  Motivated by the possibility that the laws of physics could be different in
other regions of space-time, we consider nuclear processes in universes where
the weak interaction is either stronger or weaker than observed. We focus on
the physics of both Big Bang Nucleosynthesis (BBN) and stellar evolution. For
sufficiently ineffective weak interactions, neutrons do not decay during BBN,
and the baryon-to-photon ratio $\eta$ must be smaller in order for protons to
survive without becoming incorporated into larger nuclei. For stronger weak
interactions, neutrons decay before the onset of BBN, and the early universe is
left with nearly a pure hydrogen composition. We then consider stellar
structure and evolution for the different nuclear compositions resulting from
BBN, a wide range of weak force strengths, and the full range of stellar masses
for a given universe. We delineate the range of this parameter space that
supports working stars, along with a determination of the dominant nuclear
reactions over the different regimes. Deuterium burning dominates the energy
generation in stars when the weak force is sufficiently weak, whereas
proton-proton burning into helium-3 dominates for the regime where the weak
force is much stronger than in our universe. Although stars in these universes
are somewhat different, they have comparable surface temperatures,
luminosities, radii, and lifetimes, so that a wide range of such universes
remain potentially habitable.
