Astronomy: The Science of the Heavenly BodiesTodd, David P. (David Peck)
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Astronomy: The Science of the Heavenly Bodies
Todd, David P. (David Peck)
Astronomy
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 nebulæ 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.
Public-domain text, read in full here on John Shaqi.
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