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Spectroscopic Atlas for Amateur Astronomers 65

15 Spectral Class A

15.1 Overview

Several of the best known and most striking, bright white stars, like Sirius, Vega, Castor,
Deneb, Denebola, Altair, and most of the stars in the Great Bear are classified as A-types. In
this and all the following later classes, the single stars on the Main Sequence will pass at
the end of their life a Giant phase, combined with an impressive "farewell tour" through al-
most the entire HRD. They will end their lives as extremely dense White Dwarfs. During this
final process at least some of them will repel a photogenic Planetary Nebula.

15.2 Parameters of the Early to Late A-Class Stars

The following table shows the data exclusively for the Main Sequence Stars of the A-class,
compared to the Sun ( ) and according to [701]. Compared with the enormously broad B-
Class, the low mass range (factor 1.5) is striking here. Nevertheless impressive is the huge
influence of this relatively small mass difference on the luminosity and life expectancy.

Mass Stay on main Temperature Radius Luminosity
M/M sequence [y] photosphere [K] R/R L/L

3 – 2 440M – 3bn 10,000 – 7,500 2.7 – 1.7 55 – 8

15.3 Spectral Characteristics of the A-Class

Since the beginning of spectroscopy in the 19th century this class has fascinated by their
impressive hydrogen lines, but otherwise very “tidy” and esthetically looking spectra. This
was at least one reason for numerous false hypotheses. Father Angelo Secchi already clas-
sified these spectra in the mid-19th Century as "Type I" (see appendix 34.3). Edward
Pickering labelled these in his later refined system as "A-Type". Today, this class has still
Pickering's "A-label", but inconspicuously in the upper middle of the MK–stellar classes.

These distinctive and clear spectra are very well suited as a didactic introduction to the
practical spectroscopy. Moreover, the pattern of the strong H-Balmer lines is an excellent
aid for first calibration attempts. This feature gains intensity since the late B-class and
reaches its maximum in type A2 [1]. Quantum mechanically, this can be explained with the
surface temperature of about 9,800 K. This way the hydrogen atoms are thermally excited
so that a maximum number of electrons stay already on the level n2, the “takeoff level” for
the electron transitions in the shell system n2 – n∞ of the H-Balmer series [30].

From here on, the later A-classes are characterised by a gradual, but still moderate intensity
loss of the H-Balmer series. Conversely, the two Fraunhofer H + K lines of Ca II become sig-
nificantly stronger. The Fraunhofer K-line penetrates now deeper into the continuum peak
between Hε and H8 and exceeds between A7 and F0 the intensity of the hydrogen absorp-
tion. Within the same range also the growing Fraunhofer H-line (λ 3968) overprints now the
weakening Hε absorption. In the late A-classes the "Blue Wing" of the Hγ line shows a small
kink. At the latest in the F0 class, it’s revealed as the fast growing molecular CH-absorption
of the Fraunhofer G-band (in the following graph marked with red arrow).

Higher resolved spectra show immediately that the first impression of the “simple spec-
trum” is deceptive. The continuum between the hydrogen lines is interrupted by numerous
metal absorptions. The absorptions of neutral atoms become now more intensive at the ex-
pense of the singly ionised ones. In the early subclasses, a few He I lines still appear but
very faintly. The intensity maximum of the real continuum moves here already to the “blue”
short wave edge of the visible spectrum. The graph shows the theoretical continuum for a