Goal of this Praktikum is to give an overview of the main features and stellar population present in the spectra of galaxies. For emission line galaxies (spirals and active galactic nuclei) we also analyze the systematic behaviour of the nebular emission lines to learn about the gas ionization mechanisms and galaxy star formation rates.
What follows is not a comprehensive description of the morphological and physical properties of galaxies for which one should refer for instance to a general book like ``Extragalactic astronomy'' of Mihalas and Binney (1981).
Elliptical galaxies are nearly featureless tri-dimensional stellar
systems with very little rotation and flattened by anisotropic
velocity distribution. This means that the kinetic energy
or mean squared velocity of the stars of an elliptical galaxy can
be different in different directions, in contrast to the well
known case of a thermalized gas, where the velocity distribution of
the gas molecules is a gaussian function of the squared velocity.
They appear as ellipses on the sky, with flattening 10 
(1 - b/a ) ( where a and b are the semi-major and
minor) ranging from 0 (round) to 7 (very flattened).
In contrast, spiral galaxies are two dimensional disks, rotationally supported, with a large fraction of cold (HI) and warm gas, and on-going star formation. Spirals have low central surface brightness compared to ellipticals. In their central parts on can find a bulge, or spheroidal component similar to an elliptical galaxy. As their name says, spirals show often bright spiral arms and/or barred structures. Spirals appear as circles if seen face-on (inclination angle i = 0) and ellipses more and more flattened as the inclination angle increases.
Between the two groups one finds S0 galaxies, which posses a large central bulge similar to an elliptical galaxy, a gasless disk and therefore no on-going star formation.
The morphological classification of galaxies is well correlated to their stellar populations content, i.e. their spectroscopic properties, that will be discussed in the next section.
The next pages are organized as follows. Section 2 gives an overview
of the spectral properties of elliptical-S0 (2.1) and
spiral galaxies (2.2). Spectra of active galactic nuclei
are briefly discussed in (2.3). In Section 3 we learn how
to distinguish between galaxies whose gas is ionized by stars
and those in which the ionizing source is nonthermal (3.1).
We then describe how to derive the ongoing star formation rate
in spiral galaxies, considering
the 
and 
emission line as star
formation tracers (3.2)
and taking into account the extinction caused by dust (3.2.1).
A short description of the past and future
star formation of spiral galaxies is given in Section 3.2.2.
Section 4 describes how one should perform the Praktikum.
The three Appendices provide more detail information about
the mean star formation of spiral galaxies (A),
the estimate of dust content and reddening correction (B),
and the derivation of the ongoing star formation
by means of the 
line emission (C).