The Yerkes Spectral Classification

The Harvard classification only takes into account the effect of the temperature on the spectrum. For a more precise classification, one also has to take into account the luminosity of the star, since two stars with the same effective temperature may have widely different luminosities. A two-dimensional system of spectral classification was introduced by William W. Morgan, Philip C. Keenan and Edith Kellman of Yerkes Observatory. This system is known as the MKK or Yerkes classification. (The MK classification is a modified, later version.) The MKK classification is based on the visual scrutiny of slit spectra with a dispersion of 11.5 nm/mm. It is carefully defined on the basis of standard stars and the specification of luminosity criteria. Six different luminosity classes are distinguished:

- Ia most luminous supergiants,

- Ib less luminous supergiants,

- II luminous giants,

- III normal giants,

- IV subgiants,

- V main sequence stars (dwarfs).

The luminosity class is determined from spectral lines that depend strongly on the stellar surface gravity, which is closely related to the luminosity. The masses of giants and dwarfs are roughly similar, but the radii of giants are much larger than those of dwarfs. Therefore the gravitational acceleration g = GM/R2 at the surface of a giant is much smaller than for a dwarf. In consequence, the gas density and pressure in the atmosphere of a giant is much smaller. This gives rise to luminosity effects in the stellar spectrum, which can be used to distinguish between stars of different luminosities.

1. For spectral types B-F, the lines of neutral hydrogen are deeper and narrower for stars of higher luminosities. The reason for this is that the metal ions give rise to a fluctuating electric field near the hydrogen atoms. This field leads to shifts in the hydrogen energy levels (the Stark effect), appearing as a broadening of the lines. The effect becomes stronger as the density increases. Thus the hydrogen lines are narrow in absolutely bright stars, and become broader in main sequence stars and even more so in white dwarfs (Fig. 8.6).

2. The lines from ionized elements are relatively stronger in high-luminosity stars. This is because the higher density makes it easier for electrons and ions to recombine to neutral atoms. On the other hand, the

Yerkes Luminosity Classes

Fig. 8.6. Luminosity effects in the hydrogen Hy line in A stars. The vertical axis gives the normalized intensity. HD 223385 (upper left) is an A2 supergiant, where the line is very weak, 0 Aurigae A is a giant star and a2 Geminorum is a main sequence star, where the line is very broad. [Aller, L.H. (1953): Astrophysics. The Atmospheres of the Sun and Stars (The Ronald Press Company, New York) p. 318]

Fig. 8.6. Luminosity effects in the hydrogen Hy line in A stars. The vertical axis gives the normalized intensity. HD 223385 (upper left) is an A2 supergiant, where the line is very weak, 0 Aurigae A is a giant star and a2 Geminorum is a main sequence star, where the line is very broad. [Aller, L.H. (1953): Astrophysics. The Atmospheres of the Sun and Stars (The Ronald Press Company, New York) p. 318]

rate of ionization is essentially determined by the radiation field, and is not appreciably affected by the gas density. Thus a given radiation field can maintain a higher degree of ionization in stars with more extended atmospheres. For example, in the spectral classes F-G, the relative strengths of the ionized strontium (SrII) and neutral iron (FeI) lines can be used as a luminosity indicator. Both lines depend on the temperature in roughly the same way, but the Sr II lines become relatively much stronger as the luminosity increases.

3. Giant stars are redder than dwarfs of the same spectral type. The spectral type is determined from the strengths of spectral lines, including ion lines. Since these are stronger in giants, a giant will be cooler, and thus also redder, than a dwarf of the same spectral type.

4. There is a strong cyanogen (CN) absorption band in the spectra of giant stars, which is almost totally absent in dwarfs. This is partly a temperature effect, since the cooler atmospheres of giants are more suitable for the formation of cyanogen.

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