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THE POPULAR SCIENCE MONTHLY VOLUME LXXXVI JULY TO SEPTEMBER - 1915
THE POPULAR SCIENCE MONTHLY VOLUME LXXXVI JULY TO SEPTEMBER - 1915 THE SCIENTIFIC MONTHLY VOLUME I OCTOBER TO DECEMBER Scanned by Charles Keller with OmniPage Professional OCR
NOTE: degrees A (Absolute?) is the same as the current
degrees K (Kelvin).
THE POPULAR SCIENCE MONTHLY VOLUME LXXXVI JULY TO SEPTEMBER
1915
THE SCIENTIFIC MONTHLY VOLUME I OCTOBER TO DECEMBER 1915
EDITED BY J. McKEEN CATTELL
THE SCIENTIFIC MONTHLY ------ OCTOBER 1915 -------------
THE EVOLUTION OF THE STARS AND THE FORMATION OF THE EARTH. II
BY DR. WILLIAM WALLACE CAMPBELL
DIRECTOR OF THE LICK OBSERVATORY UNIVERSITY OF CALIFORNIA
THE PRINCIPLES OF SPECTROSCOPY
THUS far our description of the stellar universe has been
confined to its geometrical properties. A serious study of the
evolution of the stars must seek to determine first of all
what the stars really are what their chemical constitutions
and physical conditions are; and how they are related to each
other as to their physical properties. The application of the
spectroscope has advanced our knowledge of the subject by leaps
and bounds. This wonderful instrument assisted by the
photographic plate enables every visible celestial body to
write its own record of the conditions existing in itself
within limits set principally by the brightness of the body.
Such records physicists have succeeded to some extent in
duplicating in their laboratories; and the known conditions
under which the laboratory experiments have been conducted are
the Rosetta Stones which are enabling us to interpret with
more or less success the records written by the stars.
It is well known that the ordinary image of a star whether
formed by the eye alone or by the achromatic telescope and the
eye combined contains light of an infinite variety of colors
corresponding speaking according to the mechanical theory of
light to waves of energy of an infinite variety of lengths
which have traveled to us from the star. In the point image of
a star these radiations fall in a confused heap. and the
observer is unable to say that radiations corresponding to any
given wave-lengths are present or absent. When the star's light
has been passed through the prism or diffracted from the
grating of a spectroscope these rays are separated one from
another and arranged side by side in perfect order ready for
the observer to survey them and to determine which ones are
present in superabundance and which other ones are lacking
wholly or in part. The following comparison is a fair one: the
ordinary point image of a star is as if all the books in the
university library were thrown together in a disorderly but
compact pile in the center of the reading room: we could say
little concerning the contents and characteristics of that
library; whether it is strong in certain fields of human
endeavor or weak in other fields. The spectrum of a star is as
the same library when the books are arranged on the shelves in
complete perfection and simplicity so that he who looks may
appraise its contents at any or all points. Let us consider the
fundamental principles of spectroscopy.
1. When a solid body a liquid or a highly-condensed gas is
heated to incandescence its light when passed through a
spectroscope forms a continuous spectrum: that is a band of
light red at one end and violet at the other uninterrupted by
either dark or bright lines.
2. The light from the incandescent gas or vapor of a chemical
element passed through a spectroscope forms a bright-line
spectrum; that is one consisting entirely of isolated bright
lines distributed differently throughout the spectrum for the
different elements or of bright lines superimposed upon a
relatively faint continuous spectrum.
3. If radiations from a continuous-spectrum source pass through
cooler gases or vapors before entering the spectroscope a
dark-line spectrum results: that is the positions which the
bright lines in the spectra of the vapors and gases would have
are occupied by dark or absorption lines. These are frequently
spoken of as Fraunhofer lines.
To illustrate: the gases and vapors forming the outer strata of
the Sun's atmosphere would in themselves produce bright-line
spectra of the elements involved. If these gases and vapors
could in effect be removed without changing underlying
conditions the remaining condensed body of the Sun should have
a continuous spectrum. The cooler overlying gases and vapors
absorb those radiations from the deeper and hotter sources
which the gases and vapors would themselves emit and thus form
the dark-line spectrum of the Sun. The stretches of spectrum
between the dark lines are of course continuous-spectrum
radiations.
These principles are illustrated in Fig. 12. The essential
parts of a spectroscope are the slit--an opening perhaps
1/100th of an inch wide and 1/10th of an inch long--to admit
the light properly; a lens to render the light rays parallel
before they fall upon the prism or grating; a prism or grating;
a lens to receive the rays after they have been dispersed by
the prism or grating and to form an image of the spectrum a
short distance in front of the eye where the eye will see the
spectrum or a sensitive dry-plate will photograph it. If we
place an alcohol lamp immediately in front of the slit and
sprinkle some common salt in the flame the two orange bright
lines of sodium will be seen in the eyepiece close together
as in the upper of the two spectra in the illustration. If we
sprinkle thallium salt in the flame the green line of that
element will be visible in the spectrum. If we take the lamp
away and place a lime light or a piece of white-hot iron in
front of the slit we shall get a brilliant continuous spectrum
not crossed by any lines either bright or dark. Insert now the
alcohol-sodium-thallium lamp between the lime light and the
slit and the observer will see the two sodium lines and one
thallium line in the same places as before but as dark lines
on a background of bright continuous spectrum as: illustrated
in the lower of the two spectra. Let us insert a screen between
the lamp and the lime light so as to cut out the latter and we
shall see the bright lines of sodium and thallium reappear as
in the upper of the two spectra. These simple facts illustrate
Kirchhoff's immortal discovery of certain fundamental
principles of spectroscopy in 1859. The gases and vapors in
the lamp flame are at a lower temperature than the lime source.
The cooler vapors of sodium and thallium have the power of
absorbing exactly those rays from the hotter lime or other
similar source which the vapors by themselves would emit to
form bright lines.
When we apply the spectroscope to celestial objects we find
apparently an endless variety of spectra. We shall illustrate
some of the leading characteristics of these spectra as in
Figs. 13 to 18 inclusive and Figs. 21 22 23 and 24. The
spectra of some nebulae consist almost exclusively of isolated
bright lines indicating that these bodies consist of luminous
gases as Huggins determined in 1864; but a very faint
continuous band of light frequently forms a background for the
brilliant bright lines. Many of the nebular lines are due to
hydrogen others are due to helium; but the majority including
the two on the extreme right in Fig. 13 which we attribute to
the hypothetical element nebulium and the close pair on the
extreme left have not been matched in our laboratories and
therefore are of unknown origin. Most of the irregular nebulae
whose spectra have been observed the ring nebulae the
planetary and stellar nebulae have very similar spectra
though with many differences in the details.[1]
[1] My colleague Wright who has been making a study of the
nebular spectra has determined the accurate positions of about
67 bright nebular lines.
The great spiral nebula in Andromeda has a continuous spectrum
crossed by a multitude of absorption lines. The spectrum is a
very close approach to the spectrum of our Sun. It is clear
that this spiral nebula is widely different from the
bright-line or gaseous nebulae in physical condition. The
spiral may be a great cluster of stars which are approximate
duplicates of our Sun or there is a chance that it consists
as Slipher has suggested of a great central sun or group of
suns and of a multitude of small bodies or particles such as
meteoric matter revolving around the nucleus; this finely
divided matter being visible by reflected light which
originates in the center of the system.
There is an occasional star like chi Carinae whose spectrum
consists almost wholly of bright lines in general bearing no
apparent relationship to the bright lines in the spectra of the
gaseous nebulae except that the hydrogen lines are there as
they are almost everywhere. There is reason to believe that
such a spectrum indicates the existence of a very extensive and
very hot atmosphere surrounding the main body or core of the
star in question. This particular star is remarkable in that it
has undergone great changes in brilliancy and is located upon a
background of nebulosity. The chances are strong that the star
has rushed through the nebulosity with high rate of speed and
that the resulting bombardment of the star has expanded and
intensely heated its atmosphere.
There are the Wolf-Rayet stars named from the French
astronomers who discovered the first three of this class whose
spectra show a great variety of combinations of continuous
spectrum and bright bands. We believe that the continuous
spectrum in such a star comes from the more condensed central
part or core and that the bright-line light proceeds from a
hot atmosphere extending far out from the core.
The great majority of the stars have spectra which are
continuous except for the presence of dark or absorption
lines: a few lines in the very blue stars and an increasing
number of lines as we pass from the blue through the yellow and
red stars to those which are extremely red.
Secchi in the late 60's classified the spectra of the brighter
stars according to the absorption lines in their spectra into
Types I II III and IV which correspond: Type I to the very
blue stars such as Spica and Sirius; Type II to the yellow
stars similar to our Sun; Type III to the red stars such as
Aldebaran; and Type IV to the extremely red stars of which
the brightest representatives are near the limit of naked-eye
vision. Secchi knew little or nothing concerning stars whose
spectra contain bright lines except as to the isolated
bright-line spectra of a few nebulae and as to the bright
hydrogen lines in gamma Cassiopeia and his system did not
include these.
One of the most comprehensive investigations ever undertaken by
a single institution was that of classifying the stars as to
their spectra over the entire sky substantially down to and
including the stars of eighth magnitude by the Harvard College
Observatory as a memorial to the lamented Henry Draper.
Professor Pickering and his associates have formulated a
classification system which is now in universal use. It starts
with the bright-line nebulae passes to the bright-line stars
and then to the stars in which the helium absorption lines are
prominent. The latter are called the helium stars or
technically the Class B stars. The next main division includes
the stars in which hydrogen absorption is prominent called
Class A. Classes B and A are blue stars. Then follows in
succession Class F composed of bluish-yellow stars which is
in a sense a transition class between the hydrogen stars and
those resembling our Sun the latter called Class G. The Class
G stars are yellow. Class K stars are the yellowish-red; Class
M the red; and Class N the extremely red. Each of these
classes has several subdivisions which make the transition from
one main class to the next main class fairly gradual and not
per saltum; though it should be said that the relationship of
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