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Spectroscopic Atlas for Amateur Astronomers 20

6 Overview and Characteristics of Stellar Spectral Classes

6.1 The Temperature Sequence

The rough, one-dimensional determination of the spectral classes O, B, A, F, G, K, M, is easy
and even feasible for slightly advanced amateurs. This sequence of letters is further subdi-
vided into decimal subclasses which follows directly the decreasing photospheric tempera-
ture, starting with the extremely hot O-type of several 10,000’s K, and proceeding down to
the cold M-class with about 2,400 – 3,500 K. The Sun is classified as G2 with 5,800 K. Not
treated here are the spectral types L and T which were first introduced in the 1990s as a
low temperature extension of the M-Class, and include, among others, the so-called Brown
Dwarfs. These cool objects are extremely faint and radiating primarily in the infrared range.
Therefore, these classes remain unreachable by amateur means.

Spectral distinction criteria are prominent features such as lines in absorption or emission,
which appear prominently in certain classes, and are entirely absent in others. The follow-
ing charts show a montage with sample spectra of the entire class sequence of
O – M. Here the original profiles of the stars are shown (200L grating), which are also used
to present the individual spectral classes. This way, this resolution already enables the
identification of the intense and therefore most documented spectral lines. Further also the
influence of some elements, ions or molecules to the different spectral types becomes
roughly visible.

Table 01 shows superposed the synthetically produced 1D-spectral stripes – Table 02
shows the same but with the according intensity profiles.

What is already clearly noticeable here?

– In the upper third of the table (B2–A5), the strong lines of the H-Balmer series, i.e. Hα,
Hβ, Hγ, etc. They appear most pronounced in the class A2 and are weakening from here
towards earlier and later spectral classes.

– In the lower quarter of the table (K5–M5) the eye-catching shaded bands of molecular
absorption spectra, mainly due to titanium oxide (TiO).

– Just underneath the half of the table some spectra (F5–K0), showing only few prominent
features, but charged with a large number of fine metal lines. Striking features here are
only the Na I double line (Fraunhofer D1, 2) and in the “blue” part the impressive Fraun-
hofer lines of Ca II (K + H), gaining strength towards later spectral classes. Fraunhofer H
at λ 3968 starts around the early F-class to overprint the weakening Hε hydrogen line at
λ 3970. In addition, the H-Balmer series is further weakening towards later classes.

– Finally on the top of the table the extremely hot O-class with very few fine lines, mostly
ionised helium (He II) and multiply ionized metals. The H-Balmer series appears here
quite weak, as a result of the extremely high temperatures. The telluric H2O and O2 ab-
sorption bands are reaching high intensities here, because the strongest radiation of the
star takes place in the ultraviolet whereas the telluric absorption bands are located in
the undisturbed domain near the infrared part of the spectrum. By contrast the maximum
radiation of the late spectral classes takes place in the infrared part, enabling the stellar
TiO absorption bands to overprint here the telluric lines.

– In the spectra of hot stars (~ classes from early A – O) the double line of neutral sodium
Na I (Fraunhofer D1,2) must imperatively be of interstellar origin. Neutral sodium Na I has
a very low ionisation energy of just 5.1 eV (see table in sect. 34) and can therefore exist
only in the atmospheres of relatively cool stars. The wavelengths of the ionised Na II lie
already in the ultraviolet range and are therefore not detectable by amateur equipment.