so the left side of equation (3.10) simplifies to n2/n0, where n0 is the number of neutrals. This extra factor of ne makes even this simpler form of the Saha equation harder to solve for (ne/n0 ) than the Boltzmann equation is to solve for the ratio of level populations. In Fig. 3.8, we show the ratio ne/n0 as a function of temperature, for a value of ne reasonable for stars like the Sun.
The ionization energies of some common atoms are given in Table 3.1. This table is useful in deciding which ions you are likely to encounter at various temperatures. In designating ionized atoms, there is a shorthand that has been adopted. The roman numeral I is used to designate the neutral species, II the singly ionized species, III the doubly ionized species, and so on. For example, neutral hydrogen is H(I), ionized hydrogen (H+) is H(II), doubly ionized carbon is C(III).
We are now in a position to discuss the intensities of various absorption lines in stars. We will take Ha as an example to see the combined effects of excitation and ionization. At low temperatures, essentially all of hydrogen is neutral, and most of it is in the ground state. Since little H will be in the second state, there will be few chances for Ha absorption. The Ha line will be weak.
As we go to moderate temperatures, most of the hydrogen is still neutral. However, more of the hydrogen is in excited states, meaning that a reasonable amount will be in level 2. Ha absorption is possible. As the temperature increases, the Ha absorption becomes stronger.
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