discovering and observing double stars whose components are
very close together, by virtue of the facts that the
spectrograph measures, velocities of approach and recession in
absolute units--so many kilometers per second--and that the
speeds of rotation in binary systems are higher the closer
together the two components are. The observations of the
brighter helium stars, especially those made at the Yerkes
Observatory by Frost and Adams, have shown that one helium star
in every two and one half on the average is a very close
double. In beta Cephei, an early Class B star, the components
are so close that they revolve around each other in 4 1/2
hours; many systems have periods in the neighborhood of a day,
of two days, of three days, and so on. Similar observations
made with the D. O. Mills spectrographs in both hemispheres
have shown that about one star in every four of the bright
stars, on the average, is a double star. In general, the
proportion of spectroscopic doubles discovered to date is
greatest in Class B and decreases as we proceed toward Class M.
The explanation is simple: in the Class B doubles the
components are close together, their orbital velocities are
very high and change rapidly, and the spectrograph is able to
discover the variations with little loss of time. As we pass
toward the yellow and red spectroscopic binaries we find the
components separated more and more, the orbital velocities are
smaller and the periods longer, the variations of velocity are
more difficult to discover, and in the wider pairs we must wait
many years before the variations become appreciable. There is a
very marked progression of the average lengths of periods of
the spectrographic double stars as we pass from the Class B to
the Class M pairs. Similarly, the eccentricities of the orbits
of the binaries increase as we proceed in the same direction.
Accumulating evidence is to the effect that the proportion of
double stars to single stars may be as great in the Classes A
to K as in Class B.
8. Kapteyn believes that he is able to divide the individual
stars--those whose proper motions are known--into the two star
streams which he has described; and he finds that the first
stream is rich in the early blue stars, less rich relatively in
yellow stars, and poor in red stars, whereas the second stream
is very poor in early blue stars, rich in yellows, and
relatively very rich in reds. His interpretation is that the
stream-one stars are effectively younger than the stream-two
stars, on the whole. Stream one still abounds in youthful
stars: they grow older and the yellow and red stars will then
predominate. Stream two abounds in stars which were once young,
but are now middle-aged and old.
The eight lines of argument outlined are in harmony to the
effect that there is a sequence of development from nebulae to
red stars.
The extremely red stars are all faint, only a very few being
visible to the naked eye, and these near the limit of vision.
Our knowledge concerning them is relatively limited. That
these, and all stars, will become invisible to our telescopes,
and ultimately be dark unshining bodies, is the logical
conclusion to which the evolutionary processes will lead. As I
have already stated, both Newcomb and Kelvin were inclined to
believe that the major part of gravitational matter in the
universe is already invisible.
It should be said that a few astronomers doubt whether the
order of evolution is so clearly defined as I have outlined it;
in fact, whether we know even the main trend of the
evolutionary process. We occasionally encounter the opinion
that the subject is still so unsettled as not to let us say
whether the helium stars are effectively young or the red stars
are effectively old. Lockyer and Russell have proposed
hypotheses in which the order of evolutionary sequence begins
with comparatively cool red stars and proceeds through the
yellow stars to the very hot blue stars, and thence back
through the yellow stars to cool red stars.
I think the essentially unanimous view of astronomers is to the
effect that the great mass of accumulated evidence favors the
order of evolution which I have described. We are all ready to
admit that there are apparent exceptions to the simple course
laid down, but that these exceptions are revolutionary in
effect, and not hopeless of removal, has not yet, in my
opinion, been established.
PHYSICAL CONDITIONS GOVERN APPEARANCES OF SPECTRA
A question frequently asked is this: if the yellow and red
stars have been developed from the blue stars, why do not the
thousands of lines in the spectra of the yellow and red stars
show in the spectra of the blue stars? Indeed, why do not the
elements so conspicuously present in the atmosphere of the red
stars show in the spectra of the gaseous nebulae? The answer is
that the conditions in the nebulae and in the youngest stars
are such that only the SIMPLEST ELEMENTS, like hydrogen and
helium, and in the nebulae nebulium, which we think are nearest
to the elemental state of matter, seem to be able to form or
exist in them; and the temperature must lower, or other
conditions change to the conditions existing in the older
stars, before what we may call the more complicated elements
can construct themselves out of the more elemental forms of
matter. The oxides of titanium and of carbon found in the red
stars, where the surface temperatures must be relatively low,
would dissociate themselves into more elemental components and
lose their identity if the temperature and other conditions
were changed back to those of the early helium stars. Lockyer's
name is closely connected with this phenomenon of dissociation.
There is no evidence, to the best of my knowledge, that the
elements known in our Earth are not essentially universal in
distribution, either in the forms which the elements have in
the Earth, or dissociated into simpler forms wherever the
temperatures or other conditions make dissociations possible
and unavoidable.
The meteorites, which have come through the atmosphere to the
Earth's surface, contain at least 25 known terrestrial
elements. That they have not been found thus far to contain all
of our elements is not surprising, for we should have
difficulty in finding a piece of our Earth weighing a few
kilograms which would contain 25 of our elements. We have not
found any elements in meteorites which are unknown to our
chemists. Our comets, which ordinarily show the presence of not
more than three elements, carbon, nitrogen and oxygen, give
certain evidence of sodium in their composition when they
approach fairly near to the Sun; and the great comet of 1882,
when very close to the Sun, developed in its spectrum many
bright lines not previously seen in comet spectra, which
Copeland said were due to iron. That the comets do not show a
greater number of elements is not in the least surprising: they
are not condensed bodies, and we think that their average
temperature is low, too low generally to develop the luminous
vapors of the more refractory elements. If their temperatures,
approximated those which exist in the stars, their spectra
would probably reveal the presence of many of the elements
which exist in the meteorites. Of course the proof of this is
lacking.
DESTINY OF THE STELLAR SYSTEM
We have said that the evolutionary processes depend primarily
upon the loss of heat. This is to the best of our knowledge a
genuine loss, except as some of the heat rays happen to strike
other celestial bodies. The flow of heat energy from a star
must be essentially continuous, always in one direction from
hotter bodies to colder bodies, or into so-called unending and
heatless space. Temperatures throughout the universe are
apparently moving toward uniformity, at the level of absolute
zero. Now, this uniformity would mean universal stagnation and
death. It is possible to have life and to do work only when
there are differences of temperature between the bodies
concerned: work is done or accompanied by a flow of heat,
always from the hotter to the colder body. We are not aware
that any compensating principle exists. Several students of the
subject, notably Arrhenius, have searched for such a principle,
a fountain of youth so to speak, in accordance with which the
vigor of stellar life should maintain itself from the beginning
of time to the end of time; but I think that nothing
approaching a satisfactory theory has yet been formulated. The
stellar universe seems, from our present point of view, to be
slowly "running down." The processes will not end, however,
when all the heat generable WITHIN the stars shall have been
radiated into an endless space. Every body within the universe,
it is conceivable, could have cooled down to absolute zero, but
the system might still be in its youth. So long as the stars,
whether intensely hot or free from all heat, are rotating
rapidly on their axes or are rushing through space with high
speeds, the system will remain VERY MUCH ALIVE. Collisions or
very close approaches of two stars are bound to occur sooner or
later, whether the stars are hot or cold, and in all such cases
a large share of the kinetic energy--the energy of motion--of
the two bodies will be converted into heat. A collision, under
average stellar conditions, should convert the two stars into a
luminous gaseous nebula, or two or more nebulae, which would
require hundreds or thousands of millions of years to evolve
again into young stars, middle-aged stars, old stars, and stars
absolutely cold. So long as any of these bodies retain motion
with reference to other bodies, they retain the power of
rebirth and another life. Not to go too far into speculative
detail, the general effect of these processes would be the
destruction of relative motions and the gradual decrease in the
number of separate bodies, through coalescence. Assume further,
however, that all existing bodies, widely scattered through the
stellar system, are absolutely cold and absolutely at rest with
reference to each other: the system might even then be only
middle-aged. The mutual gravitations of the bodies would still
be operative. They would pass each other closely, or collide,
under high generated velocities: there would be new nebulae,
and new and vigorous stellar life to continue through other
long ages. The system would not run down until all the kinetic
energy had been converted into heat, and all the heat generable
had been dissipated. This would not occur until all material in
the universe had been combined into one body, or into two
bodies in mutual revolution. However, if there are those who
say that the universe in action is eternal, through the
operation of compensating principles as yet undiscovered, no
man of science is at present equipped to prove the contrary.
THE NOVAE
The so-called new stars, otherwise known as temporary stars or
novae, present interesting considerations. These are stars
which suddenly flash out at points where previously no star was
known to exist; or, in a few cases, where a faint existing star
has in a few days become immensely brighter. Twenty-nine new
stars have been observed from the year 1572 to date; 19 of them
since 1886, when the photographic dry plate was applied
systematically to the mapping of the heavens, and 15 of the 19
stand to the credit of the Harvard observers. This is an
average of one new star in two years; and as some novae must
come and go unseen it is evident that they are by no means rare
objects. Novae pass through a series of evolutions which have
many points in common; in fact, the ones which have been
extensively studied by photometer and spectrograph have had
histories with so many identities that we are coming to look
upon them as standard products of evolutionary processes. These
stars usually rise to maximum brilliancy in a few days: some of
the most noted ones increased in brightness ten-thousand-fold
in two or three days. All of them fluctuate in brightness
irregularly, and usually in short periods of time. Several
novae have become invisible to the naked eye at the end of a
few weeks. With two or three exceptions, all have become
invisible in moderate-sized telescopes, or have become very
faint, within a few months. Two novae, found very early in
their development, had at first dark line spectra, a night
later bright lines appeared, and a night or two later the
spectra contained the broad radiation and absorption bands
characteristic of all recent novae. After the novae become
fairly faint, the bright lines of the gaseous nebula spectrum
are seen for the first time. These lines increase in relative
brilliancy until the spectra are essentially the same as those
of well-known nebulae, except that the novae lines are broad
whereas the lines of the nebulae are narrow. In a few months or
years the nebular lines diminish in brightness, and the
continuous spectrum develops. Hartmann at Potsdam, and Adams
and Pease with the 60-inch Mount Wilson reflector, have shown
that the spectra of the faint remnants of four originally
brilliant novae now contain some of the bright lines which are
characteristic of Wolf-Rayet stars.[2]
[2] After this lecture was delivered Adams of Mount Wilson
reported that in November, 1914, the chief nebular line (5007A)
and another prominent nebular line (4363A) had entirely
disappeared from the spectrum of Nova Geminorum No. 2, whereas
the second nebular line in the green (4959A) remained strong;
probably a step in progress from the nebular to the Wolf-Rayet
spectrum.
Why the novae suddenly flare up, and what their relations to
other celestial bodies may be, are questions which can not be
regarded as settled. Their distribution on the celestial sphere
is indicated in Figure 25 by the open circles. In this figure
the densest parts of the Milky Way are drawn in outline. All of
the novae have appeared in the Milky Way, with the exception of
five: and these exceptions are worthy of note. One of the five
appeared in the condensed nucleus of the great Andromeda
nebula, not far from its center; another (zeta Centauri) was
located close to the edge of a spiral nebula and quite possibly
in a faint outlying part of the nebula; a third (tau Coronae)
was observed to have a nebulous halo about it at the earliest
stage of its observed existence; a fourth (tau Scorpii)
appeared in a nebula; and the fifth (Nova Ophiuchi No. 2) in
1848 was not extensively observed. The other 24 novae appeared
within the structure of the Milky Way. Keeping the story as
short as possible, a nova is seemingly best explained on the
theory that a dark or relatively dark star, traveling rapidly
through space, has encountered resistance, such as a great
nebula or cloud of particles would afford. While passing
through the cloud the forward face of the star is bombarded at
high velocities by the resisting materials. The surface strata
become heated, the luminosity of the star increases rapidly.
The effect of the bombardment by small particles can be only
skin deep, and the brightness of the star should diminish
rapidly and therefore the spectrum change speedily from one
type to another. The new star of February, 1901, in Perseus,
afforded evidence of great strength on this question. Wolf at
Heidelberg photographed in August an irregular nebulous object
near the nova. Ritchey's photograph of September showed
extensive areas of nebulosity around the star. In October
Perrine and Ritchey discovered that the nebular structure had
apparently moved outward from the nova, from September to
October. Going back to a March 29th photograph taken for a
different purpose, Perrine found an irregular ring of
nebulosity closely surrounding the star. Apparently, the region
was full f nebulosity which is normally invisible to us. The
rushing of the star through this resisting medium made the star
the brightest one in the northern sky for two or three days.
The great wave of light going out from the star when at its
brightest traveled in five weeks as far as the ring of
nebulosity, where, falling upon non-luminous nebulous
materials, it made the ring visible. Continuing its progress,
the wave of light illuminated the material which Wolf
photographed in August, the materials which Ritchey
photographed still farther away in September, and the still
more distant materials which Perrine and Ritchey photographed
in October, November, and later. We were able to see this
material only as the very strong wave of light which left the
star at maximum brightness made the material luminous in
passing. That 24 novae should occur in the Milky Way, where the
stars are most numerous, and where the resisting materials may
preferably prevail, is not surprising; and it should be
repeated that at least three of the five occurring outside of
the Milky Way were located in nebulous surroundings.
The actual collision of two stars would necessarily be too
violent in its effect to let the reduction of brilliancy occur
so rapidly as to cause the disappearance of the nova in a few
weeks or months. The close approach of two stars might
conceivably produce the observed facts, but even this process
seems too violent in its probable results. The chances for the
collision of a rapidly traveling star with an enormously
extended nebulous cloud are vastly greater, and the apparent
mildness of the phenomenon observed is in better harmony with
expectation.
RELATION OF NOVAE, PLANETARY NEBULAE AND WOLF-RAYET STARS
Although all recent novae have been observed to become
planetary or stellar nebulae, they seem not to remain nebular
for any length of time; they have gone further and become
Wolf-Rayet stars. Whether any or all of the planetary nebulae
that have been known since Herschel's day, and have remained
apparently unchanged in form, have developed from new stars, is
uncertain and doubtful. If they have, the disturbances which
gave them their character must have been violent, such as would
result from full or glancing collisions of two stars, in order
to produce deep-seated effects which change slowly, rather than
surface effects which change rapidly.
Whether the Wolf-Rayet stars have in general been formed from
planetary nebulae is a different question: some of them
certainly have. Wright has recently shown that the stellar
nuclei of planetary nebulae are Wolf-Rayet stars, and he has
formulated several steps in the process whereby the nebulosity
in a planetary eventually condenses into the central star. The
distribution of the planetaries and the Wolf-Rayet stars on the
sphere affords further evidence of a connection. We saw. that
the novae are nearly all in the Milky Way. The irregular, ring,
planetary and stellar nebulae, plotted in Fig. 27, prefer the
Milky Way, but not so markedly. The Wolf-Rayets, without
exception, are located in the Milky Way and in the Magellanic
Clouds, and those in the Milky Way are remarkably near to its
central plane. 107 of these objects are known, 1 is in the
Lesser Magellanic Cloud, and 21 are in the Greater Magellanic
Cloud. The remaining 85 average less than 2 3/4 degrees from
the central plane of the Milky Way.
We are obliged to say that the places of the novae, of the
planetary and stellar nebulae, and of the Wolf-Rayets in the
evolutionary process are not certainly known. If the Wolf-Rayet
stars have developed from the planetaries, the planetaries from
the novae, and the novae have resulted from the close approach
or collision of two stars, or from the rushing of a dark or
faint star through a resisting medium, then the novae,
planetaries and Wolf-Rayets belong to a new and second
generation: they were born under exceptional conditions. The
velocities of the planetary nebulae seem to be an insuperable
difficulty in the way of placing them between the irregular
nebulae and the helium stars. The average radial velocity of 47
planetary nebulae is about 45 km. per second; and, if the
motions of the planetaries are somewhat at random, their
average velocities in space are twice as great, or 90 km. per
second. This is fully seven times the average velocity of the
helium stars, and the helium stars in general, therefore, could
not have come from planetary nebulae. The radial velocities of
only three Wolf-Rayet stars have been observed, and this number
is too small to have statistical value, but the average for the
three is several times as high as the average for the helium
stars. We can not say, I think, that the velocities of any
novae are certainly known.
If the planetaries have been formed from novae, especially the
novae which encountered the fiercest resistance, the high
velocities are in a sense not surprising, for those stars which
travel with abnormally high speeds are the ones whose chances
for collisions with resisting media are best; and, further, the
higher the speeds of collision the more violent the
disturbance. This line of argument also leads to the conclusion
that the novae, planetaries and Wolf-Rayets belong not in
general before the helium stars, but to another generation of
stars. They may, and I think will, develop into a small class
of helium stars having special characteristics; for example,
high velocities.
KANT'S HYPOTHESIS
Immanuel Kant's writings, published principally in 1755, are in
many ways the most remarkable contributions to the literature
of stellar evolution yet made. Curiously, Kant's papers have
not been read by the text-book makers, except in a few cases.
We have already referred to his ideas on the Milky Way and on
comets. In his hypothesis of the origin of the solar system, he
laid emphasis upon the facts that the six known planets revolve
around the Sun from west to east, nearly in the same plane and
nearly in the plane of the Sun's equator; that the then four
known moons of Jupiter, the five known moons of Saturn, and our
moon revolve around these planets from west to east, and nearly
in the same general plane; and that the Sun, our moon and the
planets, so far as known, rotate in the same direction. These
facts, he said, indicate indisputably a common origin for all
the members of the solar system. He expressed the belief that
the materials now composing the solar system were originally
scattered widely throughout the system, and in an elemental
state. This was a half century before Herschel's extensive
observations of nebuae. Kant thought of this elemental matter
as cold, endowed with gravitational power, and endowed
necessarily with some repulsive power, such as exists in gases.
He started his solar system from materials at rest. Most of the
matter, he said, drifted to the center to form the Sun. He
believed that nuclei or centers of attraction formed here and
there throughout the chaotic structure, and that in the course
of ages these centers grew by accretion of surrounding matter
into the present planets and their satellites; and that in some
manner motion in one direction prevailed throughout the whole
system. Kant's explanation of the origin of the ROTATION of the
solar system is unsound and worthless. We now know that such a
cloud of matter, free from rotation, could not of itself
generate rotation; it must get the start from outside forces.
Kant's false reasoning was due in part to the fact that some of
our most important dynamical laws were not yet discovered, in
part to his faulty comprehension of certain dynamical
principles already known, and probably in part to the
unsatisfactory state of chemical knowledge existing at that
date. This was half a century before Dalton's atomic theory of
matter was proposed.