Figure 10-9 RIVUXG Principal Types of Stellar Spectra Stars of different spectral classes and different surface temperatures have spectra dominated by different absorption lines. Notice how the lines of hydrogen (Hα, Hβ, Hγ, and Hδ) are strongest for hot stars of spectral class A, while absorption lines due to calcium (Ca) are strongest in cooler K and M stars. The spectra of M stars also have broad, dark bands caused by molecules of titanium oxide (TiO), which can only exist at relatively low temperatures. A Roman numeral after a chemical symbol shows whether the absorption line is caused by un-ionized atoms (Roman numeral I) or by atoms that have lost one electron (Roman numeral II).