THE STAR CLUSTERS
From multiple stars the transition is natural to star clusters although
the gap between these types of stellar objects is very broad. The
familiar group of the winter sky known as the Pleiades is a loose
cluster, showing relatively very few stars even in telescopes or on
photographic plates. The "Beehive," or cluster known as Praesepe in
Cancer, and a double group in the sword-handle of Perseus, both just
visible to the naked eye, are excellent examples of star clusters of the
average type. When the moon is absent, they are easily recognized
without a telescope as little patches of nebulous light; but every
increase of optical power adds to their magnificence.
Then we come in regular succession to the truly marvelous globular
clusters, that for instance in Hercules. Messier 13, a recent photograph
of which, taken by Ritchey with the 60-inch reflector on Mount Wilson,
reveals an aggregation of more than 50,000 stars. But the finest
specimens are in the southern hemisphere. Sir John Herschel spent much
time investigating them nearly a century ago at the Cape of Good Hope.
His description of the cluster in the constellation of Centaurus is as
follows: "The noble globular cluster Omega Centauri is beyond all
comparison the richest and largest object of the kind in the heavens.
The stars are literally innumerable, and as their total light when
received by the naked eye affects it hardly more than a star of the
fifth or fourth to fifth magnitude, the minuteness of each star may be
imagined."
Others of these clusters are so remote that the separate stars are not
distinguishable, especially at the center, and their distances are
entirely beyond our present powers of direct measurement, although
methods of estimating them are in process of development. If gravitation
is regnant among the uncounted components of stellar clusters, as
doubtless it is, these stars must be in rapid motion, although our
photographs of measurements have been made too recently for us to detect
even the slightest motion in any of the component stars of a cluster.
The only variations are changes of apparent magnitude, of a type first
detected in a large number of stars in Omega Centauri, by Bailey of
Harvard, who by comparison of photographs of the globular clusters was
the first to find variable stars quite numerous in these objects. Their
unexplained variations of magnitude take place with great rapidity,
often within a few hours.
There are about a hundred of these globular clusters, and the radial
velocities of ten of them have been measured by Slipher and found to
range from a recession of 410 to an approach of 225 kilometers per
second. These excessive velocities are comparable with those found for
the spiral nebulae. Shapley has estimated the distances of many of these
bodies, which contain a large number of variable stars of the Cepheid
type. By assuming their absolute magnitudes equal to those of similar
Cepheids at known distances, he finds their distance represented by the
inconceivably minute parallax of 0".00012, corresponding to 30,000
light-years. This research also places the globular clusters far
outside and independent of our Galactic system of stars. The
distribution of the globular clusters has also been investigated, and
these interesting objects are found almost exclusively in but one
hemisphere of the sky. Its center lies in the rich star clouds of
Scorpio and Sagittarius. Success in finding the distances of these
objects has made it possible to form a general idea of their
distribution in three-dimensional space.
The numerous variable stars in any one cluster are remarkable for their
uniformity. Accepting variables of this type as a constant standard of
absolute brightness, and assuming that the differences of average
magnitude of the variables in different clusters are entirely due to
differences of distance, the relative distances of many clusters were
ascertained with considerable accuracy. Then it was found that the
average absolute magnitude of the twenty-five brightest stars in a
cluster is also a uniform standard, or about 1.3 magnitudes brighter
than the mean magnitude of the variables. This new standard was employed
in ascertaining the distances of other clusters not containing many
variables.
Shapley further shows that the linear dimensions of the clusters are
nearly uniform, and the proper relative positions in space are charted
for sixty-nine of these objects. We can determine the scale of the
charts, if we know the absolute brightness of our primary standard--the
variable stars; and this is deduced from a knowledge of the distances of
variables of the same type in our immediate stellar system.
The most striking of all the globular clusters, Omega Centauri, comes
out the nearest; nevertheless it is distant 6.5 kiloparsecs. A
kiloparsec is a thousand parsecs, and is the equivalent of 3,256
light-years. At the inconceivable distance of sixty-seven kiloparsecs,
or more than 200,000 light-years, is the most remote of the globular
clusters, known to astronomers as N.G.C. 7006, from its number in the
catalogue which records its position in the sky, the New General
Catalogue of nebulae by Dreyer of Armagh.
The clusters are widely scattered, and their center of diffusion is
about twenty kiloparsecs on the Galactic plane toward the region of
Scorpio-Sagittarius. Marked symmetry with reference to this plane makes
it evident that the entire system of globular clusters is associated
with the Galaxy itself. But to conceive of this it is necessary to
extend our ideas of the actual dimensions of the Galactic system. Almost
on the circumference of the great system of globular clusters our local
stellar system is found, and it contains probably all the naked-eye
stars, with millions of fainter ones. Its size seems almost diminutive,
only about one kiloparsec in diameter. The relative location of our
local stellar system shows why the globular clusters appear to be
crowded into one hemisphere only.
Shapley suggests that globular clusters can exist only in empty space,
and that when they enter the regions of space tenanted by stars, they
dissolve into the well-known loose clusters and the star clouds of the
Milky Way. Strangely the radial velocities of the clusters already
observed show that most of them are traveling toward this region, and
that some will enter the stellar regions within a period of the order of
a hundred million years.
The actual dimensions of globular clusters are not easy to determine,
because the outer stars are much scattered. To a typical cluster,
Messier 3, Shapley assigns a diameter of 150 parsecs, which makes it
comparable with the size of the stellar cluster to which the sun
belongs. Also on certain likely assumptions, he finds that the diameter
of the great cluster in Hercules, the finest one in our northern sky, is
about 350 parsecs, and its distance no less than 30,000 parsecs; in
other words, the staggering distance that light would require 9,750,000
years to travel over. While these distances can never be verified by
direct measurement, it lends great weight to the three methods of
indirect measurement, or estimation, (1) from the diameter of the image
of the clusters, (2) from the mean magnitude of the twenty-five
brightest stars, and (3) from the mean magnitude of the short period
variables, that they are in excellent agreement.