Page 120 - spectroscopic-atlas-5_0-english_Neat

Spectroscopic Atlas for Amateur Astronomers 120

26.3 Spectroscopic Classification

The spectroscopic classification distinguishes these objects according to spectral features.
Already in the 1920ies it was recognised that such composite- or integrated spectra have a
similar information content like those of individual single stars. They also show profiles e.g.
with or without emission lines, chiefly depending on the kind and activity of the galactic nu-
clei. Such spectral profiles are powerful means to determine e.g. the content of stars, and
the development state of the galaxy [301].The relative similarity of the galactic composite
spectra to those of individual single stars, was also a convincing argument in the historical
dispute (mainly between H. Shapley and H. Curtis), that galaxies are not dust or gaseous
nebulae like M42 (see Tables 80/80A), but extremely distant, huge clusters of individual
stars!

In the second half of the 20th Century, classification systems have been developed, which
were based on the similarity with stellar spectral types. The so called Integrated Spectral
types of the galaxies were determined this way by W. Morgan, beginning with A, AF, F- and
ending by the late K-systems. The following correlations have been noticed [2]:

– The later the stellar spectral type, with which the profile shows similarities, the stronger
is the galaxy centrally concentrated.

– The elliptical classes E1 – E4, remain still dominated by absorption features of later
spectral types. From here on however, the characteristic changes until the profiles of
the types Sc/SBc and Irr look similar to those of early spectral classes. Furthermore
emission lines show up here with increasing intensity.

The following table shows another, also rather rough classification system, based on the
presence, intensity and shape of emission lines. It also includes peculiar shapes such as
Seyfert Galaxies and Quasars, standing out by an extremely high core activity and therefore
belonging to the category of AGN (Active Galactic Nuclei). The core activity increases here
from top to down.