The Bar and Spiral Arms in the Milky Way: Structure and Kinematics
  The Milky Way is a spiral galaxy with the Schechter characteristic luminosity
$L_*$, thus an important anchor point of the Hubble sequence of all spiral
galaxies. Yet the true appearance of the Milky Way has remained elusive for
centuries. We review the current best understanding of the structure and
kinematics of our home galaxy, and present an updated scientifically accurate
visualization of the Milky Way structure with almost all components of the
spiral arms, along with the COBE image in the solar perspective. The Milky Way
contains a strong bar, four major spiral arms, and an additional arm segment
(the Local arm) that may be longer than previously thought. The Galactic boxy
bulge that we observe is mostly the peanut-shaped central bar viewed nearly
end-on with a bar angle of 25-30 degrees from the Sun-Galactic center line. The
bar transitions smoothly from a central peanut-shaped structure to an extended
thin part that ends around R ~ 5 kpc. The Galactic bulge/bar contains ~ 30-40%
of the total stellar mass in the Galaxy. Dynamical modelling of both the
stellar and gas kinematics yields a bar pattern rotation speed of ~ 35-40
km/s/kpc, corresponding to a bar rotation period of ~ 160-180 Myr. From a
galaxy formation point of view, our Milky Way is probably a pure-disk galaxy
with little room for a significant merger-made, "classical" spheroidal bulge,
and we give a number of reasons why this is the case.
