_Beginnings and Endings_
We have seen how the solid substance of the material universe is
continually dissolving away into intangible radiation. The sun weighed
360,000 million tons more yesterday than to-day, the difference being
the weight of 24-hours’ emission of radiation which is now travelling
through space, and, so far as direct observation goes, is destined to
journey on through space until the end of time. The same transformation
of material weight into radiation is in progress in all the stars, and
to a lesser degree on earth, where complex atoms such as uranium are
continually changing into the simpler atoms of lead and helium, and
setting radiation free in the process. But against the sun’s daily loss
of weight of 360,000 million tons, the earth is only losing weight from
this cause at the rate of about ninety pounds a day.
CYCLIC PROCESSES. It is natural to ask whether a study of the universe
as a whole reveals these processes as part only of a closed cycle, so
that the wastage which we see in progress in the sun and stars and
on the earth is made good elsewhere. When we stand on the banks of a
river and watch its current ever carrying water out to sea, we know
that this water is in due course transformed into clouds and rain which
replenish the river. Is the physical universe a similar cyclic system,
or ought it rather to be compared to a stream which, having no source
of replenishment, must cease flowing after it has spent itself?
THERMODYNAMICS
To this question, the wide scientific principle known as the second law
of thermodynamics provides an answer in very general terms. If we ask
what is the underlying cause of all the varied animation we see around
us in the world, the answer is in every case, energy—the chemical
energy of the fuel which drives our ships, trains and cars, or of the
food which keeps our bodies alive and is used in muscular effort, the
mechanical energy of the earth’s motion which is responsible for the
alternations of day and night, of summer and winter, of high tide and
low tide, the heat-energy of the sun which makes our crops grow and
provides us with wind and rain.
The first law of thermodynamics, which embodies the principle of
“conservation of energy,” teaches that energy is indestructible; it may
change about from one form to another, but its total amount remains
unaltered through all these changes, so that the total energy of the
universe remains always the same. As the energy which is the cause of
all the life of the universe is indestructible, it might be thought
that this life could go on for ever undiminished in amount.
AVAILABILITY OF ENERGY. The second law of thermodynamics rules out any
such possibility. Energy is indestructible as regards its amount, but
it continually changes in form, and generally speaking there are upward
and downward directions of change. It is the usual story—the downward
journey is easy, while the upward is either hard or impossible. As a
consequence, more energy passes in one direction than in the other.
For instance, both light and heat are forms of energy, and a million
ergs of light-energy can be transformed into a million ergs of heat
with the utmost ease; let the light fall on any cool, black surface,
and the thing is done. But the reverse transformation is impossible;
a million ergs which have once assumed the form of heat, can never
again assume the form of a million ergs of light. This is a special
example of the general principle that radiative energy tends always to
change into a form of longer, not shorter, wave-length. In general,
for instance, fluorescence increases the wave-length of the light; it
changes blue light into green, yellow or red, but not red light into
yellow, green or blue. Exceptions to the general principle are known,
but they are of special type, admitting of special explanations, and do
not affect the general principle.
It may be objected that the everyday act of lighting a fire disproves
all this. Has not the sun’s heat been stored up in the coal we burn,
and cannot we produce light by burning coal? The answer is that the
sun’s radiation is a mixture of both light and heat, and indeed
of radiation of all wave-lengths. What is stored up in the coal
is primarily the sun’s light and other radiation of still shorter
wave-length. When we burn coal we get some light, but not as much
as the sun originally put into the coal; we also get some heat, and
this is more than the amount of heat which was originally put in. On
balance, the net result of the whole transaction is that a certain
amount of light has been transformed into a certain amount of heat.
All this shews that we must learn to think of energy, not only in terms
of quantity, but also in terms of quality. Its total quantity remains
always the same; this is the first law of thermodynamics. But its
quality changes, and tends to change always in the same direction.
Turnstiles are set up between the different qualities of energy; the
passage is easy in one direction, impossible in the other. A human
crowd may contrive to find a way round without jumping over turnstiles,
but in nature there is no way round; this is the second law of
thermodynamics. Energy flows always in the same direction, as surely as
water flows downhill.
Part of the downward path consists, as we have seen, of the transition
from radiation of short wave-length into radiation of longer
wave-length. In terms of quanta (p. 126) the transition is from a few
quanta of high energy to a large number of quanta of low energy, the
total amount of energy of course remaining unaltered. The downfall of
the energy accordingly consists in the breaking of its quanta into
smaller units. And when once the fall and breakage have taken place, it
is as impossible to reconstitute the original large quanta as it was to
put Humpty-Dumpty back on his wall.
Although this is the main part of the downward path, it is not the
whole of it. Thermodynamics teaches that all the different forms of
energy have different degrees of “availability,” and that the downward
path is always from higher to lower availability.
And now we may return to the question with which we started the present
chapter: “What is it that keeps the varied life of the universe going?”
Our original answer “energy” is seen to be incomplete. Energy is no
doubt essential, but the really complete answer is that it is the
transformation of energy from a more available to a less available
form; it is the running downhill of energy. To argue that the total
energy of the universe cannot diminish, and therefore the universe
must go on for ever, is like arguing that as a clock-weight cannot
diminish, the clock-hand must go round and round for ever.
THE FINAL END OF THE UNIVERSE
Energy cannot run downhill for ever, and, like the clock-weight, it
must touch bottom at last. And so the universe cannot go on for ever;
sooner or later the time must come when its last erg of energy has
reached the lowest rung on the ladder of descending availability, and
at this moment the active life of the universe must cease. The energy
is still there, but it has lost all capacity for change; it is as
little able to work the universe as the water in a flat pond is able to
turn a water-wheel. We are left with a dead, although possibly a warm,
universe—a “heat-death.”
Such is the teaching of modern thermodynamics. There is no reason
for doubting or challenging it, and indeed it is so fully confirmed
by the whole of our terrestrial experience, that it is difficult to
see at what point it could be open to attack. It disposes at once
of any possibility of a cyclic universe in which the events we see
are as the pouring of river water into the sea, while events we do
not see restore this water back to the river. The water of the river
can go round and round in this way, just because it is not the whole
of the universe; something extraneous to the river-cycle keeps it
continually in motion—namely, the heat of the sun. But the universe as
a whole cannot so go round and round. Short of postulating continuous
action from outside the universe, whatever this may mean, the energy
of the universe must continually lose availability; a universe in
which the energy had no further availability to lose would be dead
already. Change can occur only in the one direction, which leads to the
heat-death. With universes as with mortals, the only possible life is
progress to the grave.
Even the flow of the river to the sea, which we selected as an obvious
instance of true cyclic motion, is seen to illustrate this, as soon
as all the relevant factors are taken into account. As the river
pours seaward over its falls and cascades, the tumbling of its waters
generates heat, which ultimately passes off into space in the form of
heat radiation. But the energy which keeps the river pouring along
comes ultimately from the sun in the form mainly of light; shut off the
sun’s radiation and the river will soon stop flowing. The river flows
only by continually transforming light-energy into heat-energy, and as
soon as the cooling sun ceases to supply energy of sufficiently high
availability the flow must cease.
The same general principles may be applied to the astronomical
universe. There is no question as to the way in which energy runs
down here. It is first liberated in the hot interior of a star in
the form of quanta of extremely short wave-length and excessively
high energy. As this radiant energy struggles out to the star’s
surface, it continually adjusts itself, through repeated absorption
and re-emission, to the temperature of that part of the star through
which it is passing. As longer wave-lengths are associated with
lower temperatures (p. 140), the wave-length of the radiation is
continually lengthened; a few energetic quanta are being transformed
into numerous feeble quanta. Once these are free in space, they travel
onward unchanged until they meet dust particles, stray atoms, free
electrons, or some other form of interstellar matter. Except in the
highly improbable event of this matter being at a higher temperature
than the surfaces of the stars, these encounters still further increase
the wave-length of the radiation, and the final result of innumerable
encounters is radiation of very great wave-length. The quanta have
increased enormously in numbers, but have paid for their increase by a
corresponding decrease in individual strength. In all probability, the
original very energetic quanta had their source in the annihilation
of protons and electrons, so that the main process of the universe
consists in the energy of exceedingly high availability which is
bottled up in electrons and protons being transformed into heat-energy
at the lowest level of availability.
Many, giving rein to their fancy, have speculated that this low-level
heat-energy may in due course re-form itself into new electrons and
protons. As the existing universe dissolves away into radiation, their
imagination sees new heavens and a new earth coming into being out of
the ashes of the old. But science can give no support to such fancies.
Perhaps it is as well; it is hard to see what advantage could accrue
from an eternal reiteration of the same theme, or even from endless
variations of it.
The final state of the universe will, then, be attained when every
atom which is capable of annihilation has been annihilated, and its
energy transformed into heat-energy wandering for ever round space,
and when all the weight of any kind whatever which is capable of being
transformed into radiation has been so transformed.
We have mentioned Hubble’s estimate that matter is distributed in space
at an average rate of 1·5 × 10⁻³¹ grammes per cubic centimetre. The
annihilation of a gramme of matter liberates 9 × 10²⁰ ergs of energy,
so that the annihilation of 1·5 × 10⁻³¹ grammes of matter liberates
1·35 × 10⁻¹⁰ ergs of energy. It follows that the total annihilation
of all the substance of the existing universe would only fill space
with energy at the rate of 1·35 × 10⁻¹⁰ ergs per cubic centimetre.
This amount of energy is only enough to raise the temperature of space
from absolute zero to a temperature far below that of liquid air; it
would only raise the temperature of the earth’s surface by a 6000th
part of a degree Centigrade. The reason why the effect of annihilating
a whole universe is so extraordinarily slight is of course that
space is so extraordinarily empty of matter; trying to warm space by
annihilating all the matter in it is like trying to warm a room by
burning a speck of dust here and a speck of dust there. As compared
with any amount of radiation that is ever likely to be poured into it,
the capacity of space is that of a bottomless pit. Indeed, so far as
scientific observation goes, it is entirely possible that the radiation
of thousands of dead universes may even now be wandering round space
without our suspecting it.
Such is the final end of things to which, so far as present-day science
can see, the material universe must inevitably come in some far-off
age, unless the course of nature is changed in the meantime. Let us now
try to peer back towards the beginnings of things.
THE BEGINNINGS OF THE UNIVERSE
As we go forwards in time, material weight continually changes into
radiation. Conversely, as we go backwards in time, the total material
weight of the universe must continually increase. We have seen how the
present weights of the stars are incompatible with their having existed
for more than some 5 or 10 million million years, and that they would
need approximately the whole of this enormous period to acquire certain
signs of age which their present arrangement and motions reveal.
We have seen that the break-up of the huge extra-galactic nebulae must
result in the birth of stars, and have found that the most consistent
account of the origin of the galactic system of stars is provided by
the supposition that the whole system originated out of the break-up of
a single huge nebula some 5 to 10 million million years ago.
Let us pause for a moment to compare this with an alternative
hypothesis, which some astronomers have favoured, that stars are being
created all the time. On this hypothesis we picture the stars as
passing in an endless steady stream from creation to extinction, just
as men pass in an endless steady stream from their cradles to their
graves, a new generation always coming into being to step into the
place vacated by the old. On this view Plaskett’s star, with about a
hundred times the weight of the sun, must be a recent creation, while
Kruger 60, with only a fraction of the sun’s weight, would be very,
very old—perhaps 100 million million years older than Plaskett’s star.
At present direct observation cannot definitely decide between the two
conflicting hypotheses, but it rather frowns upon the “steady stream”
view of the stars. In a steady population the number of people in
any assigned condition is exactly proportional to the time taken to
pass through that condition. Suppose for instance that human beings
possess infant teeth for a quarter as long as they possess adult teeth.
If examination of the teeth of a population shewed that four times
as many had adult teeth as infant teeth, this would create a _prima
facie_ expectation that we were dealing with a steady population. If,
on the contrary, 100 times as many people were found with adult teeth
as with infant teeth, we should know we were not dealing with a steady
population. If other evidence pointed to the population all being of
approximately the same age, we should be inclined to accept this and
regard the 1 per cent. of cases of infant teeth as cases of arrested
development.
We do not judge the ages of stars by their teeth but by their weights
and luminosities. And the luminosities of the stars are not found
to conform to the statistical laws which would prevail in a steady
population of stars. There appear to be so many middle-aged stars and
so few infants and veterans as to make the hypothesis of a steady
continuous creation hardly tenable. Indeed there is rather distinct
evidence of a special creation of stars at about the time our sun
was born. This leads back again quite naturally to the view that the
galactic system was born out of a spiral nebula whose main activity as
a parent of stars occurred some 5 to 10 million million years ago.
PRE-STELLAR EXISTENCE. On the whole it seems likely that we must assign
ages of 5 to 10 million million years to most or all of the stars in
the galactic system. This is as far as we can probe back into time
with any reasonable plausibility. The atoms which now form the sun and
stars must no doubt have had a previous existence as atoms of a nebula,
but we cannot say for how long. The temperatures at the centres of the
spiral nebulae may be, and in all probability are, so high that atoms
are stripped bare of electrons and so shielded from annihilation. We
may in fact regard the gaseous centres of nebulae as a sort of “white
dwarfs” built on a colossal scale. This fits on to the fact that the
nebulae generate very little energy for their weights and so shine very
feebly.
We have seen that the weights of two extra-galactic nebulae can be
estimated to a reasonable degree of accuracy. The great Andromeda
nebula _M_ 31 has the weight of 3500 million suns, its total luminosity
being that of 660 million suns. The nebula N.G.C. 4594 has the weight
of 2000 million suns, and the luminosity of 260 million suns. A simple
calculation shews that the atoms in the Andromeda nebula have an
average expectation of life of 80 million million years, while the
corresponding figure in N.G.C. 4594 is 115 million million years.
From these two instances, we may guess that the average life, before
annihilation, of the atoms in such nebulae must be of the order of 100
million million years. It cannot be claimed that this calculation is
either very convincing or very exact, but it supplies the only evidence
at present available as to the probable length of life of matter in
the nebular state. We can say that the stars have existed _as such_
for from 5 to 10 million million years, and that their atoms may have
previously existed in nebulae for at least a comparable, and possibly
for a much longer, time.
Apart from detailed figures, however, it is clear that we cannot
go backward in time for ever. Each step back in time involves an
increase in the total weight of the matter of the universe, and,
just as with individual stars, we cannot go so far back that this
total weight becomes infinite. Indeed a limit may quite possibly be
set by considerations which we have already mentioned. The complete
annihilation of all the matter now in the universe would raise the
temperature of the earth’s surface by the six-thousandth part of a
degree; the annihilation of a million times as much matter would raise
it by 160 degrees. We cannot admit that as much radiation as this
can be wandering about space. The earth’s temperature is determined
by the amount of radiation it receives from the sun; it adjusts
its temperature so that it radiates away just as much energy as it
receives. A small correction is required on account of the earth’s own
radio-activity, but this need not bother us. What would bother us, and
would indeed upset the balance entirely, would be the radiation of a
million dead universes if this were for ever streaming on to us out
of space; in this event the earth’s surface would have to rise to a
temperature well above that of boiling water before it could restore
the balance between the radiation it received and that it emitted. In
a word, the radiation of a million dead universes would boil our seas,
rivers and ourselves.
THE CREATION OF MATTER. All this makes it clear that the present matter
of the universe cannot have existed for ever: indeed we can probably
assign an upper limit to its age of, say, some such round number as 200
million million years. And, wherever we fix it, our next step back in
time leads us to contemplate a definite event, or series of events, or
continuous process, of creation of matter at some time not infinitely
remote. In some way matter which had not previously existed, came, or
was brought, into being.
If we want a naturalistic interpretation of this creation of matter,
we may imagine radiant energy of any wave-length less than 1·3 ×
10⁻¹³ cms. being poured into empty space; this is energy of higher
“availability” than any known in the present universe, and the running
down of such energy might well create a universe similar to our own.
The table on p. 144 shews that radiation of the wave-length just
mentioned might conceivably crystallise into electrons and protons, and
finally form atoms. If we want a concrete picture of such a creation,
we may think of the finger of God agitating the ether.
We may avoid this sort of crude imagery by insisting on space, time,
and matter being treated together and inseparably as a single system,
so that it becomes meaningless to speak of space and time as existing
at all before matter existed. Such a view is consonant not only with
ancient metaphysical theories, but also with the modern theory of
relativity (p. 74). The universe now becomes a finite picture whose
dimensions are a certain amount of space and a certain amount of time;
the protons and electrons are the streaks of paint which define the
picture against its space-time background. Travelling as far back in
time as we can, brings us not to the creation of the picture, but to
its edge; the creation of the picture lies as much outside the picture
as the artist is outside his canvas. On this view, discussing the
creation of the universe in terms of time and space is like trying to
discover the artist and the action of painting, by going to the edge of
the picture. This brings us very near to those philosophical systems
which regard the universe as a thought in the mind of its Creator,
thereby reducing all discussion of material creation to futility.
Both these points of view are impregnable, but so also is that of the
plain man who, recognising that it is impossible for the human mind to
comprehend the full plan of the universe, decides that his own efforts
shall stop this side of the creation of matter. This last point of view
is perhaps the most justifiable of all from the purely philosophic
standpoint. It is now a full quarter of a century since physical
science, largely under the leadership of Poincaré, left off trying to
explain phenomena and resigned itself merely to describing them in the
simplest way possible. To take the simplest illustration, the Victorian
scientist thought it necessary to “explain” light as a wave-motion
in the mechanical ether which he was for ever trying to construct
out of jellies and gyroscopes; the scientist of to-day, fortunately
for his sanity, has given up the attempt and is well satisfied if he
can obtain a mathematical formula which will predict what light will
do under specified conditions. It does not matter much whether the
formula admits of a mechanical explanation or not, or whether such an
explanation corresponds to any thinkable ultimate reality. The formulae
of modern science are judged mainly, if not entirely, by their capacity
for describing the phenomena of nature with simplicity, accuracy, and
completeness. For instance, the ether has dropped out of science, not
because scientists as a whole have formed a reasoned judgment that no
such thing exists, but because they find they can describe all the
phenomena of nature quite perfectly without it. It merely cumbers the
picture, so they leave it out. If at some future time they find they
need it, they will put it back again.
This does not imply any lowering of the standards or ideals of science;
it implies merely a growing conviction that the ultimate realities of
the universe are at present quite beyond the reach of science, and may
be—and probably are—for ever beyond the comprehension of the human
mind. It is _à priori_ probable that only the artist can understand the
full significance of the picture he has painted, and that this will
remain for ever impossible for a few specks of paint on the canvas.
It is for this kind of reason that, when, as in Chapter II, we try to
discuss the ultimate structure of the atom, we are driven to speak
in terms of similes, metaphors, and parables. There is no need even
to worry overmuch about apparent contradictions. The higher unity of
ultimate reality must no doubt reconcile them all, although it remains
to be seen whether this higher unity is within our comprehension or
not. In the meantime a contradiction worries us about as much as an
unexplained fact, but hardly more; it may or may not disappear in the
progress of science.
If some such train of thought may be applied to our efforts to
understand the most minute processes of the universe (and it is the
common everyday train of thought of those who are working in this
field), then it must surely be still more applicable to our efforts
to understand the universe as a whole. Phenomena come to us disguised
in their frameworks of time and space; they are messages in cypher
of which we shall not understand the ultimate significance until we
have discovered how to decode them out of their space-time wrappings.
Whatever may be thought about our final ability to decode the difficult
messages we have recently received about the ultimate structure of the
minutest parts of matter, it seems natural that we should feel some
apprehension with regard to those about the structure of the universe
as a whole, and particularly those about its beginnings and endings.
Often enough the message itself may help us to discover the code in
which it reaches us—with sufficient skill we can often do this—but we
are now speaking of problems as to when, by whom, and for what purpose,
the code was devised. There is no reason why a code message should
throw any light on this.
The astronomer must leave the problem at this stage. The message of
astronomy is of obvious concern to philosophy, to religion and to
humanity in general, but it is not the business of the astronomer
to decode it. The observing astronomer watches and records the dots
and dashes of the needle which delivers the message, the theoretical
astronomer translates these into words—and according as they are found
to form known consistent words or not, it is known whether he has done
his job well or ill—but it is for others to try to understand and
explain the ultimate decoded meaning of the words he writes down.
LIFE AND THE UNIVERSE
Abandoning our efforts to understand the universe as a whole, let us
glance for a moment at the relation of life to the universe we know.
The old view that every point of light in the sky represented a
possible home for life is quite foreign to modern astronomy. The stars
themselves have surface-temperatures of anything from 1650 degrees to
30,000 degrees or more, and are of course at far higher temperatures
inside. By far the greater part of the matter of the universe is at a
temperature of millions of degrees, so that its molecules are broken up
into atoms, and the atoms are broken up, partially at least, into their
constituent parts. Now the very concept of life implies duration in
time; there can be no life where atoms change their make up millions of
times a second and no pair of atoms can ever stay joined together. It
also implies a certain mobility in space, and these two implications
restrict life to the small range of physical conditions in which the
liquid state is possible. Our survey of the universe has shewn how
small this range is in comparison with that exhibited by the universe
as a whole. It is not to be found in the stars, nor in the nebulae out
of which the stars are born. We know of no type of astronomical body in
which the conditions can be favourable to life except planets like our
own revolving round a sun.
Now, to the best of our present knowledge, planets are very rare. We
have seen how a single star cannot of itself produce planets. A family
of planets must have two parents; it only comes into being as the
result of the close approach of two stars, and stars are so sparsely
scattered in space that it is an inconceivably rare event for one to
pass near to a neighbour. On the Tidal Theory, explained on p. 236,
planets cannot be born except when two stars pass within about three
diameters of one another. As we know how the stars are scattered in
space, we can estimate fairly closely how often two stars will approach
within this distance of one another. The calculation shews that even
after a star has lived its life of millions of millions of years, the
chance is still about a hundred thousand to one against its being a sun
surrounded by planets.
Even so, if life is to obtain a footing, the planets must not be too
hot or too cold. In the solar system, for instance, we cannot imagine
life existing on Mercury or on Neptune; liquids boil on the former
and freeze hard on the latter. These planets are unsuitable for
life because they are too near to, or too far from, the sun. We can
imagine other planets which are unsuitable because their substance
itself generates energy at such a rate as to make them unsuitable
for habitation. The inert atoms which form our earth seem to be the
end products of a long series of atomic changes, a sort of final ash
resulting from the combustion of the universe. We have seen how such
atoms probably float to the top in every star, as being the lightest in
weight, but it is by no means a foregone conclusion that all planets
will consist of nothing but inert atoms, and so will cool down until
life can obtain a footing on them. This has happened with our earth,
but we do not know how many planets and planetary systems may be
unsuited for life because it has not happened with them.
All this suggests that only an infinitesimally small corner of the
universe can be in the least suited to form an abode of life. Primaeval
matter must go on transforming itself into radiation for millions of
millions of years to produce a minute quantity of the inert ash on
which life can exist. Then by an almost incredible accident this ash,
and nothing else, must be torn out of the sun which has produced it,
and condense into a planet. Even then, this residue of ash must not be
too hot or too cold, or life will be impossible.
Finally, after all these conditions are satisfied, will life come or
will it not? We must probably discard the at one time widely accepted
view that once life had come into the universe in any way whatsoever,
it would rapidly spread from planet to planet and from one planetary
system to another until the whole universe teemed with life; space now
seems too cold, and planetary systems too far apart. Our terrestrial
life must in all probability have originated on the earth itself. What
we would like to know is whether it originated as the result of still
another amazing accident or succession of coincidences, or whether
it is the normal event for inanimate matter to produce life in due
course, when the physical environment is suitable. We look to the
biologist for the answer, which so far he has not been able to produce.
The astronomer might be able to give a partial answer if he could
find evidence of life on some other planet, for we should then at
least know that life had occurred more than once in the history of
the universe, but so far no convincing evidence has been forthcoming.
Some astronomers interpret certain markings on Mars as canals, which
they believe to be the handiwork of intelligent beings, but this
interpretation is not generally accepted. Again, seasonal changes
necessarily occur on Mars as on the earth, and certain phenomena
accompany these which many astronomers are inclined to ascribe to the
growth and decline of vegetation, although they may represent nothing
more than rains watering the desert. There is no definite evidence of
life, and certainly no evidence of conscious life, on Mars—or indeed
anywhere else in the universe.
It seems at first somewhat surprising that oxygen figures so largely in
the earth’s atmosphere, in view of its readiness to enter into chemical
combination with other substances. We know, however, that vegetation
is continually discharging oxygen into the atmosphere, and it has
often been suggested that the oxygen of the earth’s atmosphere may be
mainly or entirely of vegetable origin. If so, the presence or absence
of oxygen in the atmospheres of other planets should shew whether
vegetation similar to that we have on earth exists on these planets or
not.
Oxygen certainly exists in the Martian atmosphere, but its amount is
small. Adams and St John estimate that there cannot be more than 15 per
cent. as much, per square mile, as on earth. On the other hand it is
either completely absent, or of negligible amount, in the atmosphere
of Venus. If any is present at all, St John estimates that the amount
above the clouds which cover the surface of Venus is less than 0·1 per
cent. of the terrestrial amount. The evidence, for what it is worth,
goes to suggest that Venus, the only planet in the solar system outside
Mars and the earth on which life could possibly exist, possesses no
vegetation and no oxygen for higher forms of life to breathe.
Apart from the certain knowledge that life exists on earth, we have
no definite knowledge whatever except that, at the best, life must
be limited to a tiny fraction of the universe. Millions of millions
of stars exist which support no life, which have never done so and
never will do so. Of the rare planetary systems in the sky, many
must be entirely lifeless, and in others life, if it exists at all,
is probably limited to a few planets. The three centuries which have
elapsed since Giordano Bruno suffered martyrdom for believing in the
plurality of worlds have changed our conception of the universe almost
beyond description, but they have not brought us appreciably nearer to
understanding the relation of life to the universe. We can still only
guess as to the meaning of this life which, to all appearances, is so
rare. Is it the final climax towards which the whole creation moves,
for which the millions of millions of years of transformation of matter
in uninhabited stars and nebulae, and of the waste of radiation in
desert space, have been only an incredibly extravagant preparation?
Or is it a mere accidental and possibly quite unimportant by-product
of natural processes, which have some other and more stupendous end
in view? Or, to glance at a still more modest line of thought, must
we regard it as something of the nature of a disease, which affects
matter in its old age when it has lost the high temperature and
capacity for generating high-frequency radiation with which younger and
more vigorous matter would at once destroy life? Or, throwing humility
aside, shall we venture to imagine that it is the only reality, which
creates, instead of being created by, the colossal masses of the stars
and nebulae and the almost inconceivably long vistas of astronomical
time?
Again it is not for the astronomer to select between these alternative
guesses; his task is done when he has delivered the message of
astronomy. Perhaps it is over-rash for him even to formulate the
questions this message suggests.
THE EARTH AND ITS FUTURE PROSPECTS
Let us leave these rather abstract regions of thought and come down to
earth. We feel the solid earth under our feet, and the rays of the sun
overhead. Somehow, but we know not how or why, life also is here; we
ourselves are part of it. And it is natural to enquire what astronomy
has to say as to its future prospects.
The central facts which dominate the whole situation are that we are
dependent on the light and heat of the sun, and that these cannot
remain for ever as they now are. So far as we can at present see, solar
conditions can hardly have changed much since the earth was born; the
earth’s 2000 million years form so small a fraction of the sun’s whole
life that we can almost suppose the sun to have stood still throughout
it. This of itself suggests that, in so far as astronomical factors
are concerned, life may look to a tenancy of the earth of far longer
duration than the total past age of the earth.
The earth, which started life as a hot mass of gas, has gradually
cooled, until it has now about touched bottom, and has almost no heat
beyond that which it receives from the sun. This just about balances
the amount it radiates away into space, so that it would stay at its
present temperature for ever if external conditions did not change, and
any changes in its condition will be forced on it by changes occurring
outside.
These external changes may be of many kinds. The sun’s loss of weight
causes the earth to recede from it at the rate of about a yard a
century, so that after a million million years, the earth will be 10
per cent. further away from the source of its light and life than now.
Consequently even if the sun then radiated as much light and heat as
now, the earth would receive 20 per cent. less of this radiation, and
its mean temperature would be some 15 degrees Centigrade or so lower
than at present. But after a million million years the sun will not
radiate as much light and heat as now; it will have lost some 6 per
cent. of its present weight through radiation, and, judging from other
stars, this loss will probably reduce its energy-generating capacity
by about 20 per cent. This will reduce the earth’s temperature by
about another 15 degrees, so that after a million million years the
inevitable course of events will have reduced the earth’s temperature
by about 30 degrees Centigrade.
It would be rash to attempt to predict how such a fall of temperature
may affect terrestrial life, and human life in particular. Given
sufficient time, life has such an enormous capacity for adapting itself
to its environment that it seems possible that, even with a temperature
30 degrees Centigrade lower than now, life may still exist on earth
a million million years hence. If so, I am glad that my life has not
fallen in this far distant future. Mountains and seas, which provide
some of the keenest pleasures of our present life, will exist only as
traditions handed down from a remote and almost incredible past. The
denudation of a million million years will have reduced the mountains
almost to plains, while seas and rivers will be frozen packs of solid
ice. We may well imagine that man will have infinitely more knowledge
than now, but he will no longer know the thrill of pleasure of the
pioneer who opens up new realms of knowledge. Disease, and perhaps
death, will have been conquered, and life will doubtless be safer and
incomparably better-ordered than now. It will seem incredible that
a time could have existed when men risked, and lost, their lives in
traversing unexplored country, in climbing hitherto unclimbed peaks, in
fighting wild beasts for the fun of it. Life will be more of a routine
and less of an adventure than now; it will also be more purposeless
when the human race knows that within a measurable space of time it
must face extinction and the eternal destruction of all its hopes,
endeavours, and achievements.
Without laying too much stress on these visionary concepts of life
a million million years hence, we may nevertheless think of this as
the period in round numbers after which the inevitable wastage of the
sun’s weight is likely to drive life off the earth. Venus, with a mean
temperature some 60 degrees higher than the earth, is probably rather
too hot for life at present. But after a million million years, the
temperature of Venus will have fallen by 40 degrees, and what the earth
is now, Venus may perhaps be somewhere between one and two million
million years hence. Whether life will then inhabit Venus we cannot
know, and it would be futile to guess, but there is at least a chance
that as the earth fails, Venus may step into its place. Possibly Venus
may be followed by Mercury in due course, but the present evidence
is that Mercury is devoid of atmosphere, in which case it is hard to
imagine it as a home for life at all resembling that which now inhabits
the earth.
So far we have considered only the normal course of events; a variety
of accidents may bring the human race to an end long before a million
million years have elapsed. To mention only possible astronomical
occurrences, the sun may run into another star, any asteroid may hit
any other asteroid and, as a result, be so deflected from its path as
to strike the earth, any of the stars in space may wander into the
solar system and, in so doing, upset all the planetary orbits to such
an extent that the earth becomes impossible as an abode of life. It is
difficult to estimate the likelihood of any of these events happening
but they all seem very improbable, and the first and last highly so.
Let us disregard them all.
A danger remains which cannot be so lightly dismissed. Let us first
state it in technical language. The sun is a main-sequence star, and
is moreover very near to the left-hand edge of the main-sequence in
the Russell diagram (p. 278). Beyond this edge is a region of the
diagram which is completely untenanted by stars. We have supposed this
region to be untenanted by stars because the stellar configurations
it represents would be unstable. Stars pass through it rapidly until
they find a stable configuration, and so end up in a region which can
be permanently tenanted by stars. Now the next stable configurations
beyond this region are those of the white dwarfs, and as these are less
massive as a class than the main-sequence stars, the general trend of
stellar evolution appears to be from main-sequence star to white dwarf.
On this view the white dwarfs must have previously been main-sequence
stars which wandered across the left-hand edge of the band of stable
configurations and then fell through the unstable region until they
resumed stability as white dwarfs.
The danger lies in the fact that the sun is already perilously near
to the left-hand edge of the main-sequence. According to Redman’s
determinations, which are probably by far the most reliable at present
available, the main-sequence belt of stable configurations for stars of
the same spectral type as the sun (G 0) extends roughly between stellar
absolute magnitudes, 4·88 and 3·54, the former marking the dangerous
left-hand edge. The sun’s present absolute magnitude is estimated as
4·85. Thus if the sun were to become 0·03 magnitudes fainter, this
representing a reduction of only 3 per cent. in its luminosity, it
would arrive exactly at the edge of the main-sequence, and would
proceed to contract precipitately to the white dwarf state. In so
doing, its light and heat would diminish to such an extent that life
would be banished from the earth. The known white dwarf star which it
would most closely resemble is the companion of Sirius, and this emits
only a four-hundredth part as much light and heat as the sun.
To put the same thing in non-technical language, the sun is in, or is
not far from, a precarious state in which stars are liable to begin to
shrink and in so doing to reduce their radiation to a tiny fraction
of that at present emitted by the sun. The shrinkage of the sun to
this state would transform our oceans into ice and our atmosphere into
liquid air; it seems impossible that terrestrial life could survive.
The vast museum of the sky must almost certainly contain examples of
shrunken suns of this type with planets like our earth revolving round
them. Whether these planets carry on them the frozen remains of a life
which was once as active as our present life on earth we can hardly
even surmise.
This may be thought to open up a startling prospect for the earth,
but we can take courage for several reasons. In the first place a 3
per cent. decrease in the sun’s luminosity can hardly occur in less
than about 150,000 million years. This in itself is not too bad, but
the prospect becomes enormously more hopeful when we reflect that the
evolution of the stars, including the sun, takes place in a direction
almost parallel to the edge of the main-sequence. The sun is not
heading for the precipice, so much as skirting along its edge. Whether
it is approaching the edge, and is ultimately destined to fall over, we
do not know, but it is in any case unlikely to reach the edge within
the next million million years.
Finally, the sun’s distance from the edge of the main-sequence cannot
be estimated with anything like the degree of accuracy assumed in the
foregoing calculations. The figure of 0·03 appeared as the difference
of two much larger numbers, and although both of these can be estimated
with fair accuracy, neither can be estimated with sufficient accuracy
to justify us in treating their small difference of 0·03 as exact. The
most we can say is that the sun is quite fairly near to the dangerous
edge, but that any appreciable motion towards this edge is a matter of
millions of millions of years.
Another danger, of a more speculative kind, must also be mentioned.
We have seen (p. 61) how every now and then a new star appears in the
sky, shines with terrific brilliancy for a short time, and then either
fades away entirely or continues to shine as an ordinary star. These
apparitions are known as “novae”—new stars. In many cases the nova has
been proved to be an ordinary star which was visible as a very faint
star long before it appeared as a nova, flashed into brilliance for a
brief span of life, and then lapsed back into commonplaceness, and it
seems reasonable to suppose that all novae are of this kind, although
the star may often escape detection until it assumes its brilliant
nova state. These apparitions are by no means rare; something like six
appear every year in the galactic system alone. Now if we suppose the
galactic system to consist of 300,000 million stars, this means that,
on the average, each star becomes a nova once in every 50,000 million
years. What we would like to know is whether our sun is in danger of
becoming a nova; for, if all kinds of stars run equal chances, it is
likely to become a nova some twenty times in the next million million
years.
So far there is no agreement among astronomers either as to the
physical causes which turn an ordinary star into a nova, or as to the
physical conditions which prevail in novae. Various suggestions are in
the field, but none of them wins general acceptance. It seems fairly
certain that if our sun were suddenly to become a nova, its emission of
light and heat would so increase as to scorch all life off the earth,
but we are completely in the dark as to whether our sun runs any risk
of entering the nova stage. If it does, this is probably the greatest
of all the risks to which life on earth is exposed.
Apart from accidents, we have seen that if the solar system is left
to the natural course of evolution, the earth is likely to remain a
possible abode of life for something of the order of a million million
years to come.
This is some five hundred times the past age of the earth, and
over three million times the period through which humanity has
so far existed on earth. Let us try to see these times in their
proper proportion by the help of yet another simple model. Take a
postage-stamp, and stick it on to a penny. Now climb Cleopatra’s
needle and lay the penny flat, postage-stamp upper-most, on top of the
obelisk. The height of the whole structure may be taken to represent
the time that has elapsed since the earth was born. On this scale,
the thickness of the penny and postage-stamp together represents the
time that man has lived on earth. The thickness of the postage-stamp
represents the time he has been civilised, the thickness of the penny
representing the time he lived in an uncivilised state. Now stick
another postage-stamp on top of the first to represent the next 5000
years of civilisation, and keep sticking on postage-stamps until
you have a pile as high as Mont Blanc. Even now the pile forms an
inadequate representation of the length of the future which, so far as
astronomy can see, probably stretches before civilised humanity, unless
an accident cuts it short. The first postage-stamp was the past of
civilisation; the column higher than Mont Blanc is its future. Or, to
look at it in another way, the first postage-stamp represents what man
has already achieved; the pile which outtops Mont Blanc represents what
he may achieve, if his future achievement is proportional to his time
on earth.
Yet we have seen that we cannot count on such a length of future with
any certainty. Accidents may happen to the race as to the individual.
Celestial collisions may occur; shrinking into a white dwarf, the sun
may freeze terrestrial life out of existence; bursting out as a nova it
may scorch our race to death. Accident may replace our Mont Blanc of
postage-stamps by a truncated column of only a fraction of the height
of Mont Blanc. Even so, there is a prospect of tens of thousands of
millions of years before our race. And the human mind, as apart from
the mind of the mathematician, can hardly distinguish clearly between
such a period as this and the million million years to which we may
look forward if accidents do not overtake us. For all practical purpose
the only statement that conveys any real meaning is that our race may
look forward to occupying the earth for a time incomparably longer than
any we can imagine.
Looked at in terms of space, the message of astronomy is at best one
of melancholy grandeur and oppressive vastness. Looked at in terms
of time, it becomes one of almost endless possibility and hope. As
denizens of the universe we may be living near its end rather than its
beginning; for it seems likely that most of the universe had melted
into radiation before we appeared on the scene. But as inhabitants
of the earth, we are living at the very beginning of time. We have
come into being in the fresh glory of the dawn, and a day of almost
unthinkable length stretches before us with unimaginable opportunities
for accomplishment. Our descendants of far-off ages, looking down
this long vista of time from the other end, will see our present age
as the misty morning of the world’s history; our contemporaries of
to-day will appear as dim heroic figures who fought their way through
jungles of ignorance, error and superstition to discover truth, to
learn how to harness the forces of nature, and to make a world worthy
for mankind to live in. We are still too much engulfed in the greyness
of the morning mists to be able to imagine, however vaguely, how this
world of ours will appear to those who will come after us and see it in
the full light of day. But by what light we have, we seem to discern
that the main message of astronomy is one of hope to the race and of
responsibility to the individual—of responsibility because we are
drawing plans and laying foundations for a longer future than we can
well imagine.
INDEX
α Aquilæ, _see_ Altair
α Canis Majoris, _see_ Sirius
α Canis Minoris, _see_ Procyon
α Centauri, distance, 33, 35
luminosity and weight, 48
system of, 40, 263, 279, 280
α Herculis, 259, 272
α Lyrae, 33
α Orionis, _see_ Betelgeux
α-particles, 112, 116, 117, 118
and Plate XIII (p. 113)
α-rays, 111
α Scorpii, _see_ Antares
Absolute temperature, 102
Absorption lines and spectrum, 51, 126
Actino-uranium, 154, 155
Adams, J. C., 18, 19
Adams, W. S., 62, 343
Age of earth, 14, 151 ff., 248
of stars, 155 ff.
of sun, 180, 184
of universe, 336
Aitken, R. G., 171, 188
Altair (α Aquilæ), 33, 278
Andromeda, Great Nebula _M_ 31 in, Plate IV (p. 30),
30, 70, 71, 72, 204, 206, 208, 215, 335
age, 335
distance, 71
rotation, 206
size, 30
weight, 71, 208, 215
Andromedid meteors, 251
Angstrom unit, defined, 256
Angular momentum, conservation of, 205, 217, 232
of solar system, 232
Annihilation of matter, 185, 191, 193, 298, 331
Antares (α Scorpii), 259, 272, 273, 278
internal constitution of, 273, 291
Aquarid meteors, 251
Aristarchus of Samos, 2, 13
Aristotle, 2, 5
Arrangement of solar system, 16, 240 ff.
Bode’s law, 19, 20
Copernican, 3, 5, 6, 27, 32
Ptolemaic, 2, 4, 6, 8, 31
Asteroids, 17, 226, 240, 247, 252, 348
Ceres, 17
distance of, 19, 20, 252
Eros, 34
origin of, 247, 252
Aston, F. W., 119, 154, 185
Atmosphere, of earth, 195, 343
of moon, 196
of planets, 196, 343, 344, 348
of sun, 195
Atomic nuclei, 108, 117
disintegration of, 111, 117
size and weight of, 109
structure of, 119, 139, 147
Atomic numbers and weights, 110, 287
in stellar matter, 306, 311
Atomic theory, 92, 103
Atoms, 103, 318
structure of, 106
synthesis of, 148, 149, 185
Availability of Energy, 326, 329
β Aurigae, 54, 58
β-particles, 112, 113, 116, 145 and Plate XIII (p. 113)
β-rays, 111, 145
B.D. 6° 1309, _see_ Plaskett’s star
Bacon, Roger, 1
Becquerel, H., 110
Bentley, R., 197
Berenice’s hair, 39
Bessel, F. W., 33
Betelgeux (α Orionis), 259, 272, 278
Bickerton, A. W., 235
Biela’s comet, 251
Binary systems, 39, 40, 52, 54
birth of, 217
eclipsing, 54, 58
loss of weight in, 188, 228
orbits of, 46, 47, 167
origins of, 169, 219, 297
relative weights of components in, 188
subdivision in, 228
weights of, 48, 55
Birth of binary systems, 217
of nebulae, 204
of planets, 232, 237
of satellites, 232, 241
of stars, 209
Blackett. P. M. S., 117, 118
Blue stars, 62
Bode’s law, 19, 20, 252
Bohr, Niels, 124, 128, 129, 131, 304, 305
Bowen, J. S., 143
Boys, C. V., 44
Brownian movements, 159, 163
Bruno, Giordano, 3, 4, 344
Buffon, G. L. L., 235
Bumstead, H. A., 113
Burton, E. F., 143
γ-rays, 111, 114, 122, 139, 144, 145
Cameron, G. H., 143
Campbell, W. W., 321
Candle-power of stars, 49, 55, 179, 188;
_see_ Luminosity
Capella, 278
Cassini, D., 23
Cavity-radiation, 123, 255
Cepheid variables, 55, 56, 57, 59, 60, 70, 209
mechanism of, 223
Ceres (asteroid), 17
Chamberlin, T. C., 235, 236
Chaos, primaeval, density of, 202
evolution from, 202, 204, 229, 230
Charlier, C. V. L., 67
Clusters, globular, 63, 64, 68, 213, 216
moving, 39, 173, 216, 217
Coal, combustion of, 175, 191
Coal sack, 30
Comets, 247, 251
origin of, 238
Condensation-chamber of C. T. R. Wilson, 113, 117
Condensations in a gas, 198, 200, 203, 209, 237
Configurations of rotating masses, 221
Conservation, of angular momentum, 205, 217, 232
of energy, 99, 189, 326, 328
of matter, 189
Constellations, 36, 39
Cooke, H. L., 143
Copernican astronomy, 3, 5, 6, 27, 32
Copernicus, 3
Creation, of matter, 336
of universe, 79, 337
Crookes, Sir W., 106
Cycles and epicycles (Ptolemaic), 2, 3, 4
Cyclic processes in nature, 325, 329, 331
δ Cephei, 56, 223
δ-rays, 113
Darwin, Sir George, 224
de Sitter, cosmology of, 79 ff., 84, 86, 174
Democritus, 93
Density, of matter in space, 77, 331, 332
of nebulae, 208
of stars, 265-8, 273, 282
Diameters of stars, 260, 263-8, 272, 277-80
Diffraction grating, 114
Dimensions, of earth, 34
of galactic system, 65
of solar system, 20, 34
of universe, 77, 83, 86, 87
Discontinuity, in physics, 124 ff.
of matter, 93
Displacements of spectral lines, (de Sitter), 81
(Doppler), 52
(Einstein), 78, 323
Distances, of globular clusters, 63
of nebulae, 71, 73
of stars, 33, 35, 36, 61, 62, 63
Doppler effect, 52
Dwarf and Giant stars, 276
Dwarfs, white, 280, 283, 284, 292, 317, 319, 320, 322, 323, 335
Earth, age of, 14, 151, 155, 173, 247
as a planet, 16, 19, 230, 237, 243
birth of, 237, 243
dimensions of, 34
future of, 225, 226, 250, 346, 352
-moon system, 34, 225, 226, 240, 243, 250
orbit of, 35, 36, 227, 346
Eccentricity of binary orbits, 171
of ellipse (defined), 46
Eclipsing binaries, 54, 55, 56, 58
Eddington, A. S., 67, 84, 179, 184, 187, 224, 287, 288, 295
Einstein, A., (Cosmology), 74, 77, 79, 84
(Gravitation), 45, 75, 79, 161
(Quantum theory), 124, 126, 127
(Relativity), 74 ff., 186, 261
Electricity, positive and negative, 107
attraction and repulsion, 107, 129
Electromagnetic energy, 120, 121
Electron, 107, 112, 119, 121
orbits in atom, 108, 128
Ellipse, defined, 46
as atomic orbit, 129
as gravitational orbit, 46, 47, 227
Ellis, C. D., 139
Emden, R., 286, 288
Energy, 98, 120, 189, 326 ff.
availability of, 326, 328
conservation of, 99, 189
source of stellar, 184
weight of, 120
Epicycles, 2, 3, 4
Equipartition of energy, in a gas, 156
stellar, 160, 164, 170, 172
Eros, 34
Ether, existence of, 338
Evaporation, 94
Evening stars, 17
Evolution, from primaeval chaos, 202, 205, 229, 230
stellar, 313 ff.
Expanding universe, 84, 86 ff.
Extra-galactic nebulae, 30, 70, 82
densities of, 208
distance of, 71, 82
evolution of, 204 ff. and Plates XVI to XXI in order
number of, 72
rotation of, 205, 206
size of, 31
velocities of, 82, 84, 86
weights of, 71, 215
Faraday, M., 122
Final end of universe, 329 ff.
Fireball, 248
Fission of stars, 169, 217, 219, 222, 229
Fizeau, H. L., 261
Fluorescence, 115, 327
Fraunhofer, J. von, 105
Free-path of molecules, 98
Frequency of radiation, 115, 126, 127
Full radiator, 123, 255, 261
Galactic latitude and longitude, 24, 25
Galactic nebulae, 29, Plates III (p. 29), VI (p. 37), VII (p. 44)
Galactic system of stars, 23, 25, 65 ff., 211
number of, 68, 69, 215
rotation of, 66 ff., 214
weight of, 69, 215
Galileo, 1, 3, 4, 5, 8, 16, 24
Galle, J. G., 19
Gaseous masses in rotation, 206, 221
Gaseous state, nature of, 95
equipartition of energy in, 156
Gaunt, J. A., 309
Geodesy, 34
Geology, 150, 152
Gerasimovič, B. P., 321
Giant and Dwarf stars (defined), 276
Giant stars, internal constitution, 273, 290, 291, 310, 319
Globular clusters, 63, 65, 213, 216 and Plate IX (p. 63)
origin of, 216
shape of, 213
Goldsbrough, G. R., 250
Graham, T., 158
Gravitation, law of, 41, 45, 78, 161
cause of, 76
Gravitational instability, 197, 210, 237, 245, 246
Great Bear (cluster), 173, 216, 217
(constellation), 32, 39
Great nebulae, _see_ Extra-galactic nebulae
H.D. 1337, _see_ Pearce’s star
Halley, E., 151
Halley’s comet, 247, 251
Halm, J., 163
Hayford, J. F., 34
Heat, nature of, 100
effect of, on electrical structures, 141, 144
transport of, in a star, 307
Heat-death, 329, 330
Helium atom, 110, 119, 185, 305
Helmholtz, H., 177, 285
Henderson, T., 33
Herschel, Sir John, 23, 24, 211
Herschel, Sir William, 18, 23, 24, 25, 26, 28, 65, 70, 211
Hertzsprung, E., 59, 223, 275
Hesperus, 17
Hess, V. F., 143
Highly penetrating radiation, 143, 144, 191, 298, 299
Hinks, A. R., 65
Holmes, A., 152
Hubble, E., 71, 73, 77, 83, 84, 202, 204, 206, 208, 210
Humason, M. L., 82
Huyghens, C., 23
Hydrogen atom, 110, 119, 130, 135, 146
annihilation of, 146, 147, 192
electron orbits in, 128, 130, 192
Hyperbola, 46
Instability, dynamical, 202
gravitational, 197, 210, 237, 245, 246
Interferometer, 261, 272
Interstellar matter, 29, 30, 63
Interstellar space, temperature of, 102
Inverse square law, (electric), 129
(gravitational), 44, 45, 47
(luminosity), 24, 33
Invisible radiation, 115, 258, 259
Irregular nebulae, 206
Irregular variables, 55, 58
Island universes, 28, 31, 68
Isotopes, 118
Jeffreys, H., 155, 225, 226, 239
Jupiter, 4, 16, 17, 19, 207
birth of, 240 ff., 243, 244
rotation of, 168, 225
satellites of, 4, 23, 196, 240, 241, 243, 250
size of, 17
temperature of, 21
weight of, 45
_K_-ring of electrons in atom, 134, 138, 289, 291,
292, 302, 304, 318
Kant, I., 176, 230, 231
Kapteyn, J. C., 27, 65, 212, 214
Kelvin, Lord, 177, 285
Kepler, J., 5, 33, 46
Klein, O., 146, 148
Kolhörster, W., 143
Kramers, H. A., 309, 310
Kruger 60, 48, 278, 279, 280, 317
system of, 267, 279, 280
_L_-ring of electrons in atom, 134, 138, 289, 292, 302
Lalande 21185, 36, 265
Lane’s law, 286
Laplace, P. S., 230, 233 ff.
nebular hypothesis of, 230, 231, 232 ff.
Lead, radio-active, 154
Leavitt, Miss, 57
Lemaître, G., 84, 86, 174
Lenard, P., 106
Leonid shooting-stars, 250
Leslie, Sir John, 158
Leucippus, 93
Leverrier, U. J. J., 18, 19
Life in solar system, 341
in universe, 340, 342, 345
on Earth, 12, 342, 345
on Mars, 343
on Venus, 344, 347
Light, nature and composition of, 37, 50, 114
speed of, 37, 115
wave-length of, 115, 138
Light-year, 37
Lindblad, B., 67, 69
Lippershey, 1
Liquid masses in rotation, 219 ff.
Liquid stars, 222, 296 ff., 300, 304, 310
Lives of stars, _see_ Stars
Lockyer, Sir N., 314
Long-period variables, 56, 58, 61, 271, 319
Lowell, P., 19
Lucifer, 17
Lucretius, 93
Luminosity of stars, 48, 49, 179, 269, 278, 279, 280
_M_-ring of electrons in atom, 134, 289, 292, 293, 302
McLennan, J. C., 143
Magellanic clouds, 57, 210 and Plate XXI (p. 214)
Main-sequence stars, 279, 291, 295
Major planets, 16, 17
atmospheres on, 197
Man’s life on earth: past, 12, 16
future, 345 ff.
Marius, 31
Mars, 18, 19, 35
atmosphere of, 196, 343
birth of, 240, 243, 244
life on, 343
orbit of, 20, 239
rotation of, 225
satellites of, 240, 243, 250
temperature of, 21
Matter, annihilation of, 185, 191, 192, 315
conservation of, 189
Maxwell, J. C., 122, 157
Mayer, R., 176
Mercury, 4, 5, 16, 17, 19, 22, 35
absence of satellites, 243
atmosphere of, 196, 348
birth of, 240, 243
life on, 348
orbit of, 20, 45, 239
rotation of, 22, 224
Meteor, 238, 247, 248
Meteorite, 248
Michelson, A. A., 261
Milky way, 3, 23, 26, 27, 28, 30
Millikan, R. A., 143, 148
Minor planets, 16
Molecule, 94
collisions of, 97, 98, 102
equipartition of energy, 156
size of, 95
speed of, 96, 195
Moon, 4, 43
atmosphere of, 196
birth of, 243
distance of, 34, 43
eclipse of, 34
future of, 225, 226, 251
rotation of, 225
Moore, J. H., 321
Morning star, 17
Moulton, F. R., 235, 236
Moving clusters, 40, 173, 216
shape of, 216, 217
Munich 15040, 36, 264
Nebulae, classification of, 28
Nebulae, distance and size of, 31, 71, 72
extra-galactic, 28, 30, 70, 73, 82, 204 ff.
motions of, 82
rotation of, 205
weights of, 71, 204, 208, 215
_M_ 31, _see_ Andromeda, Great Nebula in
_M_ 33, 71, 210 and Plate XX (p. 213)
N.G.C. 4594, 71, 72, 204, 208 and Plate XV (p. 204)
Nebulae, galactic, 29 and Plates III (p. 29),
VI (p. 37), VII (p. 44)
Nebulae, planetary, 29, 273, 321, 323 and Plate II (p. 28)
Nebulae, spiral, 30, 236
Nebular hypothesis of Laplace, 230, 231, 232 ff.
Neptune, 16, 17, 18, 19
birth of, 240, 243, 244
rotation of, 225
satellite of, 240, 243
New stars (novae), 61, 351
Newton, Sir Isaac, (cosmogony), 197, 203
(distances of stars), 33
(law of gravitation), 44, 45, 46, 161, 197
(light and optics), 50, 105
Nicholas of Cusa, 3
Nishina, Y., 146, 148
Novae, 61, 351
Nucleus, atomic, _see_ Atomic
ο Ceti, 58, 272, 280
ο₂ Eridani, 268, 278, 280
_Ο_-type stars, 51, 321, 323
Obscuring matter in galactic system, 24, 30
Oort, J. H., 67, 84, 214
Opacity of stellar matter, 309
Orbit, of binary systems, 47, 167 ff., 224 ff.
of earth, 34, 36, 227
of moon, 34, 43, 226, 250
of planets, 45, 46, 237, 239, 245
of stars, 211, 213, 216
Oresme, 2
Orion, constellation of, 32, 39, 173
Paneth, F., 247
Parallactic measures of stellar distances, 33, 61, 63
Parallactic motion, defined, 33
Parallax, spectroscopic, 62, 303
Pearce’s star (H.D. 1337), 49, 54, 317
Period-luminosity law (Cepheid variables), 59-61
Perrin, J., 159, 184
Perseid shooting-stars, 251
Phases of moon and inner planets, 4, 5
Philolaus, 2
Phosphoros, 17
Photo-electric action, 128
Photography in astronomy, 37
Piazzi, G., 17
Planck, M., 123, 124, 126, 255
“Planet X,” 19
Planetary motions, 36, 45, 46, 78
Planetary nebulae, 29, 273, 321, 323 and Plate II (p. 28)
Planetary orbits, 45, 46, 237, 245
Planetesimal Hypothesis, 235, 236
Planets, arrangement of, 16, 19, 240
birth of (Laplace), 231, 234
birth of (Tidal Theory), 237, 240, 244
in the universe, 341, 349
motions and orbits of, 36, 45, 46, 78, 237, 239, 245
Plaskett, J. S., 67, 214
Plaskett’s star (B.D. 6° 1309), 49, 55, 269, 270, 272
Pleiades, 32, 39, 40, 173, 216
Plummer, H. C., 223
Pluto, 16, 18, 19, 20, 239, 240, 241, 243
birth of, 240, 243
temperature of, 20
Poincaré, H., 66, 285, 338
Poincaré’s theorem, 285, 291
Pressure in a gas, 97
in a star, 306
of radiation, 120
of radiation in a star, 290, 306
Primaeval chaos, evolution from, 202, 205, 229, 230
Proctor, R. A., 235
Procyon, 48, 266, 278, 279, 280
Proton, 119, 121
Proxima Centauri, 36, 40, 264, 278
Ptolemy, 2, 3, 4, 5, 6, 8, 31
Pythagoras, 2, 3, 5, 13
Quantum, defined, 126
Quantum theory, 122 ff., 137 ff., 286, 330, 331
Quotations, Arnold, Matthew, 5
Bede, 7
Bruno, Giordano, 3
Cornford, Frances, 11
Cusa, Nicholas of, 3
Galileo, 5
Meredith, George, 283
Newton, Sir Isaac, 197
Sackville-West, Victoria, 7
Shakespeare, 193
Radiation, 114 ff., 137 ff., 254 ff., 307
distribution by wave-length, 253
highly penetrating, 143, 144, 191, 298, 299
mechanical effects of, 120, 137, 290, 306
of sun, 55, 175, 256
pressure of, 120;
in a star, 290, 306
visible and invisible, 258, 259
wave-length of, 122, 255, 258
weight of, 120, 122, 136
Radio-activity, 110 ff., 153, 298
Rainbow, 50, 51, 105
Rayleigh, Lord, 94
Redman, H. O., 277, 349
Relativity, theory of, 74 ff., 186, 261, 323, 337
Ring nebula in Lyra, 29, 274 and Plate II (p. 28)
Rings of electrons, in atom, 134, 138, 289, 291 ff., 302, 304, 305
Roche, E., 245, 249
Roche’s limit, 246, 250, 251, 252
Rosse, Lord, 30
Rotating masses, of gas, 206, 221
of liquid, 219, 221
Rotating nebulae, 205
Rotating systems of stars, 168, 213
Rotation, of astronomical bodies, 168, 216
of galaxy, 66 ff., 214
of nebulae, 205
of stars, 217
Royds, T., 112
Russell, H. N., 187, 229, 276, 293, 299, 314
Russell diagram, 276, 277, 278, 282, 283, 291,
295, 302, 303, 314, 338
Rutherford, Sir E., 107, 111, 112, 117, 143, 154
_S_ Doradus, 182, 269, 270
St John, C. E., 343, 344
Salinity of oceans, 151
Sampson, R. A., 17, 308
Satellites, atmospheres on, 196
birth of (Laplace), 231
birth of (Tidal Theory), 241, 243, 246
discovery of, 23
Saturn, 16, 17, 18, 19, 32, 45
birth of, 240, 242, 243
rings of, 226, 231, 246, 247, 250
rotation of, 225
satellites of, 23, 196, 240, 241 ff., 250
temperature of, 21
Schrödinger, E., 305
Schwarzschild, K., 308
Seares, F. H., 26, 163, 281, 282
Sedimentation, 151
Shapley, H., 26, 59, 63, 65, 213, 216, 223, 247
Shooting-stars, nature of, 176, 247
origin of, 238, 247
swarms of, 247, 248, 251
Sirius, 36, 48, 259, 278, 279, 280, 316
system of, 265
Sirius _B_, 48, 262, 280, 292
Soddy, F., 111
Solar spectrum, 51, 105, 125, 256
Solar system, arrangement of, Bode’s law, 19
arrangement of (Copernican), 3, 5, 6, 27, 32
arrangement of (Ptolemaic), 2, 4, 6, 8, 31
atmospheres in, 194 ff., 343, 344, 347
orbits in, 45, 46, 224 ff., 238
origin of, 230 ff.
Sound, 96, 198
Spectra, stellar, 50, 125, 130 and Plate VIII (p. 51).
_See also_ Displacements
Spectral types, 51
relation to stellar weights, 164
relation to surface-temperatures, 257
Spectroscope, 50
Spectroscopic binaries, 52
orbits of, 171, 172
origin of, 169, 222, 223
Spectroscopic parallaxes, 62, 303
Spectroscopic velocities, 50
Spectroscopy, 105
Spectrum, 50
of sun, 50, 105, 125, 256
Spherical nebulae, 207, 218, 219
Spiral nebulae, 30, 236
arms of, 236
Stability of stellar structures, 295, 296, 300 ff.
Stars, ages of, 84, 155, 167, 173, 174, 186
arrangement of, 24, 65
birth of, 209
density of, 263 ff., 273, 282, 292
diameters of, 260 ff., 277, 278, 321
distances of, 31, 35, 37
evolution of, 313 ff.
internal constitution of, 285 ff.
internal temperatures of, 286
liquid, 222, 296 ff., 298, 301, 310
luminosity of, 49, 179, 180, 269
motions of, 36, 50, 66 ff., 164, 212, 213
number of, 68, 69, 88
spectra of, 51, 125, 130 and Plate VIII (p. 51).
_See also_ Displacements
stability of, 295, 296, 300 ff.
surface-temperatures of, 254, 257, 263 ff., 270, 271
variable, 55 ff., 223
weights of, 46, 48, 49, 164, 179, 268, 280 ff.
Star-streaming, 212, 214
Statistical methods in astronomy, 212, 213
Stellar, _see_ Stars
Struve, Otto, 223
Struve, W., 33
Sun, age of, 181, 184
candle-power of, 55
distance from earth, 34, 227, 346
future of, 348, 350
internal constitution of, 286, 290
internal temperature of, 286, 288
loss of weight of, 178, 227, 346
past history of, 180
position in galactic system, 26, 66
radiation of, 55, 175, 189, 190
rotation of, 168, 205, 245
surface-temperature of, 256
Surface-temperatures of planets, 21 ff., 346, 347
Surface-temperatures of stars, 254, 257, 263 ff., 270, 271
of sun, 256
Taylor’s comet, 251
Telescope, 1
Galileo’s, 2, 13
Herschels’, 25
Lord Rosse’s, 30
100-inch, 2, 20, 68, 72, 73
200-inch, 2, 73
Temperature, scale of, 102
Temperature-radiation, 140, 255
Thermodynamics, 326 ff.
first and second laws of, 326, 328
Thomson, Sir J. J., 106
Tidal friction, 224 ff.
Tidal theory of solar system, 236 ff.
Tides in binary stars, 224, 226
Titan (satellite of Saturn), 23, 196
Transit of Venus, 34
Transport of energy in a star, 307
Ultra-violet radiation, 116, 143, 144
Universe, age of, 334, 336
apparent expansion of, 84, 86 ff.
beginning of, 77, 332 ff.
final end of, 329 ff.
size of, 77, 84, 87
structure of, 74 ff., 87
Uranium atom, 110
disintegration of, 116, 121, 153
Uranus, 16, 17, 18, 19, 20
birth of, 240, 243, 244
rotation of, 225
satellites of, 23, 240, 243, 244
temperature of, 21
_V_ Puppis, 278, 317
van Maanen, A., 273
van Maanen’s star, 268, 274, 278, 292
van Rhijn, P. J., 26
Variable stars, 55 ff., 222, 271, 319
Vega (α Lyrae), 33
Velocities of stars, 36, 68, 163 ff.
Venus, 4, 5, 16, 17, 19, 34
absence of satellites, 243
atmosphere on, 197, 344
birth of, 240, 243
life on, 344, 347
phases of, 4, 5
rotation of, 224
temperature of, 22, 347
transit of, 34
Virgo, nebula N.G.C. 4594 in, 71, 72, 204, 208
and Plate XV (p. 204)
Visual binaries, orbits of, 170, 171
origin of, 169, 284
Wave-length of radiation, 115, 122
Wave-mechanics, 136, 305
White dwarf stars, 280, 283, 288, 317, 318,
319, 320, 323, 348, 349
Wilson, C. T. R., 113, 117
Wireless transmission, 115
Wolf 359, 36, 264, 269, 270
X-rays, 116, 138, 144, 307, 311
Zero (absolute) of temperature, 102
Zodiacal light, 238
Zwicky, F., 87
CAMBRIDGE: PRINTED BY
W. LEWIS, M.A.
AT THE UNIVERSITY PRESS