Fundamental Astronomy

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17. The Milky Way
Later investigations using a variety of methods, both
optical and radio have confirmed that a four-armed pattern
gives the best description of the spiral structure in
the Sun’s vicinity (Fig. 17.22). The pitch angle of the
spiral in this model is about 11.3 ◦ . Three of the arms
start at the position of the galactic bar.
The cause of the spiral structure is a long-standing
problem. A small perturbation in the disc will quickly
be stretched into a spiral shape by differential rotation.
However, such a spiral would disappear in a few galactic
revolutions, a few hundred million years.
An important step forward in the study of the spiral
structure was the density wave theory developed by
Chia-Chiao Lin and Frank H. Shu in the 1960’s. The
spiral structure is taken to be a wavelike variation of the
density of the disc. The spiral pattern rotates as a solid
body with an angular velocity smaller than that of the
galactic rotation, while the stars and gas in the disc pass
through the wave.
Fig. 17.21. The central parts of the Milky Way in a radio
picture. The observations were made with the VLA telescope.
(Image Kassim et. al., Naval Research Laboratory)
Fig. 17.22. General view of the spiral pattern of the Milky Way.
Different tracers of spiral arms lead to somewhat different
patterns, but they tend to agree that a four-armed pattern like
the the one indicated here gives the best overall representation.
The names of the arms are those most commonly used. See
also Fig. 17.11. (Y.Xu et al. 2006, Science 311,54)
The density wave theory explains in a natural way
why young objects, like molecular clouds, H II regions
and bright young stars are found in the spiral arms. As
gas passes through the wave, it is strongly compressed.
The internal gravity of the gas clouds then becomes
more important and causes them to collapse and form
stars.
It takes about 10 7 years for the material to pass
through a spiral arm. By that time, the hot, bright stars
have finished their evolution, their ultraviolet radiation
has ceased and the H II regions have disappeared. The
less massive stars formed in the spiral arms are spread
out in the disc by their peculiar velocities.
It is not yet clear what gives rise to the spiral wave.
For some further discussion of spiral structure, see
Sect. 18.4.
The Galactic Centre. Our knowledge of the centre of
the Milky Way is mostly based on radio and infrared
observations. In the optical region the view to the centre
is blocked by the dark clouds in the Sagittarius spiral
arm about 2 kpc from us. The galactic centre is inter-