As we move toward later types along the main sequence after G0, we
note that the hydrogen lines continue to fade, while the strength of the
general
metallic-line spectrum continues to increase. The G-band continues to increase
in strength until the early K-type stars (about K2), and then begins to fade.
The Ca I 4227 line grows gradually in strength until the early K stars,
and then
becomes dramatically stronger by mid-K. The ratios Fe I
4046/H
and Fe I
4325/H
are useful
in estimating the temperature type, reversing at a spectral
type near G8. Unfortunately, these ratios are not reliable in metal-weak or
metal-strong stars. The temperature type may be estimated with precision, even
in metal-weak stars by using the ratio of the Cr I 4254 line with the two
neighboring Fe I lines at 4250 and 4260. The position of these lines is
indicated on the illustration by the three tick marks immediately below the G0
V spectrum. Notice that the Cr I line (which arises from a low-lying level)
becomes stronger in ratio with the two flanking Fe I lines, being clearly
stronger than both by K5.
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