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 
105(3 He/H) = 1.1 ± 0.2.