Interpretation of the Spectral Sequence

The preceding discussion of ionization indicates that the spectral sequence is primarily a temperature sequence (more precisely, a surface temperature sequence, since the spectral lines are generated by absorption in the surface region of the star). The following diagram illustrates qualitatively the dominant spectral lines for each spectral class.

For example, a dominant characteristic of spectral class A stars is the presence of strong hydrogen lines, but ionized helium lines are present only in class O stars. 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 found only for spectral classes K and M. This is because these classes correspond to low surface temperatures, and molecules can hold together only in cool stars because chemical bonds are relatively fragile.

We give a more complete characterization of the spectral sequence in the following table, including the surface temperatures for each spectral class and example stars:

The Harvard Spectral Sequence
Spectral Class Distinguishing
Spectral Characteristics		 Temperature (K) Examples
O Ionized He and metals;
weak H		 28,000-60,000 zeta Orionis (O9.5)
B Neutral He, ionized metals, stronger H 10,000-28,000 Rigel (B8)
Spica (B1)
A Balmer H dominant, singly-ionized metals 7500-10,000 Sirius (A1)
Deneb (A2)
F H weaker, neutral and singly-ionized metals 6000-7500 Procyon (F5)
Canopus (F0)
G Singly ionized Ca, H weaker, neutral metals 5000-6000 Sun (G2)
Capella (G5)
K Neutral metals, molecular
bands appear		 3500-5000 Aldebaran (K5)
Arcturus (K1.5)
M Ti oxide molecular lines;
neutral metals		 < 3500 Antares (M1.5)
Betelgeuse (M1)

Animation: the Harvard spectral sequence