form 3He or He. As nuclear processes in stars thus tend to destroy D, this nuclide must have been generated in another process, i.e. in the earliest prestellar nucleosynthesis.

This hypothesis can be tested by measuring the prestellar D/H ratio of galaxies. This can be achieved by the spectroscopy of interstellar clouds lying along the line of sight to a remote very bright object. Some of these clouds represent nearly virgin samples of prestellar cosmic material with ∼ 1000 times lower metallicity than that in the solar system (the metallicity of a system is a measure of the abundance of elements heavier than helium). High-resolution spectroscopic measurements of the H and D abundances in these clouds gave D/H = (30 ± 2) × 10−6 (Schramm and Turner, 1998).