  The compositions of stars are a critical diagnostic tool for many topics in
astronomy such as the evolution of our Galaxy, the formation of planets, and
the uniqueness of the Sun. Previous spectroscopic measurements indicate a large
intrinsic variation in the elemental abundance patterns of stars with similar
overall metal content. However, systematic errors arising from inaccuracies in
stellar models are known to be a limiting factor in such studies, and thus it
is uncertain to what extent the observed diversity of stellar abundance
patterns is real. Here we report the abundances of 30 elements with precisions
of 2% for 79 Sun-like stars within 100 parsecs. Systematic errors are minimized
in this study by focusing on solar twin stars and performing a line-by-line
differential analysis using high-resolution, high-signal-to-noise spectra. We
resolve [X/Fe] abundance trends in galactic chemical evolution at precisions of
$10^{-3}$ dex Gyr$^{-1}$ and reveal that stars with similar ages and
metallicities have nearly identical abundance patterns. Contrary to previous
results, we find that the ratios of carbon-to-oxygen and magnesium-to-silicon
in solar metallicity stars are homogeneous to within 10% throughout the solar
neighborhood, implying that exoplanets may exhibit much less compositional
diversity than previously thought. Finally, we demonstrate that the Sun has a
subtle deficiency in refractory material relative to >80% of solar twins (at
2$\sigma$ confidence), suggesting a possible signpost for planetary systems
like our own.
