Lectures on Stellar Statistics | Page 7

the absolute magnitude of these stars nearly coincides with that of the
stars of type B.
The type is grouped into five subdivisions represented by the letters Oa,
Ob, Oc, Od and Oe. These subdivisions are conditioned by the varying
intensities of the bright bands named above. The due sequence of these
sub-types is for the present an open question.
Among interesting stars of this type is [zeta] Puppis (Od), in the
spectrum of which PICKERING discovered a previously unknown
series of helium lines. They were at first attributed (by RYDBERG) to
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] Andromedæ, [beta] Aquilæ, Arcturus, [alpha]
Cassiopeiæ, 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