Lectures on Stellar Statistics | Page 7

hydrogen and were called "additional lines of hydrogen".
Type B (Orion type, Helium stars). All lines are here dark. Besides the hydrogen series we here find the He-lines (396, 403, 412, 414, 447, 471, 493 [mu][mu]).
To this type belong all the bright stars ([beta], [gamma], [delta], [epsilon], [zeta], [eta] and others) in Orion with the exception of Betelgeuze. Further, Spica and many other bright stars.
On plate III [epsilon] Orionis is taken as representative of this type.
Type A (Sirius type) is characterized by the great intensity of the hydrogen lines (compare plate III). The helium lines have vanished. Other lines visible but faintly.
The greater part of the stars visible to the naked eye are found here. There are 1251 stars brighter than the 6th magnitude which belong to this type. Sirius, Vega, Castor, Altair, Deneb and others are all A-stars.
Type F (Calcium type). The hydrogen lines still rather prominent but not so broad as in the preceding type. The two calcium lines H and K (396.9, 393.4 [mu][mu]) strongly pronounced.
Among the stars of this type are found a great many bright stars (compare the third chapter), such as Polaris, Canopus, Procyon.
Type G (Sun type). Numerous metallic lines together with relatively faint hydrogen lines.
To this class belong the sun, Capella, [alpha] Centauri and other bright stars.
Type K. The hydrogen lines still fainter. The K-line attains its maximum intensity (is not especially pronounced in the figure of plate III).
This is, next to the A-type, the most numerous type (1142 stars) among the bright stars.
We find here [gamma] Andromed?, [beta] Aquil?, Arcturus, [alpha] Cassiopei?, Pollux and Aldebaran, which last forms a transition to the next type.
Type M. The spectrum is banded and belongs to SECCHI's third type. The flutings are due to titanium oxide.
Only 190 of the stars visible to the naked eye belong to this type. Generally they are rather faint, but we here find Betelgeuze, [alpha] Herculis, [beta] Pegasi, [alpha] Scorpii (Antares) and most variables of long period, which form a special sub-type Md, characterized by bright hydrogen lines together with the flutings.
Type M has two other sub-types Ma and Mb.
Type N (SECCHI's fourth type). Banded spectra. The flutings are due to compounds of carbon.
Here are found only faint stars. The total number is 241. All are red. 27 stars having this spectrum are variables of long period of the same type as Md.
The spectral types may be summed up in the following way:--
White stars:--SECCHI's type I:--Harvard B and A, Yellow " :-- " " II:-- " F, G and K, Red " :-- " " III:-- " M, " " :-- " " IV:-- " N.
The Harvard astronomers do not confine themselves to the types mentioned above, but fill up the intervals between the types with sub-types which are designated by the name of the type followed by a numeral 0, 1, 2, ..., 9. Thus the sub-types between A and F have the designations A0, A1, A2, ..., A9, F0, &c. Exceptions are made as already indicated, for the extreme types O and M.
11. Spectral index. It may be gathered from the above description that the definition of the types implies many vague moments. Especially in regard to the G-type are very different definitions indeed accepted, even at Harvard.[6] It is also a defect that the definitions do not directly give quantitative characteristics of the spectra. None the less it is possible to substitute for the spectral classes a continuous scale expressing the spectral character of a star. Such a scale is indeed implicit in the Harvard classification of the spectra.
Let us use the term spectral index (s) to define a number expressing the spectral character of a star. Then we may conveniently define this conception in the following way. Let A0 correspond to the spectral index s = 0.0, F0 to s = +1.0, G0 to s = +2.0, K0 to s = +3.0 M0 to s = +4.0 and B0 to s = -1.0. Further, let A1, A2, A3, &c., have the spectral indices +0.1, +0.2, +0.3, &c., and in like manner with the other intermediate sub-classes. Then it is evident that to all spectral classes between B0 and M there corresponds a certain spectral index s. The extreme types O and N are not here included. Their spectral indices may however be determined, as will be seen later.
Though the spectral indices, defined in this manner, are directly known for every spectral type, it is nevertheless not obvious that the series of spectral indices corresponds to a continuous series of values of some attribute of the stars. This may be seen to be possible from a comparison with another attribute which may be rather markedly graduated, namely the colour of the stars. We shall discuss this point in another paragraph. To obtain a well graduated scale of the spectra