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Spectroscopic Atlas for Amateur Astronomers 77

17 Spectral Class G

17.1 Overview

The yellow shining stars of the G-class have, from a spectroscopic view, a special status,
because their spectra are more or less similar to that of our Sun, which is probably one of

the best explored and documented. α Centauri in the southern sky is with G2V equally

classified as the Sun. It shows therefore nearly the same surface temperature like our cen-
tral star. In the northern sky Muphrid (η Boo) with G0 IV is classified relatively close to the
Sun. Otherwise among the bright stars no further similar classified can be found. Capella
with its two binary components of G5llle and G0lll is already settled on the Giant Branch of
the HRD. The same applies for Sadalsuud (β Aqr) with G0lb.

Other well-known G-stars can only be found in the classes G7 and later, such as
Kornephoros (β Her), γ Leo, γ Per, δ Boo, Vindemiatrix (ε Vir).

17.2 Parameters of the Early to Late G-Class Stars

The following table shows the data exclusively for the Main Sequence Stars of the G-class,
compared to the Sun ( ) and according to [701].

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

1.05 – 0.9 7 bn – 15 bn 6,000 – 5,500 1.1 – 0.85 1.5 – 0.66

Striking is the percental very low mass range, which is covered by the G-class. Neverthe-
less, the life expectancy and luminosity of the star respond almost grotesquely sensitive to
this difference. Our Sun with a surface temperature of about 5,800 K (G2V) belongs to the
early G-class, spending some 7 billion years on the Main Sequence.

17.3 Spectral Characteristics of the G-Class

The Fraunhofer H + K lines of ionised Ca II become here impressively strong achieving theo-
retically the maximum intensity in the late G classes. This becomes also evident in the chart
in sect. 7. In the solar spectrum (G2V) they are by far the strongest lines generated by the
star itself. For Main Sequence Stars of the G-Class, the K-line is always slightly more in-
tense than the H-line.

The H-Balmer series becomes significantly weaker, so these lines are now surpassed even
by various metal absorptions. Therefore they lose from here on their function as welcome
orientation marks, e.g. for the calibration and line identification. The intensity of the so-
called Magnesium Triplet (λλ 5169-83) has increased during the F-Class and achieves a
considerable strength here, so with "b" it’s even labelled with its own Fraunhofer letter. The
Ca l line by λ 4227 also impressively gained intensity since the early F classes and be-
comes here a striking spectral feature also with an own Fraunhofer letter “g”.

In general, the trend here continues by growing intensity of neutral metals e.g. Fe I and the
Fraunhofer-D lines (Na I). Towards the later subclasses they increasingly replace the ab-
sorption of the ionised elements. Due to the dominance of fine metal lines, the spectra be-
come now more and more complex. Therefore, our Sun is not really a suitable object for be-
ginners. The temperature here is sunken to such a low level that simple and robust diatomic
molecules can survive in these stellar atmospheres. Most prominent of such features is the
Fraunhofer G-band of the CH molecule, which already surpassed in the late F-class the in-
tensity of the Hγ line. Further mentionable are also the strong CN and CH absorption bands