The spectrum of stars is useful for classification purposes along with determining their atmosphere constituents and radial velocities.
The behavior of Hd and surrounding metal absorption lines upon spectral class:
Determination of atomic, ionic and molecular constituents in the atmospheres of some stars:
Epsilon Virgo a G8III star HD192913,a silicon Ap star Unusual metals in CH subgiant HD216219
HD1461 a G0 V Super Heavy Metal (SHM) star
Some of the atomic-ionic composition of Antares
The more exotic:
In addition, some halo stars exhibit the presence of Lithium:
Paper: Amateur Spectroscopy: Those Peculiar C and S Type stars
Detection of technetium in 19 Piscium a C5III star. Since technetium has no stable isotopes and the longest lived isotope is 2.6 million years, detection in the atmosphere of this star means it has been synthesized recently by neutron capture and subsequently dredged up from lower layers onto the surface where it can be detected.
Technetium is often, but not always detected in M, MS, MC, S and C type stars which are also often long period variables. Shown below is the long term variable R Andromeda which is an S type star where I have been able to identify a number of technetium lines along with enhanced C13 ratios compared to C12. These enhanced carbon isotope ratios are not as great as is seen in C type stars such as 19 Piscium shown above. I was unable to find the C13-C13 lines as I could in the spectra of C type star 19 Piscium. The spectra of R Andromeda was obtained on November 4, 2000 when the star was at a magnitude of ~9.
In addition, 19 Piscium is noted for its unusual carbon isotope composition. The ratio of C13 to C12 is much greater than that found in the Solar system (~80). This isotope anomaly is particularly well observed in diatomic carbon where the absorption lines for C12-C12, C12-C13 and C13-C13 are significantly different.
More on C12-C12, C12-C13 and C13-C13 ratios:This figure shows a variety of R and C type stars that possess varying ratios of C12 to C13 as reflected in the indicated by the absorption lines for diatomic carbon as C12-C12, C12-C13 and C13-C13. HD112869 has near solar system values for this ratio (~80), while C12-C13 makes its appearance in HD32736 and HD44987 with ratios around 20-30. Finally, with HD70138 and HD19557, these ratios are in the region of 5-10 and are reflected in the presence of the C13-C13 line.