Normal stars

The spectra of stars of class M are characterized by strong absorption bands of TiO, and by the great number and strength of metallic lines which practically block the spectrum for λ < 4000 Å.

In the Harvard system four subdivisions of M-type stars were kept: Ma, Mb, Mc and Md. The Mt Wilson observers replaced these types by the decimal subdivisions M0, 1,…, 6 in which Ma corresponds to M0–M2, Mb to M3–M5, and Mc to M5 and later. The last group, Md, is characterized by variable emission lines, so that any Md object should be denoted Mxe, x standing for the decimal subtype and e for emission. The Mt Wilson system uses mainly the intensities of the TiO bands for classification and this is also done in the Yerkes system. The reason is that the bands strengthen so rapidly with advancing type that it becomes difficult to find stretches of continuum where atomic lines can be compared reliably, except for the earlier subtypes. The most important TiO bands are listed in table 14.1.

The first bands which appear in the yellow and red are λ5167 and λ7054, and λ4954 in the blue at M0. After M2 many others can be seen; the ones especially useful are marked with an asterisk in table 14.1.