Fundamental Astronomy

17. The Milky Way
On clear, moonless nights a nebulous band of light
can be seen stretching across the sky. This is the
Milky Way (Fig. 17.1). The name is used both for the
phenomenon in the sky and for the large stellar system
causing it. The Milky Way system is also called the Galaxy
− with a capital letter. The general term galaxy is used to
refer to the countless stellar systems more or less like our
Milky Way.
The band of the Milky Way extends round the whole
celestial sphere. It is a huge system consisting mostly
of stars, among them the Sun. The stars of the Milky
Way form a flattened disc-like system. In the direction
of the plane of the disc huge numbers of stars are visible,
whereas relatively few are seen in the perpendicular direction.
The faint light of distant stars merges into a uniform
glow, and therefore the Milky Way appears as a nebulous
band to the naked eye. A long-exposure photograph
reveals hundreds of thousands of stars (Fig. 17.2).
Hannu Karttunen et al. (Eds.), The Milky Way.
In: Hannu Karttunen et al. (Eds.), Fundamental Astronomy, 5th Edition. pp. 347–366 (2007)
DOI: 11685739_17 © Springer-Verlag Berlin Heidelberg 2007
In the early 17th century Galileo Galilei, using his first
telescope, discovered that the Milky Way consists of innumerable
stars. In the late 18th century William Herschel
attempted to determine the size and shape of the Milky
Way by means of star counts. Only early in the 20th century
did the Dutch astronomer Jacobus Kapteyn obtain the
first estimate of the size of the Milky Way. The true size
of the Milky Way and the Sun’s position in it became clear
in the 1920’s from Harlow Shapley’s studies of the space
distribution of globular clusters.
In studying the structure of the Milky Way, it is convenient
to choose a spherical coordinate system so that
the fundamental plane is the symmetry plane of the
Milky Way. This is defined to be the symmetry plane
of the distribution of neutral hydrogen, and it agrees
quite closely with the symmetry plane defined by the
distribution of stars in the solar neighbourhood (within
a few kpc).
Fig. 17.1. The nebulous band of the Milky Way stretches across the entire sky. (Photograph M. and T. Kesküla, Lund Observatory)
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