and Cosmology

2. The Milky Way as a Galaxy
35
The Earth is orbiting around the Sun, which itself is
orbiting around the center of the Milky Way. Our Milky
Way, the Galaxy, is the only galaxy in which we are able
to study astrophysical processes in detail. Therefore, our
journey through extragalactic astronomy will begin in
our home Galaxy, with which we first need to become
familiar before we are ready to take off into the depths
of the Universe. Knowing the properties of the Milky
Way is indispensable for understanding other galaxies.
2.1 Galactic Coordinates
On a clear night, and sufficiently far away from cities,
one can see the magnificent band of the Milky Way
on the sky (Fig. 2.1). This observation suggests that the
distribution of light, i.e., that of the stars in the Galaxy,
is predominantly that of a thin disk. A detailed analysis
of the geometry of the distribution of stars and gas
confirms this impression. This geometry of the Galaxy
suggests the introduction of two specially adapted coordinate
systems which are particularly convenient for
quantitative descriptions.
Spherical Galactic Coordinates (l, b). We consider
a spherical coordinate system, with its center being
“here”, at the location of the Sun (see Fig. 2.2). The
Galactic plane is the plane of the Galactic disk, i.e., it
is parallel to the band of the Milky Way. The two Galactic
coordinates l and b are angular coordinates on
the sphere. Here, b denotes the Galactic latitude, the
angular distance of a source from the Galactic plane,
with b ∈[−90 ◦ , +90 ◦ ]. The great circle b = 0 ◦ is then
located in the plane of the Galactic disk. The direction
b = 90 ◦ is perpendicular to the disk and denotes
the North Galactic Pole (NGP), while b =−90 ◦ marks
the direction to the South Galactic Pole (SGP). The
second angular coordinate is the Galactic longitude l,
with l ∈[0 ◦ , 360 ◦ ]. It measures the angular separation
between the position of a source, projected perpendicularly
onto the Galactic disk (see Fig. 2.2), and the
Galactic center, which itself has angular coordinates
b = 0 ◦ and l = 0 ◦ .Givenl and b for a source, its location
on the sky is fully specified. In order to specify its
three-dimensional location, the distance of that source
from us is also needed.
The conversion of the positions of sources given in
Galactic coordinates (b,l)to that in equatorial coordinates
(α, δ) and vice versa is obtained from the rotation
between these two coordinate systems, and is described
by spherical trigonometry. 1 The necessary formulae can
be found in numerous standard texts. We will not reproduce
them here, since nowadays this transformation
is done nearly exclusively using computer programs.
Instead, we will give some examples. The following
figures refer to the Epoch 2000: due to the precession
1 The equatorial coordinates are defined by the direction of the Earth’s
rotation axis and by the rotation of the Earth. The intersections of the
Earth’s axis and the sphere define the northern and southern poles. The
great circles on the sphere through these two poles, the meridians, are
curves of constant right ascension α. Curves perpendicular to them
and parallel to the projection of the Earth’s equator onto the sky are
curves of constant declination δ, with the poles located at δ =±90 ◦ .
Fig. 2.1. An unusual optical
image of the Milky
Way. This total view of
the Galaxy is composed
of a large number of
individual images
Peter Schneider, The Milky Way as a Galaxy.
In: Peter Schneider, Extragalactic Astronomy and Cosmology. pp. 35–85 (2006)
DOI: 10.1007/11614371_2 © Springer-Verlag Berlin Heidelberg 2006