Normal stars

According to the Harvard system an F-type star is characterized by strong Ca ii (K and H) lines, which become much stronger than the hydrogen lines of the Balmer series. A multitude of fainter metallic lines accompanies both features. At F0, as already mentioned I(K) = I(H + Hε); at G0, I(K) » I(H). Whereas at 100 Å/mm in A-type stars the Balmer lines are remarkable for their strength, in F-type stars they are no longer conspicuous. Another feature which appears at this dispersion is the G-band (near λ4300), which is due to the molecule CH; this feature appears around F3 and strengthens toward the later subtypes. The feature is constituted by the head of a molecular band and tends to dissolve when observed at lower plate factors.

In order to fix these ideas, table 11.1 provides the equivalent widths of some strong lines.

Besides the strong lines, there exists a host of weak lines, which produce, as we have seen, an increasing blocking. They also become so numerous that the undisturbed continuum is hard to see, except at low plate factors.

The spectral type is obtained from intensity ratios involving medium intensity features mostly from neutral elements. Attention has to be paid to two facts. The first is that because of the large number of lines present, any feature is a blend of several lines, except at very small plate factors.