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.

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.

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.