Temperature, Density, and Ionization
At first the 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.
The key to understanding stellar spectra is that the degree of ionization for
various atoms depends strongly on the temperature, as
illustrated in the adjacent figure for hydrogen
and helium. These plots show the relative abundance of different ionization states of these two atoms
as a function of temperature. Thus, hydrogen is present almost entirely in the form of neutral hydrogen (H I)
for temperatures below about 7000 K, but above that temperature there is a rapid transition so that above
about 10,000 K the hydrogen is present almost completely as ionized hydrogen (H II). In the region
7000 - 10,000 K the gas is a mixture of H I and H II.
Similar considerations hold for helium, but now there are
three possibilities. Below about 12,000 K, helium is present primarily as neutral helium (He I).
From that temperature up to about 29,000 K, helium is primarily in the first ionized state (He II),
and above about 29,000 K most helium is completely ionized (He III).
In addition, there is a much weaker dependence of the ionization on pressure (or equivalently,
density). This is
illustrated for hydrogen in the following diagram, which shows the degree of ionization
versus temperature for two different assumed pressures in the surface layers of the star (a dyne is a
standard unit of force).
Summary: Ionization and Surface Temperature and Density
From these diagrams we conclude that
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).
Since the spectrum of an ion generally differs from 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.
Helium requires higher temperatures to ionize than hydrogen.
There is a weak dependence of the degree of ionization on the
pressure (and hence the density) at a given temperature.
These observations permit us to interpret the spectral sequence as primarily a temperature
sequence, modified by small effects associated with the density.
Here is an
animation illustrating the dependence of ionization on temperature.