|Themes > Science > Astronomy > The Stars > Ordinary Stars > The Spectral Sequence as a Temperature Sequence|
Ionization and Surface TemperatureThe key to understanding stellar spectra is that the degree of ionization for various atoms depends strongly on the temperature and weakly on the density, as illustrated in the following image for hydrogen
and in this image for helium:
From these diagrams we conclude that
Interpretation of the Harvard Spectral SequenceThus, we may interpret the Harvard Spectral Sequence as primarily a temperature sequence. The following diagram illustrates, giving qualitatively the dominant spectral lines for different spectral classes.
For example, a dominant characteristic of spectral class A stars is the presence of strong hydrogen lines, but ionized helium lines are only present in the class O stars. As noted above, since helium ionizes only at high temperatures, this tells us that class O stars must have very high surface temperatures. On the other hand, spectral lines associated with molecules are only found for spectral classes K and M. This is because these correspond to low surface temperatures, and molecules can only hold together in stars with relatively low surface temperatures.
Spectral Class and Surface TemperatureGenerally, the above sequence is a surface temperature sequence, with high temperatures toward the left (O and B stars) and low temperatures to the right (K and M stars). Since the color index is also a measure of surface temperature, this sequence may also be interpreted in terms of color index, with algebraically smaller values of the color index to the left and larger values to the right.