35. The M Stars

Discussion of the M dwarfs is outside the range of the present Atlas. Since no stars have been observed intermediate between M dwarfs and giants, the latter can be considered separately.

Plate 51. The M Giant Sequence

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The titanium oxide bands in the photographic region increase smoothly in intensity with decreasing temperature, and spectral classification from the intensity of the bands is a temperature classification (Pl.52). The four stars illustrated in Plate 51 as standards of the M-giant sequence are on the Mount Wilson system. We are greatly indebted to Dr. Joy for checking our types at Mount Wilson. He has noted that some M stars probably vary slightly in spectral type, so that some of the standards illustrated may have a slightly different appearance at times.

Plate 52. The M Sequence as a Temperature Sequence

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The absolute magnitudes of some of the giant M stars have been discussed recently by Keenan, ⁽¹⁾ and the details of the luminosity classification are given there. Keenan's spectral types require systematic corrections to reduce them to the Mount Wilson system. Some luminosity effects in the early M giants are illustrated in Plate 53.

Table 30 gives a selection of stars whose luminosity classes have been taken from Keenan`s paper. The spectral types are from the Mount Wilson catalogue of spectroscopic parallaxes. Luminosity line ratios are 4045: 4077, 4215: 4250, 4376: 4383 and 4383: 4390.

TABLE 30
STANDARD M GIANTS

Star	MKK	a		m	HD	Notes
RW Cep .	M0: Ia	22:19	+55°27'	6.2-7.6*	Ma	1
µ Cep ....	M2 Ia	21:40	+58 19	4.4*	Ma
SU Per ...	M4 Ia-Ib	02:15	+56 09	7.3*	Ma
a Ori ......	M2 Ib	05:49	+07 23	0.9*	Ma
a Sco ......	M1 Ib	16:23	-26 13	1.2	Ma
5 Lac ......	M0 II	22:25	+47 12	4.6	K0
Aur .....	M3 II	05:52	+45 56	4.6	Ma
Peg .....	M2 II-III	22:58	+27 32	2.6*	Ma
Peg .....	M2 III	00:09	+19 39	4.9	Ma
And ....	M0 III	01:04	+35 05	2.4	Ma
Gem ....	M3 III	06:08	+22 32	3.7*	Ma
NOTES
1. The spectrum indicates that the absolute magnitude is brighter than µ Cep. Spectral type by Keenan.
* Light variable.

Plate 54. A Carbon Star and a Long Period Variable

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Plate 55. The Banded Stars in the Visual Region

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