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Analysis and Interpretation of Astronomical Spectra 47

13 The Spectral Classes

13.1 Preliminary Remarks

Basic knowledge of the spectral classes is an indispensable prerequisite for a reasonable
spectroscopic activity. Combined with appropriate skills and tools, this classification sys-
tem contains a considerable qualitative and quantitative information potential about the
classified objects. The average-equipped amateur will hardly ever come into the embar-
rassment, that he really must determine an unknown classification of a star, unless of cer-
tainly recommendable didactic reasons. The spectral classes can nowadays be obtained
from Internet sources [100], planetarium programs, etc. For a deeper understanding of the
classification system, a rough knowledge of the historical development is very useful since
from each development step something important remained until to date!

13.2 The Fraunhofer Lines

At the beginning of the 19th Century, the physicist and Line Element Wavelength Å
optician Joseph von Fraunhofer (1787-1826) investi- A – Band
gated, based on the discovery of Wollaston, the sunlight B – Band O2 7594 – 7621
with his home-built prism spectroscope. He discovered C O2 6867 – 6884
over 500 absorption lines in this very complex spectrum. a – Band H (α) 6563
The more intensive of them he denoted with the letters D 1, 2 O2 6276 – 6287
A – K, at that time still unaware of the physical context. E Na 5896 & 5890
Picture below: Original drawing by Fraunhofer from Inter- b 1, 2 Fe 5270
net sources. This line names can frequently be found even F Mg 5184 & 5173
in recent papers! D H (β) 4861
E Fe 4668
Fraunhofer has been studied the brighter stars with this F Fe 4384
spectroscope, and already recognised that in the spectrum G – Band H (γ) 4340
of Sirius broad strong lines are dominating and Pollux G CH 4300 - 4310
shows a similar pattern as the spectrum of the sun! Further H Ca 4227
he observed the spectrum of Betelgeuse which shows H H (δ) 4102
barely discrete absorption lines but broad absorption K Ca II 3968
bands. Ca II 3934
The table on the right and the graphics below show how
this system was expanded later on. (Source: NASA).