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4. THE SUPERGIANTS OF CLASSES B8-M2

The general appearance of the spectra of the supergiants of types A-K is different from that of stars of lower luminosity; and, when an attempt is made to classify the high-luminosity stars by the ratios used for ordinary giants and dwarfs, a number of difficulties are encountered. Ratios which include a hydrogen line are strongly affected by absolute-magnitude effects in classes B8-F0 and G8-K5; in the first spectral interval the H lines are greatly weakened in the supergiants, and in the second they are considerably strengthened. The lines used to classify the A5-F5 spectra are disturbed by blends in the supergiants which have a marked absolute-magnitude effect. In addition, the G band appears as a fairly continuous absorption only for types later than F8 in the supergiants; while in ordinary giants and dwarfs it is present at F5 on plates similar to the ones used in preparing the Atlas.

For these reasons, if a highly accurate system is to be defined for supergiants and Cepheids, it is important to set up a sequence of standard supergiants by criteria suitable for the high-luminosity stars. The system defined by the supergiants in Table 31 is in fairly good systematic agreement with the Henry Draper Catalogue. The stars listed define the system accurately to about a tenth of a class, except in the case of the late A and early F subdivisions, where the accuracy is appreciably lower.

Some ratios useful in determining the spectral type of the supergiants are: lambda 4128-lambda 4130: lambda 4172-lambda 4179 (A0-F0), lambda 4226: Hdelta (F5-G5), lambda 4045: Hlambda (F5-G8), lambda 4226: Hlambda (F5-K5), lambda 4325: Hlambda (F5-G2), blend at lambda 4176: blend at lambda 4200 (G5-K5), lambda 4383: lambda 4406 (G8-K5), and the appearance of the region of the G band (F0-K5).

Plate 4. Supergiants O9.5 - A0
Plate 4
High resolution image

Plate 20. Supergiants A0 - F0
Plate 20
High resolution image

Plate 30. Supergiants F0 - K5
Plate 30
High resolution image

Plate 38. High Luminosity Stars at F8
Plate 38
High resolution image

Plate 47. High Luminosity Stars at K1
Plate 47
High resolution image

Plate 49. High Luminosity Stars at K3
Plate 49
High resolution image

Plate 53. Luminosity Effects in the Early M Giants
Plate 53
High resolution image

No stars have been classified as Ia between F8 and M2; it is possible that certain luminous irregular variable stars may belong to this class in the G and K types. It is also possible that stars of the highest luminosity develop TiO bands at slightly lower temperatures than the F8 Ia stars delta CMa and rho Cas; they might then be classified among the M stars, while their line spectra correspond to class G or K.

TABLE 31
THE SUPERGIANTS OF CLASSES B8-M2

Star MKK a delta m HD Notes

beta Ori ..... B8 Ia 05:09 -08°19' 0.3 B8p
4 Lac ..... B8 Ib 22:20 +48 58 4.6 B8p
sigma Cyg ..... B9 Ia 21:13 +38 59 4.3 A0p
HR 1040 .. A0 Ia 03:21 +58 32 4.8 A0p 1
13 Mon .... A0 Ib 06:27 +07 24 4.5 A0p
eta Leo ..... A0 Ib 10:01 +17 15 3.6 A0p
a Cyg ..... A2 Ia 20:38 +44 55 1.3 A0p
nu Cep ..... A2 Ia 21:42 +60 40 4.5 A0p
phi Cas ..... A5 Ia 01:13 +57 42 5.3 F5P
epsilon Aur ..... F0 Ia 04:54 +43 41 (3.3) F5p
a Lep ..... F0 Ib 05:28 -17 54 2.7 F0
a Per ..... F5 Ib 03:17 +49 30 1.9 F5
delta CMa ..... F8 Ia 07:04 -26 14 2.0 F8p
rho Cas ..... F8 Ia 23:49 +56 57 (4.4) F8p
gamma Cyg ..... F8 Ib 20:18 +39 56 2.3 F8p
beta Aqr ..... G0 Ib 21:26 -06 01 3.1 G0
epsilon Leo ..... G0 I-II 09:40 +24 14 3.1 G0p
a Aqr ..... G1 Ib 22:00 -00 48 3.2 G0
zeta Cap ..... G4 Ib? 21:21 -22 51 3.9 G5p 2
9 Peg ..... G5 Ib 21:39 +16 53 4.5 G5
epsilon Gem ..... G8 Ib 06:37 +25 14 3.2 G5
56 Peg .... G8 Ib 23:02 +24 56 5.0 K0
zeta Cep ..... K1 Ib 22:07 +57 42 3.6 K0
epsilon Peg ..... K3 Ib 21:39 +09 25 2.5 K0
gamma Aql ..... K3 I-II 19:41 +10 22 2.8 K2
xi Cyg ..... K5 Ib 21:01 +43 32 3.9 K5
a Sco ..... M1 Ib 16:23 -26 13 1.2 Ma 3
µ Cep ..... M2 Ia 21:40 +58 19 var Ma 3
a Ori ..... M2 Ib 05:49 +07 23 var Ma 3

NOTES
1. The H lines are slightly stronger than in beta Ori.
2. The line Sr II 4077 is very Strong.
3. The Mount Wilson spectral types of the M giants have been assumed.

Plate 30. Supergiants F0 - K5
Plate 30
High resolution image

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