The New Heavens | Page 4

I endeavored to ascertain this
point by counting many fields, and computing from a mean of them,
what a certain given portion of the Milky Way might contain." By this
means, applied not only to the Milky Way but to all parts of the
heavens, Herschel determined the approximate number and distribution
of all the stars within reach of his instrument.
By comparing many hundred gauges or counts of stars visible in a field
of about one-quarter of the area of the moon, Herschel found that the
average number of stars increased toward the great circle which most
nearly conforms with the course of the Milky Way. Ninety degrees
from this plane, at the pole of the Milky Way, only four stars, on the
average, were seen in the field of the telescope. In approaching the
Milky Way this number increased slowly at first, and then more and
more rapidly, until it rose to an average of 122 stars per field.
[Illustration: Fig. 5. Erecting the polar axis of the 100-inch telescope.]
These observations were made in the northern hemisphere, and
subsequently Sir John Herschel, using his father's telescope at the Cape
of Good Hope, found an almost exactly similar increase of apparent
star density for the southern hemisphere. According to his estimates,
the total number of stars in both hemispheres that could be seen
distinctly enough to be counted in this telescope would probably be
about five and one-half millions.
The Herschels concluded that "the stars of our firmament, instead of
being scattered in all directions indifferently through space, form a
stratum of which the thickness is small, in comparison with its length
and breadth; and in which the earth occupies a place somewhere about
the middle of its thickness, between the point where it subdivides into
two principal laminæ inclined at a small angle to each other." This view
does not differ essentially from our modern conception of the form of
the Galaxy; but as the Herschels were unable to see stars fainter than

the fifteenth magnitude, it is evident that their conclusions apply only
to a restricted region surrounding the solar system, in the midst of the
enormously extended sidereal universe which modern instruments have
brought within our range.
MODERN METHODS
The remarkable progress of modern astronomy is mainly due to two
great instrumental advances: the rise and development of the
photographic telescope, and the application of the spectroscope to the
study of celestial objects. These new and powerful instruments,
supplemented by many accessories which have completely
revolutionized observatory equipment, have not only revealed a vastly
greater number of stars and nebulæ: they have also rendered feasible
observations of a type formerly regarded as impossible. The chemical
analysis of a faint star is now so easy that it can be accomplished in a
very short time--as quickly, in fact, as an equally complex substance
can be analyzed in the laboratory. The spectroscope also measures a
star's velocity, the pressure at different levels in its atmosphere, its
approximate temperature, and now, by a new and ingenious method, its
distance from the earth. It determines the velocity of rotation of the sun
and of nebulæ, the existence and periods of orbital revolution of binary
stars too close to be separated by any telescope, the presence of
magnetic fields in sunspots, and the fact that the entire sun, like the
earth, is a magnet.
[Illustration: Fig. 6. Lowest section of tube of 100-inch telescope, ready
to leave Pasadena for Mount Wilson.]
Such new possibilities, with many others resulting from the application
of physical methods of the most diverse character, have greatly
enlarged the astronomer's outlook. He may now attack two great
problems: (1) The structure of the universe and the motions of its
constituent bodies, and (2) the evolution of the stars: their nature, origin,
growth, and decline. These two problems are intimately related and
must be studied as one.[*]
[Footnote *: A third great problem open to the astronomer, the study of

the constitution of matter, is described in Chapter III.]
If space permitted, it would be interesting to survey the progress
already accomplished by modern methods of astronomical research.
Hundreds of millions of stars have been photographed, and the
boundaries of the stellar universe have been pushed far into space, but
have not been attained. Globular star clusters, containing tens of
thousands of stars, are on so great a scale (according to Shapley) that
light, travelling at the rate of 186,000 miles per second, may take 500
years to cross one of them, while the most distant of these objects may
be more than 200,000 light-years from the earth. The spiral nebulæ,
more than a million in number, are vast whirling masses in process of
development, but we are not yet certain whether they should be
regarded as "island universes"