or stars of Class B. Proceeding through the subdivisions of
Class B, the helium lines increase to a maximum of intensity
and then decrease. The dark hydrogen lines are more and more in
evidence, with intensities increasing slowly. In the middle and
later subdivisions of the helium stars silicon, oxygen and
nitrogen are usually represented by a few absorption lines.

Just as the gaseous nebulae radiate heat into space and
condense, so must the stars, with this difference: the nebulae
are highly rarified bodies, with surfaces enormously large in
proportion to the heat contents; and the radiation from them
must be relatively rapid. In fact, some of the nebulae seem to
be so highly rarified that radiation may take place from their
interiors almost as well as from their surfaces. The radiation
from a star just formed must occur at a much slower rate. The
continued condensation of the star, following the loss of heat,
must lead to a change of physical condition, which will be
apparent in the spectrum. It should pass from the so-called
helium group, to the hydrogen, or Class A group, not suddenly
but by insensible gradations of spectrum. In the Class A stars
the hydrogen lines are the most prominent features. The helium
lines have disappeared, except in a few stars where faint
helium remnants are in evidence. The magnesium lines have
become prominent and the calcium lines are growing rapidly in
strength. The so-called metallic lines, usually beginning with
iron and titanium lines, which have a few extremely faint
representatives in the last of the helium stars, become visible
here and there in the Class A spectra, but they are not
conspicuous.