Effects of Bound Diprotons and Enhanced Nuclear Reaction Rates on
  Stellar Evolution
  Deuterium represents the only bound isotope in the universe with atomic mass
number $A=2$. Motivated by the possibility of other universes, where the strong
force could be stronger, this paper considers the effects of bound diprotons
and dineutrons on stars. We find that the existence of additional stable nuclei
with $A=2$ has relatively modest effects on the universe. Previous work
indicates that Big Bang Nucleosynthesis (BBN) produces more deuterium, but does
not lead to catastrophic heavy element production. This paper revisits BBN
considerations and confirms that the universe is left with an ample supply of
hydrogen and other light nuclei for typical cosmological parameters. Using the
$MESA$ numerical package, we carry out stellar evolution calculations for
universes with stable diprotons, with nuclear cross sections enhanced by large
factors $X$. This work focuses on $X=10^{15}-10^{18}$, but explores the wider
range $X$ = $10^{-3}-10^{18}$. For a given stellar mass, the presence of stable
diprotons leads to somewhat brighter stars, with the radii and photospheric
temperatures roughly comparable to thoese of red giants. The central
temperature decreases from the characteristic value of
$T_c\approx1.5\times10^7$ K for hydrogen burning down to the value of
$T_c\approx10^6$ K characteristic of deuterium burning. The stellar lifetimes
are smaller for a given mass, but with the extended possible mass range, the
smallest stars live for trillions of years, far longer than the current cosmic
age. Finally, the enhanced cross sections allow for small, partially degenerate
objects with mass $M_\ast=1-10M_J$ to produce significant steady-state
luminosity and thereby function as stars.
