COSMOLOGY OF THE UNIVERSE 
45 
In the bulletin of the National Research Council, May, 1921, 
the same author says : “We know some of these important correla- 
tions with greater certainty in the star clusters than in the solar 
neighborhood. We now have the spectra of many individual 
stars, the colors and spectral type of variables and something also 
of the absolute luminosity of the brightest stars of the clusters 
from the appearance of the spectra. Is it surprising therefore that 
we venture to determine the distance of this cluster “Messier 13” 
and similar systems with more Confidence than was possible when 
none of these facts was known or even seriously considered in 
cosmic speculation ?” 
In a report on “The Physical Members of the Pleiades Group,” 
Lv. O. Bulletin No. 333, Mr. Robert Trumbler who has given ex- 
tensive study to this cluster through the application of these mod- 
ern criteria, says: “With respect to the distribution of the stars 
among the different spectral types, the relation between luminosity, 
spectral type and color, the proportion of double stars, etc., this 
cluster resembles our stellar system on the average, with the 
exception however that yellow and red giants are completely mis- 
sing in the Pleiades and that the stellar density is about ten times 
greater than in the immediate neighborhood of our sun.” 
As a result of these statistical studies we find that the stars 
differ greatly in absolute luminosity and absolute magnitude. By 
absolute magnitude is meant the apparent magnitude of the stars 
if they were all placed at the same distance, that is, at a distance 
corresponding to a parallax of one-tenth of a second of arc or 
thirty-two and six-tenths light years. This relation of absolute 
magnitude to spectral type shows that our own sun is a typical 
star of about the fifth absolute magnitude and neither a giant nor 
a dwarf, but is classed as a “G” type star. 
Today astronomers are divided, as to the extent and form of 
our stellar system, into two camps. One, whose leader is Dr. 
Shapley, conceives of our stellar system as practically unlimited 
in extent and similar in arrangement to star clusters. To quote 
again from Dr. Shapley’s paper, “One consequence of accepting 
the theory that clusters, outline the form and extent of the galactic 
system is that the sun is found to be very distant from the middle 
of the galaxy. It appears that we are not far from the center 
of a large local cluster or cloud, but that cloud is at least fifteen 
thousand light years from the center of the “Milky Way.” An- 
swering the suggestion made by Newcomb in The Structure of 
the Universe that the appearance of the “Milky Way” is due to 
