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At very high temperatures, the hydrogen becomes ionized. Since there is less neutral hydrogen, the Ha line becomes weaker. This explains why the Ha line is strongest in middle-temperature stars, and why the original scheme of classifying by hydrogen line strengths did not produce a sequence ordered in temperature.

We can apply a similar analysis to other elements. The details will differ because of different energy level structures and different ionization energies. It should be noted that, after hydrogen and helium, the abundances of the elements fall off drastically (see Appendix F for the abundances of the elements). In fact, astronomers often refer to hydrogen, helium and 'everything else'. The 'everything else' are collectively called metals, even though many of the elements don't fit our common definition of a metal.

We now look at the properties of different spectral types, in order of increasing temperature. Sample spectra are shown in Fig. 3.9, and the behaviors of a few spectral lines are shown in Fig. 3.10.

M Temperatures in M stars are below 3500 K, explaining their red color. The temperature is not high enough to produce strong Ha absorption, but some lines from neutral metals are seen. The stars are cool enough for simple molecules to form, and many lines are seen from molecules such as CN (cyanogen) and TiO (titanium oxide). If cool stars show strong CH lines, we designate them as C-type or 'carbon stars'. If any M

Samples of spectra from stars of different spectral types.The name of the star appears on the right of each spectrum, and the spectral type appears on the left. In each spectrum, the wavelength increases from left to right. Hotter stars are at the top. [NOAO/AURA/NSF]

Samples of spectra from stars of different spectral types.The name of the star appears on the right of each spectrum, and the spectral type appears on the left. In each spectrum, the wavelength increases from left to right. Hotter stars are at the top. [NOAO/AURA/NSF]

The relative strengths of spectral lines from important species as a function of spectral type. Each species shows the effects of excitation and ionization. For example, the increase in H line strengths from K to A stars occurs because the increasing temperature results in more hydrogen in the n = 2 (and higher) levels. However, the higher temperatures of the B and O stars ionize much of the hydrogen and the lines get much weaker.

The relative strengths of spectral lines from important species as a function of spectral type. Each species shows the effects of excitation and ionization. For example, the increase in H line strengths from K to A stars occurs because the increasing temperature results in more hydrogen in the n = 2 (and higher) levels. However, the higher temperatures of the B and O stars ionize much of the hydrogen and the lines get much weaker.

star has strong ZrO (zirconium oxide) lines as opposed to TiO lines, we call it an S-type.

K Temperatures range from 3500 to 5000 K. There are many lines from neutral metals. The H lines are stronger than in M stars but most of the H is still in the ground state.

G Temperatures in the range 5000-6000 K. The Sun is a G2 star. The H lines are stronger than in K stars, as more atoms are in excited states. The temperature is high enough for metals with low ionization energies to be partially ionized. Two prominent lines are from Ca(II). When Fraunhofer studied the solar spectrum, he gave the strongest lines letter designations. These Ca(II) lines are the H and K lines in his sequence.

F Temperatures range from 6000 to 7500 K. The H lines are a little stronger than in G stars. The ionized metal lines are also stronger.

A Temperatures range from 7500 to 10 000 K. These stars are white-blue in color. They have the strongest H lines. Lines of ionized metals are still present.

B Temperatures are in the range 10 000-30 000 K, and the stars appear blue. The H lines are beginning to weaken because the temperatures are high enough to ionize a significant fraction of the hydrogen. The lines of neutral and singly ionized helium begin to appear. Otherwise there are relatively few lines in the spectrum.

O Temperatures range from 30 000 to over 60 000 K, and the stars appear blue. The earliest spectral types that have been seen are O3 stars and there are very few O3 and 04 stars. The hydrogen lines fall off very sharply because of the high rate of ionization. The lines of singly ionized helium are still present, but there are very few lines overall in the visible part of the spectrum. There are several lines in the ultraviolet.

Some stars have emission as well as absorption lines in their spectra. These stars are designated with an 'e' after the spectral class, for example, Oe, Be, Ae, etc. 0 stars with very broad emission lines are called Wolf-Rayet stars. These stars probably have circumstellar material that has been ejected from the star. (Wolf-Rayet stars are not the only stars with such outflowing material.)

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