and Cosmology

1.2 Overview
parameters of the Milky Way (like its total luminosity)
are difficult to measure. These parameters are estimated
much better from outside, i.e., in other similar spiral galaxies.
In order to understand the large-scale properties
of our Galaxy, a comparison with similar galaxies which
we can examine in their entirety is extremely helpful.
Only by combining the study of the Milky Way with
that of other galaxies can we hope to fully understand
the physical nature of galaxies and their evolution.
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Fig. 1.6. The galaxy Dwingeloo 1 is only five times more
distant than our closest large neighboring galaxy, Andromeda,
yet it was not discovered until the 1990s because it hides
behind the Galactic center. The absorption in this direction and
numerous bright stars prevented it being discovered earlier.
The figure shows an image observed with the Isaac Newton
Telescope in the V-, R-, and I-bands
1.2.2 The World of Galaxies
Next we will discuss the properties of other galaxies.
The two main types of galaxies are spirals (like the
Milky Way, see also Fig. 1.7) and elliptical galaxies
(Fig. 1.8). Besides these, there are additional classes
such as irregular and dwarf galaxies, active galaxies, and
starburst galaxies, where the latter have a very high starformation
rate in comparison to normal galaxies. These
classes differ not only in their morphology, which forms
the basis for their classification, but also in their physical
properties such as color (indicating a different stellar
content), internal reddening (depending on their dust
Fig. 1.7. NGC 2997 is a typical spiral galaxy,
with its disk inclined by about 45 ◦ with
respect to the line-of-sight. Like most spiral
galaxies it has two spiral arms; they are
significantly bluer than other parts of the
galaxy. This is caused by ongoing star formation
in these regions so that young, hot
and thus blue stars are present in the arms,
whereas the center of the galaxy, especially
the bulge, consists mainly of old stars