The branch of physics which at the present day has assumed such mighty
and far-reaching proportions in astronomical work is that dealing with
spectrum analysis, which, although suggested as early as the time of
Kepler, did not receive any impetus as regards its application to
celestial bodies until the beginning of the present century at the
hands of Wollaston and Fraunhofer. Then, however, it still lacked the
chemical touch supplied afterwards by Kirchhoff and Bunsen. They showed
us that the spectrum observed when the light from any heated body is
passed through a prism is an index to the chemical composition of the
light source; the constitution of a vapor when in a condition to absorb
light can be determined by an extension of the same principle, first
demonstrated by Stokes, Angström, and Balfour Stewart, when the century
was about half completed.
The first celestial body towards which the spectroscope was turned was
our central luminary, the sun.
Wollaston first discovered that its spectrum was crossed by a few
dark lines; we learned next from Fraunhofer, who in 1814 worked with
instruments of greater power, that the solar spectrum was crossed
not only by a few dark lines, but by some hundreds. Not content with
examining the light of the sun, Fraunhofer turned his instrument
towards the stars, the light of which he also examined, so that he may
be justly called the inventor of stellar spectrum analysis. It is not
to the credit of modern science that from this time forward spectrum
analysis did not become a recognized branch of scientific inquiry, but,
as a matter of fact, Fraunhofer’s observations were buried in oblivion
for nearly half a century. The importance of them was not recognized
till the origin of the dark lines, both in sun and stars, had been
explained by Stokes and others, as before stated. The lines in the
solar spectrum were mapped with great diligence by Kirchhoff in 1861
and 1862, and later by Angström and Thalen, and this was done side by
side with chemical work in the laboratory. The chemistry of the sun was
thus to a great extent revealed; it was no longer a habitable globe,
but one with its visible boundary at a fierce heat, surrounded by an
atmosphere of metallic vapors, chief among them iron, also in a state
of incandescence. To these metallic vapors Angström added hydrogen
shortly afterwards.
Here, then, was established a firm link between the heavens and the
earth; the first step to the problem of the chemistry of space had been
taken.
It was only natural that as advances were made the instrumental
equipment should keep pace with them. Spectroscopes were built on
a larger scale; more prisms, which meant greater dispersion, were
employed to render the measurements of the lines in spectra more
accurate. The growth of our knowledge especially necessitated the
making of maps of the lines in the solar spectrum, and in the spectra
of the chemical elements which had been compared with it on a natural
scale. This was done by Angström, who utilized for this purpose the
diffraction grating invented by Fraunhofer, and defined the position of
all lines in spectra by their “wave lengths,” in ten-millionths of a
millimetre or “tenth-metres.”
In 1862 Rutherfurd extended Fraunhofer’s work on the stars by a first
attempt at classification. Two years later Huggins and Miller produced
maps of the spectra of some stars. Donati demonstrated that comets gave
radiation spectra, and Huggins did the same for nebulæ.
By these observations comets and nebulæ were shown to be
spectroscopically different from stars, which at that time were studied
by their dark lines only.
Chiefly by the labors of Pickering, the energetic head of the Harvard
Observatory, science has been enriched during the later years by
observations of thousands of stellar spectra, the study of which has
brought about the most marvellous advance in our knowledge.
These priceless data have enabled us now to classify the stars not only
by their brightness, or their color, but by their chemistry.
Next to be chronicled is the application of the so-called
Doppler-Fizeau principle, which teaches us that when a light source
is approaching or receding from us the light waves are crushed
together or drawn out, so that the wave length is changed. The amount
of change gives us the velocity of approach or recess, so that the
rate of movement of stars towards or from the earth, or the up-rush
or down-rush of the solar vapors on the sun’s disk can be accurately
determined. A further utilization of this principle is found when the
stars are so close together that they appear as one if the plane of
motion passes near the earth. A line common to the spectra of both
stars will appear double twice in each revolution, when the motion to
or from the earth, or, as it is termed, “in the line of sight,” is
greatest. “Spectroscopic doubles,” as these stars are called, yield
up many of their secrets which otherwise would elude us. Their time
of revolution, the size of the orbit, and the combined mass can be
determined.
To return from the stars to the sun.
By the device of throwing an image of the sun on the slit of the
spectroscope the spectra of solar spots have been studied from 1866
onward, and a little later the brighter portions of the sun’s outer
envelopes, revealed till then only during eclipses, were brought within
our ken spectroscopically, so that they are now studied every day.