The Spectral Sequence as a
Temperature Sequence

At first the Harvard Spectral Sequence was thought to reflect different compositions for different stars. We now know that the different spectral types are primarily a consequence of different surface temperatures for the stars, with composition differences playing only a minor role.

Ionization and Surface Temperature

The 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

Ionization of Hydrogen

and in this image for helium:

Ionization of Helium

From these diagrams we conclude that

  1. The higher the temperature, the more likely that higher states of ionization are produced. For example, we see that at a temperature of 5000 K almost all hydrogen is present as neutral hydrogen (H I), but at 10,000 K almost all hydrogen is in the form of ions (H II).
  2. Since the spectrum of an ion is generally different than the spectrum of the corresponding atom, what lines appear in the spectrum will depend strongly on the temperature. For example, the above figures indicate that neutral hydrogen lines would be present at a temperature of 5000 K, but almost absent at 15,000 K.
  3. Helium requires higher temperatures to ionize than hydrogen.
  4. There is a weak but not zero dependence of the degree of ionization on the pressure (and hence the density) at a given temperature.

Interpretation of the Harvard Spectral Sequence

Thus, 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.

Ionization vs. Spectral Class

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 Temperature

Generally, 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.