,
are very faint and thin, or quite invisible, and the blue and white
parts are very intensely bright. Vogel subdivides the class into three
groups. In the first (I.a) the hydrogen lines are present, and are
remarkably broad and intense; Sirius, Vega, and Regulus are examples of
this group. The great breadth of the lines probably indicates that these
stars are surrounded by hydrogen atmospheres of great dimensions. It is
generally acknowledged that stars of this group must be the hottest of
all, and support is lent to this view by the appearance in their
spectra of a certain magnesium line, which, as Sir Norman Lockyer showed
many years ago, by laboratory experiments, does not appear in the
ordinary spectrum of magnesium, but is indicative of the presence of the
substance at a very high temperature. In the spectra of stars of Group
I.b the hydrogen lines and the few metallic lines are of equal breadth,
and the magnesium line just mentioned is the strongest of all. Rigel and
several other bright stars in Orion belong to this group, and it is
remarkable that helium is present at least in some of these stars, so
that (as Professor Keeler remarks) the spectrum of Rigel may almost be
regarded as the nebular spectrum reversed (lines dark instead of
bright), except that the two chief nebular lines are not reversed in the
star. This fact will doubtless eventually be of great importance to our
understanding the successive development of a star from a nebula; and a
star like Rigel is no doubt also of very high temperature. This is
probably not the case with stars of the third subdivision of Type I.
(I.c), the spectra of which are distinguished by the presence of bright
hydrogen lines and the bright helium line D3. Among the stars having
this very remarkable kind of spectrum is a very interesting variable
star in the constellation Lyra (b) and the star known as g
Cassiopeiae, both of which have been assiduously observed, their spectra
possessing numerous peculiarities which render an explanation of the
physical constitution of the stars of this subdivision a very difficult
matter.

Passing to _Type II._, we find spectra in which the metallic lines are
strong. The more refrangible end of the spectrum is fainter than in the
previous Class, and absorption bands are sometimes found towards the red
end. In its first subdivision (II.a) are contained spectra with a large
number of strong and well-defined lines due to metals, the hydrogen