The Scientific Monthly, October to December, 1915

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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
Class N to Class M spectra is not clear. The illustration, Fig.
17, brings out the principal features of the spectra of Classes
B to M. The spectrum becomes more complicated as we pass from
Class B to the Class M, and the color changes from blue to
extreme red, because the violet and blue radiations become
rapidly weaker as we pass through the various classes.

GENERAL COURSE OF EVOLUTIONARY PROCESS

The general course of the evolutionary processes as applied to
the principal classes of celestial bodies is thought to be
fairly well known. With very few exceptions astronomers are
agreed as to the main trend of this order, but this must not be
interpreted to mean that there are no outstanding differences
of opinion. There are, in fact, some items of knowledge which
seem to run counter to every order of evolution that has been
proposed.

The large irregular nebulae, such as the great nebula in Orion,
the Trifid nebula, and the background of nebulosity which
embraces a large part of the constellation of Orion, are
thought to represent the earliest form of inorganic life known
to us. The material appears to be in a chaotic state. There is
no suggestion of order or system. The spectroscope shows that
in many cases the substance consists of glowing gases or
vapors; but whether they are glowing from the incandescence
resulting from high temperature, or electrical condition, or
otherwise, is unknown, though heat origin of their light is the
simplest hypothesis now available. Whether such nebulae are
originally hot or cold, we must believe that they are endowed
with gravitational power, and that their molecules or particles
are, or will ultimately be, in motion. It will happen that
there are regions of greater density, or nuclei, here and there
throughout the structure which will act as centers of
condensation, drawing surrounding materials into combination
with them. The processes of growth from nuclei originally small
to volumes and masses ultimately stupendous must be slow at
first, relatively more rapid after the masses have grown to
moderate dimensions and the supplies of outlying materials are
still plentiful, and again slow after the supplies shall have
been largely exhausted. By virtue of motions prevailing within
the original nebular structure, or because of inrushing
materials which strike the central masses, not centrally but
obliquely, low rotations of the condensed nebulous masses will
occur. Stupendous quantities of heat will be generated in the
building-up process. This heat will radiate rapidly into space
because the gaseous masses are highly rarefied and their
radiating surfaces are large in proportion to the masses. With
loss of heat the nebulous masses will contract in volume and
gradually assume forms more and more spherical. When the forms
become approximately spherical, the first stage of stellar life
may be said to have been reached.

It was Herschel’s belief that by processes of condensation,
following the loss of heat by radiation into surrounding space,
formless nebulae gravitated into nebula of smaller and smaller
volumes until finally the planetary form was reached, and that
planetaries were the ancestors of stars in general. That the
planetaries do develop into stars, we have every reason to
believe; but that all nebulae, or relatively many nebulae, pass
through the planetary stage, or that many of our stars have
developed from planetaries, we shall later find good reason for
doubting. The probabilities are immensely stronger that the
stars in general have been formed directly from the irregular
nebulae, without the intervention of the planetaries. The
planetary nebula seem to be exceptional cases, but to this
point we shall return later.

It is quite possible, and even probable, that gaseous masses
have not in all cases passed directly to the stellar state. The
materials in a gaseous nebula may be so highly attenuated, or
be distributed so irregularly throughout a vast volume of
space, that they will condense into solids, small meteoric
particles for example, before they combine to form stars. Such
masses or clouds of non-shining or invisible matter are thought
to exist in considerable profusion within the stellar system.
The nebulosity connected more or less closely with the brighter
Pleiades stars may be a case in illustration. Slipher has
recently found that the spectra of two small regions observed
in this nebula are continuous, with absorption lines of
hydrogen and helium. This spectrum is apparently the same as
that of the bright Pleiades stars. Slipher’s interpretation is
that the nebula is not shining by its own light, but is
reflecting to us the light of the Pleiades stars. That this
material will eventually be drawn into the stars already
existing in the neighborhood, or be condensed into new centers
and form other stars, we can scarcely doubt. The condensation
of such materials to form stars large enough to be seen from
the great distance of the Pleiades cluster must generate heat
in the process, and cause these stars in their earliest youth
to be substantially as hot as other stars formed directly from
gaseous materials. It is possible, also, that the spiral
nebulae will develop into stars, perhaps each such object into
many, or some of the larger ones into multitudes, of stars.

Let us attempt to visualize the conditions which we think exist
in a newly-formed star of average mass. It should be
essentially spherical, with surface fairly sharply defined. Our
Sun has average specific gravity of 1.4, as compared with that
of water. The average density of the very young star must
certainly be vastly lower; perhaps no greater than the density
of our atmosphere at the Earth’s surface; it may even be
considerably lower than this estimate. The diameter of our Sun
is 1,400,000 kilometers. The diameter of the average young star
may be ten or twenty or forty times as great. The central
volume or core of the star is undoubtedly a great deal denser
than the surface strata, on account of pressure due to the
star’s own gravitational forces. The conditions in the outer
strata should bear some resemblance to those existing in the
gaseous nebula. The star may or may not have a corona closely
or remotely similar to our Sun’s corona. The deep interior of
the star must be very hot, though not nearly so hot as the
interiors of older stars; but the surface strata of the young
star should be remarkably hot; for, being composed of highly
attenuated gases, any lowering of the temperature by radiation
into surrounding space will be compensated promptly through the
medium of highly-heated convection currents which can travel
more rapidly from the interior to the surface than in the case
of stars in middle or old age. Even though the star, as
observed in our most powerful telescopes, is a point of light,
without apparent diameter, its outer strata should supply some
bright lines in the spectrum, because these strata project out
beyond what we may call the core of the star and themselves act
as sources of light. The spectrum should, therefore, consist of
some of the bright lines which were observed in the nebular
spectrum, these proceeding from the outer strata of the star;
and of a continuous spectrum made up of radiations proceeding
from the deeper strata or core of the star, in which a few dark
lines may be introduced by the absorption from those parts of
the outer gaseous strata which lie between us and the core.

A few hundred stellar spectra resembling this description are
well known, discovered mostly at the Harvard Observatory. Their
details differ greatly, but they have certain features in
common. The bright lines of helium are extremely rare in stars,
but they have been observed in a few stellar spectra. The
bright lines of nebulium have never been observed in a true
star: they and the radiations in the ultra-violet known as at
3726A, seem to be confined to the nebular state; and the
absorption lines of nebulium have never been observed in any
spectrum. As soon as the stellar state is reached nebulium is
no longer in evidence. Stellar spectra containing bright lines
seem always to include hydrogen bright lines. This is as we
should expect; hydrogen is the lightest known gas, and it is
probably the substance which can best exist in the outer strata
of stars in general. The extensive outer strata of very young
stars seem to be composed largely of hydrogen, though other
elements are in some cases present, as indicated by the weaker
bright lines in a few cases. This preference of hydrogen for
the outermost strata is illustrated by several very interesting
observations of the nebulae. The nebulium lines are relatively
strong in the central denser parts of the Orion and Trifid
nebulae, but the hydrogen bright-lines are relatively very
strong in the faint outlying parts of these nebulae. The
planetary nebula B.D.—12 degrees.1172 is seen in the ordinary
telescope to consist of a circular disc (probably a sphere or
spheroid) of light and a faint star in its center. When this
nebula is observed with a slitless spectrograph the hydrogen
and nebulium components are seen as circular discs, but the
hydrogen discs are larger than the nebulium discs. In other
words, the hydrogen forms an atmosphere about the central star
which extends out into space in all directions a great deal
farther than the nebulium discs extend. The Wolf-Rayet
star-planetary nebula D. M. + 30 degrees.3639 looks hazy in a
powerful telescope, and when examined in a spectroscope the
haziness is seen to be due to a sharply defined globe of
hydrogen 5 seconds of arc in diameter surrounding the star in
its center. Wolf and Burns have shown that in the Ring Nebula
in Lyra the 3726A and the hydrogen images are larger as to
outer diameter than the nebulium images, but that the latter
are the more condensed on the inner edge of the ring. Wright
has in the present year examined these and other nebulae with
special reference to the distribution of the principal
ingredients. He finds in general that the radiations at 4363A
and 4686A, of unknown or possibly helium origin, are most
closely compressed around the central nuclei of nebulae; that
the matter definitely known to be helium is more extended in
size; that the nebulium structure is still larger; and that the
hydrogen uniformly extends out farther than the nebulium; and
that the ultra violet radiation at 3726A seems to proceed from
the largest volume of all. The 37726A line, like the nebulium
line, is unknown in stellar spectra; it seems also to be
confined to true nebulosity. Neglecting the elements which have
never been observed in true stars, we may say that all these
observations are in harmony with the view that hydrogen should
be and is the principal element in the outer stratum of the
very young star. A few of the stars whose spectra contain
bright hydrogen lines have also a number of bright lines whose
chemical origin is not known. They appear to exist exactly at
this state of stellar life: several of them have not been found
in the spectra of the gaseous nebulae, and they are not
represented in the later types of stellar spectra. The strata
which produce these bright lines are thought to be a little
deeper in the stars than the outer hydrogen stratum.

A slightly older stage of stellar existence is indicated by the
type of spectrum in which some of the lines of hydrogen, always
those at the violet end, are dark, and the remaining hydrogen
lines, always those toward the red end, are bright. The
brightest star in the Pleiades group, Alcyone, presents
apparently the last of this series, for all of the hydrogen
lines are dark except H alpha, in the red. In some of the
bright-line stars which we have described, technically known as
Oe5, Harvard College Observatory found that the dark helium and
hydrogen lines exist, and apparently increase in intensity, on
the average, as the bright lines become fainter. Wright has
observed the absorption lines of helium and hydrogen in the
spectra of the nuclei of some planetary nebulae, although the
helium and hydrogen lines are bright in the nebulosity
surrounding the nuclei. We may say that when all of the bright
lines have disappeared from the spectra of stars, the helium
lines, and likewise the hydrogen lines, have in general become
fairly conspicuous. These stars are known as the helium stars,
or stars of Class B. Proceeding through the subdivisions of
Class B, the helium lines increase to a maximum of intensity
and then decrease. The dark hydrogen lines are more and more in
evidence, with intensities increasing slowly. In the middle and
later subdivisions of the helium stars silicon, oxygen and
nitrogen are usually represented by a few absorption lines.

Just as the gaseous nebulae radiate heat into space and
condense, so must the stars, with this difference: the nebulae
are highly rarified bodies, with surfaces enormously large in
proportion to the heat contents; and the radiation from them
must be relatively rapid. In fact, some of the nebulae seem to
be so highly rarified that radiation may take place from their
interiors almost as well as from their surfaces. The radiation
from a star just formed must occur at a much slower rate. The
continued condensation of the star, following the loss of heat,
must lead to a change of physical condition, which will be
apparent in the spectrum. It should pass from the so-called
helium group, to the hydrogen, or Class A group, not suddenly
but by insensible gradations of spectrum. In the Class A stars
the hydrogen lines are the most prominent features. The helium
lines have disappeared, except in a few stars where faint
helium remnants are in evidence. The magnesium lines have
become prominent and the calcium lines are growing rapidly in
strength. The so-called metallic lines, usually beginning with
iron and titanium lines, which have a few extremely faint
representatives in the last of the helium stars, become visible
here and there in the Class A spectra, but they are not
conspicuous.

In the next main division, the Class F spectra, the metallic
lines increase rapidly in prominence, and the hydrogen lines
decrease slightly in strength. These stars are not so blue as
the helium and hydrogen stars. They are intermediate between
the blue stars and the yellow stars, which begin with the next
class, G, of which our Sun is a representative.

The metallic lines are in Class G spectra in great number and
intensity, and the hydrogen lines are greatly reduced in
prominence. The calcium bands are very wide and intense.

Another step brings us to the very yellow and the

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