xtended 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 nebulae: 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 nebulae, 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