Binding the Diproton in Stars: Anthropic Limits on the Strength of
  Gravity
  We calculate the properties and investigate the stability of stars that burn
via strong (and electromagnetic) interactions, and compare their properties
with those that, as in our Universe, include a rate-limiting weak interaction.
It has been suggested that, if the diproton were bound, stars would burn
~10^{18} times brighter and faster via strong interactions, resulting in a
universe that would fail to support life. By considering the representative
case of a star in our Universe with initially equal numbers of protons and
deuterons, we find that stable, "strong-burning" stars adjust their central
densities and temperatures to have familiar surface temperatures, luminosities
and lifetimes. There is no "diproton disaster". In addition, strong-burning
stars are stable in a much larger region of the parameter space of fundamental
constants, specifically the strength of electromagnetism and gravity. The
strongest anthropic bound on stars in such universes is not their stability, as
is the case for stars limited by the weak interaction, but rather their
lifetime. Regardless of the strength of electromagnetism, all stars burn out in
mere millions of years unless the gravitational coupling constant is extremely
small, \alpha_G < 10^{-30}.
