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Analysis and Interpretation of Astronomical Spectra 109

temperature of 5800 K, Ca II is an extremely effective absorber. The optimum conditions
for the hydrogen lines, however, are reached not until nearly 10,000 K (see sect. 9.2). In
the professional area the element abundance is also determined by the iterative compari-
son of the spectrum with simulated synthetic profiles of different chemical composition
[11].

23.4 Relative Abundance-Comparison at Stars of Similar Spectral Class
A simplified special case is formed by stars with similar spectral- and luminosity class and
comparable rotational velocities. Thus the physical parameters of the photospheres are
very similar. Here the equivalent widths EW of certain lines can simply be compared and
thus the relative abundance differences at least qualitatively be seen. In the Spectroscopic
Atlas [33] this is demonstrated at the classical example of the two main-sequence stars Sir-
ius A1Vm and Vega A0V. The basic principle is the so called Curve of Growth. It shows that
within its unsaturated and somewhat linearly running part, the equivalent width EW of a
certain spectral line of an element, behaves roughly proportional to its number of atoms
within a plasma mixture.

Equivalent width EW [Å] Saturated line

Curve of Growth

Linear region
Linie profile deepening

Number of atoms