The Solar System and Its Place in the Galaxy 25
FIGURE 15 The spiral
structure of the Milky Way
galaxy as inferred from the
positions of HII regions (clouds
of ionized hydrogen) in the
galaxy. The Sun and solar
system are located at the upper
center, as indicated by the
symbol. (Reprinted with kind
permission from Kluwer
Academic Publishers, Forbes
and Shuter, in “Kinematics,
Dynamics, and Structure of the
Milky Way,” p. 221, Fig. 3,
copyright©C1983.)the disk, and with a radius of about 1.5 kpc perpendicular
to the disk. The bar rotates more slowly than the disk and
consists largely of densely packed older stars and interstellar
clouds. It does not display spiral structure. At the center of
the bar is the nucleus, a complex region only 4–5 pc across,
which appears to have a massive black hole at its center.
The mass of the central black hole has been estimated at
2.6 millionM.
The halo surrounds both of these structures and extends
∼20–30 kpc from the galactic center. The halo has an oblate
spheroid shape and contains older stars and globular clus-
ters of stars. The corona appears to be a yet more distant
halo extending 60–100 kpc and consists of dark matter, un-
observable except for the effect it has on the dynamics of
observable bodies in the galaxy. The corona may be several
times more massive than the other three galactic compo-
nents combined.
The galactic disk is visible in the night sky as the Milky
Way, a bright band of light extending around the celestial
sphere. When examined with a small telescope, the Milky
Way is resolved into thousands or even millions of individual
stars and numerous nebulae and star clusters. The direction
to the center of the galaxy is in the constellation Sagittarius
(best seen from the southern hemisphere in June), and the
disk appears visibly wider in that direction, which is the view
of the central bulge and bar.
The disk is not perfectly flat; there is evidence for warp-
ing in the outer reaches of the disk, between 15 and 25 kpc.
The warp may be the result of gravitational perturbations
due to encounters with other galaxies and/or with the
Magellanic clouds, two nearby, irregular dwarf galaxies that
appear to be in orbit around the Milky Way. In addition, the
Milky Way’s central bar appears to be tilted relative to the
plane of the galactic disk. The nonspherical shape of the bar
and the tilt have important implications for understanding
stellar dynamics and the long-term evolution of the galaxy.
Stars in the galactic disk have different characteristic ve-
locities as a function of their stellar classification, and hence
age. Low mass, older stars, like the Sun, have relatively high
random velocities and, as a result, can move farther out of