The name is absent

58 Recent Advances in Stellar Astronomy
elucidation of the real relationship was first provided, a
dozen years or so ago, by the investigation of the spectra
of faint stars at Harvard; and, as soon as this became
available the main features of the situation were pointed
out, independently, by Hertzsprung and myself. If we
group the stars according to their spectra, we find that
those of Class B are always bright. Among the long
list studied by Kapteyn, those of Class B2 or “earlier”
are almost all brighter than the absolute magnitude zero,
that is, more than 100 times more luminous than the Sun.
The “later” B’s average fainter, and some of the A’s
come down to less than ten times the Sun’s light. In
Class F some stars come down to two or three times the
Sun’s light, yet Canopus, which belongs to this class, is
enormously bright. So the list continues, the lower limit
of brightness growing steadily fainter, by about two mag-
nitudes from each class to the next, while there is no in-
dication of a change in the upper limit. As far as Class
G the stars seem to be distributed all through the range
of brightness in which they occur; but in Class K, there
appears a division into two groups—brighter and fainter
—which in Class M has become very sharp and definite.
There are few, if any, stars of this spectral type which
are comparable with the Sun in brightness, while there
are many whose light is from 20 to 100 times the Sun’s,
and others with a brightness of from 1/50 to 1/500 that
of the Sun.

Combining all these results, we find that practically all
the stars fall into one or other of two great divisions. In
the first of these the brightness is very much the same,
whatever the spectral type, and averages about fifty times
that of the Sun : in the other the brightness falls off very
rapidly with increasing redness, stars of Class G averag-

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