64 Recent Advances in Stellar Astronomy
are as much as five thousand times as bright as the Sun.
Their distances range from sixty parsecs (for the Pole-
star, which shows a slight variation of this type) to more
than five thousand parsecs in a few instances. But these
are not the greatest distances that can be measured by this
very powerful photometric method. Shapley derives a
distance for the Smaller Magellanic Cloud, of 19,000
parsecs. More recently Seares, working on similar prin-
ciples, but utilizing our knowledge of the absolute magni-
tudes of blue stars (whose color indicates that they are
of Class B) has estimated that the star clouds in the
Milky Way itself lie at distances ranging from 6,000 to
15,000 parsecs.
Even these distances are surpassed by those of the great
globular clusters of stars, which have been the object of
one of Shapley’s most striking investigations. From a
study of the Cepheid variables which appear in these clus-
ters, and by a most ingenious combination of other data,
he has succeeded in obtaining reliable estimates of the dis-
tances of all the known objects of this type, 69 in all.
These clusters are extraordinarily similar in size and
constitution. The denser central portion of any one is
some 10 parsecs in diameter, while the outlying stragglers
extend to a distance of more than 50 parsecs in all direc-
tions. Within this region there are at least 40,000 stars
brighter than the Sun, and probably a still greater num-
ber of fainter ones. The brightest stars, which are almost
all red giants, average about 1,000 times the Sun’s
luminosity.
The nearest of these gigantic systems—which is visible
to the naked eye in southern latitudes as a hazy star,
Omega Centauri—is 6,500 parsecs away, while the re-
motest of them is at the enormous distance of 65,000 par-