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4.2. Helium-3

Unlike D, the post-BBN evolution of 3He and 7Li are quite complex. 3He is destroyed in the hotter interiors of all but the least massive (coolest) stars, but it is preserved in the cooler, outer layers of most stars. In addition, hydrogen burning in low mass stars results in the production of significant amounts of new 3He (Iben 1967; Rood 1972; Dearborn, Steigman & Schramm 1986; Vassiliadis & Wood 1993; Dearborn, Steigman & Tosi 1996). To follow the post-BBN evolution of 3He, it is necessary to account for all these effects - quantitatively - in the material returned by stars to the interstellar medium (ISM). As indicated by the existing Galactic data (Geiss & Gloeckler 1998; Bania, Rood & Balser 2002), a very delicate balance exists between net production and net destruction of 3He in the course of the evolution of the Galaxy. As a consequence, aside from noting an excellent qualitative agreement between the SBBN predicted and observed 3He abundances, 3He has - at present - little role to play as a quantitatively useful baryometer. In this spirit, it is noted that an uncertain estimate of the primordial abundance of 3He may be inferred from the observation of an outer-Galaxy (less evolved) HII region (Bania et al. 2002): y3 ident 105(3 He/H) = 1.1 ± 0.2.