  Studies of the ages, abundances, and motions of individual stars in the Milky
Way provide one of the best ways to study the evolution of disk galaxies over
cosmic time. The formation of the Milky Way's barred inner region in particular
is a crucial piece of the puzzle of disk galaxy evolution. Using data from
APOGEE and Gaia, we present maps of the kinematics, elemental abundances, and
age of the Milky Way bulge and disk that show the barred structure of the inner
Milky Way in unprecedented detail. The kinematic maps allow a direct, purely
kinematic determination of the bar's pattern speed of 41+/-3 km/s/kpc and of
its shape and radial profile. We find the bar's age, metallicity, and abundance
ratios to be the same as those of the oldest stars in the disk that are formed
in its turbulent beginnings, while stars in the bulge outside of the bar are
younger and more metal-rich. This implies that the bar likely formed ~8 Gyr
ago, when the decrease in turbulence in the gas disk allowed a thin disk to
form that quickly became bar-unstable. The bar's formation therefore stands as
a crucial epoch in the evolution of the Milky Way, a picture that is in line
with the evolutionary path that emerges from observations of the gas kinematics
in external disk galaxies over the last ~10 Gyr.

Astrophysics of Galaxies;Cosmology and Nongalactic Astrophysics