The Cosmic Saga of 3 He

We recall the emergence of the “He problem”, its currently accepted solution, and we summarize the presently available constraints on models of stellar nucleosynthesis and studies of Galactic chemical evolution from measurements of the abundance of He in the Galaxy. 1 In the beginning was tralphium The isotope He probably first entered the astrophysical arena in 1949 with the (unpublished) calculations of Fermi & Turkevich concerning the chemical evolution of the first half-hour of the Universe. The names “tralphium” and “tralpha particles” invented by George Gamow for this isotope and its nuclei, have survived only in his humourous description of nucleogenesis: And God said: “Let there be mass three.” And there was mass three. And God saw tritium and tralphium, and they were good”. And so on to transuranium elements, with Fred Hoyle’s help to bridge the gap at mass five (Kragh 1996). The rough estimate of by Fermi & Turkevich (He ≃ 10 by mass) was later refined by more detailed calculations, like e.g. those of Wagoner, Fowler, & Hoyle (1967) who showed that He could be produced at levels comparable to its terrestrial abundance (∼ 5×10 by mass) during the evolution of a “universal fireball or a supermassive object”, or, as we say today, in the big bang . Thus, at least in principle, the abundance of He could be used (together with D, He and Li) to test theoretical predictions, and, in particular, to constrain the baryon density of the Universe. Having gained the special status of “cosmic baryometer” and caught the attention of cosmologists, the interest in He spread rapidly in the astronomical