The Harvard Spectral Classification

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The spectral classification scheme in present use was developed at Harvard Observatory in the United States in the early 20th century. The work was begun by Henry Draper who in 1872 took the first photograph of the

spectrum of Vega. Later Draper's widow donated the observing equipment and a sum of money to Harvard Observatory to continue the work of classification.

The main part of the classification was done by Annie Jump Cannon using objective prism spectra. The Henry Draper Catalogue (HD) was published in 1918-1924. It contains 225,000 stars extending down to 9 magnitudes. Altogether more than 390,000 stars were classified at Harvard.

The Harvard classification is based on lines that are mainly sensitive to the stellar temperature, rather than to gravity or luminosity. Important lines are the hydrogen Balmer lines, the lines of neutral helium, the iron lines, the H and K doublet of ionized calcium at 396.8 and 393.3 nm, the G band due to the CH molecule and some metals around 431 nm, the neutral calcium line at 422.7 nm and the lines of titanium oxide (TiO).

The main types in the Harvard classification are denoted by capital letters. They were initially ordered in alphabetical sequence, but subsequently it was noticed that they could be ordered according to temperature. With the temperature decreasing towards the right the sequence is

Additional notations are Q for novae, P for planetary nebulae and W for Wolf-Rayet stars. The class C consists of the earlier types R and N. The spectral classes C and S represent parallel branches to types G-M, differing in their surface chemical composition. The most recent addition are the spectral classes L and T continuing the sequence beyond M, representing brown dwarfs. There is a well-known mnemonic for the spectral classes, but due to its chauvinistic tone we refuse to tell it.

The spectral classes are divided into subclasses denoted by the numbers 0... 9; sometimes decimals are used, e.g. B0.5 (Figs. 8.3 and 8.4). Spectra of brown dwarfs are shown in Fig. 8.4a compared with those of M dwarfs.

The main characteristics of the different classes are:

O Blue stars, surface temperature 20,000-35,000 K.

Spectrum with lines from multiply ionized atoms, e.g. He II, CIII, NIII, OIII, SiV. He I visible, HI

lines weak.

Fig. 8.3a,b. Spectra of early and late spectral type stars between 375 and 390 nm. (a) The upper star is Vega, of spectral type A0, and (b) the lower one is Aldebaran, of spectral type

K5. The hydrogen Balmer lines are strong in the spectrum of Vega; in that of Aldebaran, there are many metal lines. (Lick Observatory)

Fig. 8.3a,b. Spectra of early and late spectral type stars between 375 and 390 nm. (a) The upper star is Vega, of spectral type A0, and (b) the lower one is Aldebaran, of spectral type

B Blue-white stars, surface temperature about 15,000 K. He II lines have disappeared, He I (403 nm) lines are strongest at B2, then get weaker and have disappeared at type B9. The K line of Call becomes visible at type B3. H I lines getting stronger. OII, Si II and Mg II lines visible.

A White stars, surface temperature about 9000 K. The HI lines are very strong at A0 and dominate the whole spectrum, then get weaker. H and K lines of CaII getting stronger. He I no longer visible. Neutral metal lines begin to appear.

F Yellow-white stars, surface temperature about 7000K. HI lines getting weaker, H and K of CaII getting stronger. Many other metal lines, e. g. Fe I, Fe II, Cr II, Ti II, clear and getting stronger.

G Yellow stars like the Sun, surface temperature about 5500 K. The H I lines still getting weaker, H and K lines very strong, strongest at G0. Metal lines getting stronger. G band clearly visible. CN lines seen in giant stars.

K Orange-yellow stars, surface temperature about 4000 K. Spectrum dominated by metal lines. HI lines insignificant. Ca 1422.7 nm clearly visible. Strong H and K lines and G band. TiO bands become visible at K5.

M Red stars, surface temperature about 3000 K. TiO bands getting stronger. CaI 422.7 nm very strong. Many neutral metal lines.

L Brown (actually dark red) stars, surface temperature about 2000 K. The TiO and VO bands disappear for early L class. Very strong and broad lines of Na I and KI.

T Brown dwarfs, surface temperature about 1000 K. Very strong molecular absorption bands of CH4 and H2O.

K5. The hydrogen Balmer lines are strong in the spectrum of Vega; in that of Aldebaran, there are many metal lines. (Lick Observatory)

C Carbon stars, previously R and N. Very red stars, surface temperature about 3000 K. Strong molecular bands, e.g. C2, CN and CH. No TiO bands. Line spectrum like in the types K and M. S Red low-temperature stars (about 3000 K). Very clear ZrO bands. Also other molecular bands, e.g. YO, LaO and TiO.

The main characteristics of the classification scheme can be seen in Fig. 8.5 showing the variations of some typical absorption lines in the different spectral classes. Different spectral features are mainly due to different effective temparatures. Different pressures and chemical compositions of stellar atmospheres are not very important factors in the spectral classification, execpt in some peculiar stars. The early, i. e. hot, spectral classes are characterised by the lines of ionized atoms, whereas the cool, or late, spectral types have lines of neutral atoms. In hot stars molecules dissociate into atoms; thus the absorption bands of molecules appear only in the spectra of cool stars of late spectral types.

To see how the strengths of the spectral lines are determined by the temperature, we consider, for example, the neutral helium lines at 402.6 nm and 447.2 nm. These are only seen in the spectra of hot stars. The reason for this is that the lines are due to absorption by excited atoms, and that a high temperature is required to produce any appreciable excitation. As the stellar temperature increases, more atoms are in the required excited state, and the strength of the helium lines increases. When the temperature becomes even higher, helium begins to be ionized, and the strength of the neutral helium lines begins to decrease. In a similar way one can understand the variation with temperature of other important lines, such as the calcium H and K

Optical Spectra DwarfsHarvard Classification Spectral Type

Fig. 8.4. (a) Intensity curves for various spectral classes showing characteristic spectral features. The name of the star and its spectral and luminosity class are given next to each curve, and the most important spectral features are identified. (Drawing by J. Dufay) (b) Optical spectra of M stars and brown dwarfs. In an approximate sense, the brown dwarfs continue the spectral sequence towards lower temperatures, although in many respects they differ from the earlier spectral types. (J.D. Kirkpatrick 2005, ARAA43, 205)

Fig. 8.5. Equivalent widths of some important spectral lines in the various spectral classes. [Struve, O. (1959): Elementary Astronomy (Oxford University Press, New York) p. 259]

Fig. 8.5. Equivalent widths of some important spectral lines in the various spectral classes. [Struve, O. (1959): Elementary Astronomy (Oxford University Press, New York) p. 259]

Spectral Class

lines. These lines are due to singly ionized calcium, and the temperature must be just right to remove one electron but no more.

The hydrogen Balmer lines Hp, HY and H5 are stongest in the spectral class A2. These lines correspond to transitions to the level the principal quantum number of which is n = 2. If the temprature is too high the hydrogen is ionized and such transitions are not possible.

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