here
describe the principal forms as they are defined in Part. II of Vol.
XXVIII of the Annals of the Harvard Observatory. It may be remarked that
PICKERING first arranged the types in alphabetical order A, B, C, &c.,
supposing that order to correspond to the temperature of the stars.
Later this was found to be partly wrong, and in particular it was found
that the B-stars may be hotter than those of type A. The following is
the temperature-order of the spectra according to the opinion of the
Harvard astronomers.

_Type O_ (WOLF-RAYET stars). The spectra of these stars consist mainly
of bright lines. They are characterized by the bright bands at
wave-lengths 463 [mu][mu] and 469 [mu][mu], and the line at 501 [mu][mu]
characteristic of gaseous nebulae is sometimes present.

This type embraces mainly stars of relatively small apparent brightness.
The brightest is [gamma] Velorum with _m_ = 2.22. We shall find that 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 precedi