Sunday, May 10, 2009

The Outline of Science, Vol. 1 by J. Arthur Thomson




§ 1

The story of the triumphs of modern science naturally opens with
Astronomy. The picture of the Universe which the astronomer offers to us
is imperfect; the lines he traces are often faint and uncertain. There
are many problems which have been solved, there are just as many about
which there is doubt, and notwithstanding our great increase in
knowledge, there remain just as many which are entirely unsolved.

The problem of the structure and duration of the universe [said the
great astronomer Simon Newcomb] is the most far-reaching with which
the mind has to deal. Its solution may be regarded as the ultimate
object of stellar astronomy, the possibility of reaching which has
occupied the minds of thinkers since the beginning of civilisation.
Before our time the problem could be considered only from the
imaginative or the speculative point of view. Although we can to-day
attack it to a limited extent by scientific methods, it must be
admitted that we have scarcely taken more than the first step toward
the actual solution.... What is the duration of the universe in
time? Is it fitted to last for ever in its present form, or does it
contain within itself the seeds of dissolution? Must it, in the
course of time, in we know not how many millions of ages, be
transformed into something very different from what it now is? This
question is intimately associated with the question whether the
stars form a system. If they do, we may suppose that system to be
permanent in its general features; if not, we must look further for
our conclusions.

The Heavenly Bodies

The heavenly bodies fall into two very distinct classes so far as their
relation to our Earth is concerned; the one class, a very small one,
comprises a sort of colony of which the Earth is a member. These bodies
are called _planets_, or wanderers. There are eight of them, including
the Earth, and they all circle round the sun. Their names, in the order
of their distance from the sun, are Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, Neptune, and of these Mercury, the nearest to
the sun, is rarely seen by the naked eye. Uranus is practically
invisible, and Neptune quite so. These eight planets, together with the
sun, constitute, as we have said, a sort of little colony; this colony
is called the Solar System.

The second class of heavenly bodies are those which lie _outside_ the
solar system. Every one of those glittering points we see on a starlit
night is at an immensely greater distance from us than is any member of
the Solar System. Yet the members of this little colony of ours, judged
by terrestrial standards, are at enormous distances from one another. If
a shell were shot in a straight line from one side of Neptune's orbit to
the other it would take five hundred years to complete its journey. Yet
this distance, the greatest in the Solar System as now known (excepting
the far swing of some of the comets), is insignificant compared to the
distances of the stars. One of the nearest stars to the earth that we
know of is Alpha Centauri, estimated to be some twenty-five million
millions of miles away. Sirius, the brightest star in the firmament, is
double this distance from the earth.

We must imagine the colony of planets to which we belong as a compact
little family swimming in an immense void. At distances which would take
our shell, not hundreds, but millions of years to traverse, we reach
the stars--or rather, a star, for the distances between stars are as
great as the distance between the nearest of them and our Sun. The
Earth, the planet on which we live, is a mighty globe bounded by a crust
of rock many miles in thickness; the great volumes of water which we
call our oceans lie in the deeper hollows of the crust. Above the
surface an ocean of invisible gas, the atmosphere, rises to a height of
about three hundred miles, getting thinner and thinner as it ascends.

[Illustration: LAPLACE

One of the greatest mathematical astronomers of all time and the
originator of the nebular theory.]

[Illustration: _Photo: Royal Astronomical Society._


who, anticipating the great French mathematician, Le Verrier, discovered
the planet Neptune by calculations based on the irregularities of the
orbit of Uranus. One of the most dramatic discoveries in the history of

[Illustration: _Photo: Elliott & Fry, Ltd._


Professor of Astronomy at Cambridge. The most famous of the English
disciples of Einstein.]



(Drawn approximately to scale)

The isolation of the Solar System is very great. On the above scale the
_nearest_ star (at a distance of 25 trillions of miles) would be over
_one half mile_ away. The hours, days, and years are the measures of
time as we use them; that is: Jupiter's "Day" (one rotation of the
planet) is made in ten of _our hours_; Mercury's "Year" (one revolution
of the planet around the Sun) is eighty-eight of _our days_. Mercury's
"Day" and "Year" are the same. This planet turns always the same side to
the Sun.]

approximately to scale)

On this scale the Sun would be 17-1/2 inches in diameter; it is far
greater than all the planets put together. Jupiter, in turn, is greater
than all the other planets put together.]

Except when the winds rise to a high speed, we seem to live in a very
tranquil world. At night, when the glare of the sun passes out of our
atmosphere, the stars and planets seem to move across the heavens with a
stately and solemn slowness. It was one of the first discoveries of
modern astronomy that this movement is only apparent. The apparent
creeping of the stars across the heavens at night is accounted for by
the fact that the earth turns upon its axis once in every twenty-four
hours. When we remember the size of the earth we see that this implies a
prodigious speed.

In addition to this the earth revolves round the sun at a speed of more
than a thousand miles a minute. Its path round the sun, year in year
out, measures about 580,000,000 miles. The earth is held closely to this
path by the gravitational pull of the sun, which has a mass 333,432
times that of the earth. If at any moment the sun ceased to exert this
pull the earth would instantly fly off into space straight in the
direction in which it was moving at the time, that is to say, at a
tangent. This tendency to fly off at a tangent is continuous. It is the
balance between it and the sun's pull which keeps the earth to her
almost circular orbit. In the same way the seven other planets are held
to their orbits.

Circling round the earth, in the same way as the earth circles round the
sun, is our moon. Sometimes the moon passes directly between us and the
sun, and cuts off the light from us. We then have a total or partial
eclipse of the sun. At other times the earth passes directly between the
sun and the moon, and causes an eclipse of the moon. The great ball of
the earth naturally trails a mighty shadow across space, and the moon is
"eclipsed" when it passes into this.

The other seven planets, five of which have moons of their own, circle
round the sun as the earth does. The sun's mass is immensely larger than
that of all the planets put together, and all of them would be drawn
into it and perish if they did not travel rapidly round it in gigantic
orbits. So the eight planets, spinning round on their axes, follow their
fixed paths round the sun. The planets are secondary bodies, but they
are most important, because they are the only globes in which there can
be life, as we know life.

If we could be transported in some magical way to an immense distance in
space above the sun, we should see our Solar System as it is drawn in
the accompanying diagram (Fig. 1), except that the planets would be mere
specks, faintly visible in the light which they receive from the sun.
(This diagram is drawn approximately to scale.) If we moved still
farther away, trillions of miles away, the planets would fade entirely
out of view, and the sun would shrink into a point of fire, a star. And
here you begin to realize the nature of the universe. _The sun is a
star. The stars are suns._ Our sun looks big simply because of its
comparative nearness to us. The universe is a stupendous collection of
millions of stars or suns, many of which may have planetary families
like ours.

§ 2

The Scale of the Universe

How many stars are there? A glance at a photograph of star-clouds will
tell at once that it is quite impossible to count them. The fine
photograph reproduced in Figure 2 represents a very small patch of that
pale-white belt, the Milky Way, which spans the sky at night. It is true
that this is a particularly rich area of the Milky Way, but the entire
belt of light has been resolved in this way into masses or clouds of
stars. Astronomers have counted the stars in typical districts here and
there, and from these partial counts we get some idea of the total
number of stars. There are estimated to be between two and three
thousand million stars.

Yet these stars are separated by inconceivable distances from each
other, and it is one of the greatest triumphs of modern astronomy to
have mastered, so far, the scale of the universe. For several centuries
astronomers have known the relative distances from each other of the sun
and the planets. If they could discover the actual distance of any one
planet from any other, they could at once tell all the distances within
the Solar System.

The sun is, on the latest measurements, at an average distance of
92,830,000 miles from the earth, for as the orbit of the earth is not a
true circle, this distance varies. This means that in six months from
now the earth will be right at the opposite side of its path round the
sun, or 185,000,000 miles away from where it is now. Viewed or
photographed from two positions so wide apart, the nearest stars show a
tiny "shift" against the background of the most distant stars, and that
is enough for the mathematician. He can calculate the distance of any
star near enough to show this "shift." We have found that the nearest
star to the earth, a recently discovered star, is twenty-five trillion
miles away. Only thirty stars are known to be within a hundred trillion
miles of us.

This way of measuring does not, however, take us very far away in the
heavens. There are only a few hundred stars within five hundred trillion
miles of the earth, and at that distance the "shift" of a star against
the background (parallax, the astronomer calls it) is so minute that
figures are very uncertain. At this point the astronomer takes up a new
method. He learns the different types of stars, and then he is able to
deduce more or less accurately the distance of a star of a known type
from its faintness. He, of course, has instruments for gauging their
light. As a result of twenty years work in this field, it is now known
that the more distant stars of the Milky Way are at least a hundred
thousand trillion (100,000,000,000,000,000) miles away from the sun.

Our sun is in a more or less central region of the universe, or a few
hundred trillion miles from the actual centre. The remainder of the
stars, which are all outside our Solar System, are spread out,
apparently, in an enormous disc-like collection, so vast that even a ray
of light, which travels at the rate of 186,000 miles a second, would
take 50,000 years to travel from one end of it to the other. This, then
is what we call our universe.

Are there other Universes?

Why do we say "our universe"? Why not _the_ universe? It is now believed
by many of our most distinguished astronomers that our colossal family
of stars is only one of many universes. By a universe an astronomer
means any collection of stars which are close enough to control each
other's movements by gravitation; and it is clear that there might be
many universes, in this sense, separated from each other by profound
abysses of space. Probably there are.

For a long time we have been familiar with certain strange objects in
the heavens which are called "spiral nebulæ" (Fig 4). We shall see at a
later stage what a nebula is, and we shall see that some astronomers
regard these spiral nebulæ as worlds "in the making." But some of the
most eminent astronomers believe that they are separate
universes--"island-universes" they call them--or great collections of
millions of stars like our universe. There are certain peculiarities in
the structure of the Milky Way which lead these astronomers to think
that our universe may be a spiral nebula, and that the other spiral
nebulæ are "other universes."

[Illustration: _Photo: Harvard College Observatory._


Note the cloud-like effect.]


The diagram shows the Moon partially eclipsed.]

[Illustration: _From a photograph taken at the Yerkes Observatory_


Vast as is the Solar System, then, it is excessively minute in
comparison with the Stellar System, the universe of the Stars, which is
on a scale far transcending anything the human mind can apprehend.



§ 1

But now let us turn to the Solar System, and consider the members of our
own little colony.

Within the Solar System there are a large number of problems that
interest us. What is the size, mass, and distance of each of the
planets? What satellites, like our Moon, do they possess? What are their
temperatures? And those other, sporadic members of our system, comets
and meteors, what are they? What are their movements? How do they
originate? And the Sun itself, what is its composition, what is the
source of its heat, how did it originate? Is it running down?

These last questions introduce us to a branch of astronomy which is
concerned with the physical constitution of the stars, a study which,
not so very many years ago, may well have appeared inconceivable. But
the spectroscope enables us to answer even these questions, and the
answer opens up questions of yet greater interest. We find that the
stars can be arranged in an order of development--that there are stars
at all stages of their life-history. The main lines of the evolution of
the stellar universe can be worked out. In the sun and stars we have
furnaces with temperatures enormously high; it is in such conditions
that substances are resolved into their simplest forms, and it is thus
we are enabled to obtain a knowledge of the most primitive forms of
matter. It is in this direction that the spectroscope (which we shall
refer to immediately) has helped us so much. It is to this wonderful
instrument that we owe our knowledge of the composition of the sun and
stars, as we shall see.

"That the spectroscope will detect the millionth of a milligram of
matter, and on that account has discovered new elements, commands
our admiration; but when we find in addition that it will detect the
nature of forms of matter trillions of miles away, and moreover,
that it will measure the velocities with which these forms of matter
are moving with an absurdly small per cent. of possible error, we
can easily acquiesce in the statement that it is the greatest
instrument ever devised by the brain and hand of man."

Such are some of the questions with which modern astronomy deals. To
answer them requires the employment of instruments of almost incredible
refinement and exactitude and also the full resources of mathematical
genius. Whether astronomy be judged from the point of view of the
phenomena studied, the vast masses, the immense distances, the æons of
time, or whether it be judged as a monument of human ingenuity,
patience, and the rarest type of genius, it is certainly one of the
grandest, as it is also one of the oldest, of the sciences.

The Solar System

In the Solar System we include all those bodies dependent on the sun
which circulate round it at various distances, deriving their light and
heat from the sun--the planets and their moons, certain comets and a
multitude of meteors: in other words, all bodies whose movements in
space are determined by the gravitational pull of the sun.

The Sun

Thanks to our wonderful modern instruments and the ingenious methods
used by astronomers, we have to-day a remarkable knowledge of the sun.

Look at the figure of the sun in the frontispiece. The picture
represents an eclipse of the sun; the dark body of the moon has screened
the sun's shining disc and taken the glare out of our eyes; we see a
silvery halo surrounding the great orb on every side. It is the sun's
atmosphere, or "crown" (corona), stretching for millions of miles into
space in the form of a soft silvery-looking light; probably much of its
light is sunlight reflected from particles of dust, although the
spectroscope shows an element in the corona that has not so far been
detected anywhere else in the universe and which in consequence has been
named Coronium.

We next notice in the illustration that at the base of the halo there
are red flames peeping out from the edges of the hidden disc. When one
remembers that the sun is 866,000 miles in diameter, one hardly needs to
be told that these flames are really gigantic. We shall see what they
are presently.

Regions of the Sun

The astronomer has divided the sun into definite concentric regions or
layers. These layers envelop the nucleus or central body of the sun
somewhat as the atmosphere envelops our earth. It is through these
vapour layers that the bright white body of the sun is seen. Of the
innermost region, the heart or nucleus of the sun, we know almost
nothing. The central body or nucleus is surrounded by a brilliantly
luminous envelope or layer of vaporous matter which is what we see when
we look at the sun and which the astronomer calls the photosphere.

Above--that is, overlying--the photosphere there is a second layer of
glowing gases, which is known as the reversing layer. This layer is
cooler than the underlying photosphere; it forms a veil of smoke-like
haze and is of from 500 to 1,000 miles in thickness.

A third layer or envelope immediately lying over the last one is the
region known as the chromosphere. The chromosphere extends from 5,000
to 10,000 miles in thickness--a "sea" of red tumultuous surging fire.
Chief among the glowing gases is the vapour of hydrogen. The intense
white heat of the photosphere beneath shines through this layer,
overpowering its brilliant redness. From the uppermost portion of the
chromosphere great fiery tongues of glowing hydrogen and calcium vapour
shoot out for many thousands of miles, driven outward by some prodigious
expulsive force. It is these red "prominences" which are such a notable
feature in the picture of the eclipse of the sun already referred to.

During the solar eclipse of 1919 one of these red flames rose in less
than seven hours from a height of 130,000 miles to more than 500,000
miles above the sun's surface. This immense column of red-hot gas, four
or five times the thickness of the earth, was soaring upward at the rate
of 60,000 miles an hour.

These flaming jets or prominences shooting out from the chromosphere are
not to be seen every day by the naked eye; the dazzling light of the sun
obscures them, gigantic as they are. They can be observed, however, by
the spectroscope any day, and they are visible to us for a very short
time during an eclipse of the sun. Some extraordinary outbursts have
been witnessed. Thus the late Professor Young described one on September
7, 1871, when he had been examining a prominence by the spectroscope:

It had remained unchanged since noon of the previous day--a long,
low, quiet-looking cloud, not very dense, or brilliant, or in any
way remarkable except for its size. At 12:30 p.m. the Professor left
the spectroscope for a short time, and on returning half an hour
later to his observations, he was astonished to find the gigantic
Sun flame shattered to pieces. The solar atmosphere was filled with
flying debris, and some of these portions reached a height of
100,000 miles above the solar surface. Moving with a velocity which,
even at the distance of 93,000,000 miles, was almost perceptible to
the eye, these fragments doubled their height in ten minutes. On
January 30, 1885, another distinguished solar observer, the late
Professor Tacchini of Rome, observed one of the greatest prominences
ever seen by man. Its height was no less than 142,000
miles--eighteen times the diameter of the earth. Another mighty
flame was so vast that supposing the eight large planets of the
solar system ranged one on top of the other, the prominence would
still tower above them.[1]

[1] _The Romance of Astronomy_, by H. Macpherson.


Compare with frontispiece.]

[Illustration: _Photo: Royal Observatory, Greenwich._


The small Corona is also visible.]


A photograph taken at the Mount Wilson Observatory of the Carnegie
Institution at Washington.]

[Illustration: FIG. 8.--THE SUN

Photographed in the light of glowing hydrogen, at the Mount Wilson
Observatory of the Carnegie Institution of Washington: vortex phenomena
near the spots are especially prominent.]

The fourth and uppermost layer or region is that of the corona, of
immense extent and fading away into the surrounding sky--this we have
already referred to. The diagram (Fig. 5) shows the dispositions of
these various layers of the sun. It is through these several transparent
layers that we see the white light body of the sun.

§ 2

The Surface of the Sun

Here let us return to and see what more we know about the
photosphere--the sun's surface. It is from the photosphere that we have
gained most of our knowledge of the composition of the sun, which is
believed not to be a solid body. Examination of the photosphere shows
that the outer surface is never at rest. Small bright cloudlets come and
go in rapid succession, giving the surface, through contrasts in
luminosity, a granular appearance. Of course, to be visible at all at
92,830,000 miles the cloudlets cannot be small. They imply enormous
activity in the photosphere. If we might speak picturesquely the sun's
surface resembles a boiling ocean of white-hot metal vapours. We have
to-day a wonderful instrument, which will be described later, which
dilutes, as it were, the general glare of the sun, and enables us to
observe these fiery eruptions at any hour. The "oceans" of red-hot gas
and white-hot metal vapour at the sun's surface are constantly driven by
great storms. Some unimaginable energy streams out from the body or
muscles of the sun and blows its outer layers into gigantic shreds, as
it were.

The actual temperature at the sun's surface, or what appears to us to be
the surface--the photosphere--is, of course, unknown, but careful
calculation suggests that it is from 5,000° C. to 7,000° C. The interior
is vastly hotter. We can form no conception of such temperatures as must
exist there. Not even the most obdurate solid could resist such
temperatures, but would be converted almost instantaneously into gas.
But it would not be gas as we know gases on the earth. The enormous
pressures that exist on the sun must convert even gases into thick
treacly fluids. We can only infer this state of matter. It is beyond our
power to reproduce it.


It is in the brilliant photosphere that the dark areas known as
sun-spots appear. Some of these dark spots--they are dark only by
contrast with the photosphere surrounding them--are of enormous size,
covering many thousands of square miles of surface. What they are we
cannot positively say. They look like great cavities in the sun's
surface. Some think they are giant whirlpools. Certainly they seem to be
great whirling streams of glowing gases with vapours above them and
immense upward and downward currents within them. Round the edges of the
sun-spots rise great tongues of flame.

Perhaps the most popularly known fact about sun-spots is that they are
somehow connected with what we call magnetic storms on earth. These
magnetic storms manifest themselves in interruptions of our telegraphic
and telephonic communications, in violent disturbances of the mariner's
compass, and in exceptional auroral displays. The connection between the
two sets of phenomena cannot be doubted, even although at times there
may be a great spot on the sun without any corresponding "magnetic
storm" effects on the earth.

A surprising fact about sun-spots is that they show definite periodic
variations in number. The best-defined period is one of about eleven
years. During this period the spots increase to a maximum in number and
then diminish to a minimum, the variation being more or less regular.
Now this can only mean one thing. To be periodic the spots must have
some deep-seated connection with the fundamental facts of the sun's
structure and activities. Looked at from this point of view their
importance becomes great.

[Illustration: _Reproduction from "The Forces of Nature"_ (_Messrs.


The aurora borealis is one of the most beautiful spectacles in the sky.
The colours and shape change every instant; sometimes a fan-like cluster
of rays, at other times long golden draperies gliding one over the
other. Blue, green, yellow, red, and white combine to give a glorious
display of colour. The theory of its origin is still, in part, obscure,
but there can be no doubt that the aurora is related to the magnetic
phenomena of the earth and therefore is connected with the electrical
influence of the sun.]

It is from the study of sun-spots that we have learned that the sun's
surface does not appear to rotate all at the same speed. The
"equatorial" regions are rotating quicker than regions farther north or
south. A point forty-five degrees from the equator seems to take about
two and a half days longer to complete one rotation than a point on the
equator. This, of course, confirms our belief that the sun cannot be a
solid body.

What is its composition? We know that there are present, in a gaseous
state, such well-known elements as sodium, iron, copper, zinc, and
magnesium; indeed, we know that there is practically every element in
the sun that we know to be in the earth. How do we know?

It is from the photosphere, as has been said, that we have won most of
our knowledge of the sun. The instrument used for this purpose is the
spectroscope; and before proceeding to deal further with the sun and the
source of its energy it will be better to describe this instrument.


The spectroscope is an instrument for analysing light. So important is
it in the revelations it has given us that it will be best to describe
it fully. Every substance to be examined must first be made to glow,
made luminous; and as nearly everything in the heavens _is_ luminous the
instrument has a great range in Astronomy. And when we speak of
analysing light, we mean that the light may be broken up into waves of
different lengths. What we call light is a series of minute waves in
ether, and these waves are--measuring them from crest to crest, so to
say--of various lengths. Each wave-length corresponds to a colour of the
rainbow. The shortest waves give us a sensation of violet colour, and
the largest waves cause a sensation of red. The rainbow, in fact, is a
sort of natural spectrum. (The meaning of the rainbow is that the
moisture-laden air has sorted out these waves, in the sun's light,
according to their length.) Now the simplest form of spectroscope is a
glass prism--a triangular-shaped piece of glass. If white light
(sunlight, for example) passes through a glass prism, we see a series of
rainbow-tinted colours. Anyone can notice this effect when sunlight is
shining through any kind of cut glass--the stopper of a wine decanter,
for instance. If, instead of catching with the eye the coloured lights
as they emerge from the glass prism, we allow them to fall on a screen,
we shall find that they pass, by continuous gradations, from red at the
one end of the screen, through orange, yellow, green, blue, and indigo,
to violet at the other end. _In other words, what we call white light is
composed of rays of these several colours. They go to make up the effect
which we call white._ And now just as water can be split up into its two
elements, oxygen and hydrogen, so sunlight can be broken up into its
primary colours, which are those we have just mentioned.

This range of colours, produced by the spectroscope, we call the solar
spectrum, and these are, from the spectroscopic point of view, primary
colours. Each shade of colour has its definite position in the spectrum.
That is to say, the light of each shade of colour (corresponding to its
wave-length) is reflected through a certain fixed angle on passing
through the glass prism. Every possible kind of light has its definite
position, and is denoted by a number which gives the wave-length of the
vibrations constituting that particular kind of light.

Now, other kinds of light besides sunlight can be analysed. Light
from any substance which has been made incandescent may be observed with
the spectroscope in the same way, and each element can be thus
separated. It is found that each substance (in the same conditions of
pressure, etc.) gives a constant spectrum of its own. _Each metal
displays its own distinctive colour. It is obvious, therefore, that the
spectrum provides the means for identifying a particular substance._ It
was by this method that we discovered in the sun the presence of such
well-known elements as sodium, iron, copper, zinc, and magnesium.

[Illustration: _Yerkes Observatory._


[Illustration: _From photographs taken at the Yerkes Observatory._


These are about 60,000 miles in height. The two photographs show the
vast changes occurring in ten minutes. October 10, 1910.]

[Illustration: _Photo: Mount Wilson Observatory._

FIG. 11.--MARS, October 5, 1909

Showing the dark markings and the Polar Cap.]

[Illustration: FIG. 12.--JUPITER

Showing the belts which are probably cloud formations.]

[Illustration: _Photo: Professor E. E. Barnard, Yerkes Observatory._

FIG. 13.--SATURN, November 19, 1911

Showing the rings, mighty swarms of meteorites.]

Every chemical element known, then, has a distinctive spectrum of its
own when it is raised to incandescence, and this distinctive spectrum is
as reliable a means of identification for the element as a human face is
for its owner. Whether it is a substance glowing in the laboratory or in
a remote star makes no difference to the spectroscope; if the light of
any substance reaches it, that substance will be recognised and
identified by the characteristic set of waves.

The spectrum of a glowing mass of gas will consist in a number of bright
lines of various colours, and at various intervals; corresponding to
each kind of gas, there will be a peculiar and distinctive arrangement
of bright lines. But if the light from such a mass of glowing gas be
made to pass through a cool mass of the _same_ gas it will be found that
dark lines replace the bright lines in the spectrum, the reason for this
being that the cool gas absorbs the rays of light emitted by the hot
gas. Experiments of this kind enable us to reach the important general
statement that every gas, when cold, absorbs the same rays of light
which it emits when hot.

Crossing the solar spectrum are hundreds and hundreds of dark lines.
These could not at first be explained, because this fact of
discriminative absorption was not known. We understand now. The sun's
white light comes from the photosphere, but between us and the
photosphere there is, as we have seen, another solar envelope of
relatively cooler vapours--the reversing layer. Each constituent
element in this outer envelope stops its own kind of light, that is, the
kind of light made by incandescent atoms of the same element in the
photosphere. The "stoppages" register themselves in the solar spectrum
as dark lines placed exactly where the corresponding bright lines would
have been. The explanation once attained, dark lines became as
significant as bright lines. The secret of the sun's composition was
out. We have found practically every element in the sun that we know to
be in the earth. We have identified an element in the sun before we were
able to isolate it on the earth. We have been able even to point to the
coolest places on the sun, the centres of sun-spots, where alone the
temperature seems to have fallen sufficiently low to allow chemical
compounds to form.

It is thus we have been able to determine what the stars, comets, or
nebulæ are made of.

A Unique Discovery

In 1868 Sir Norman Lockyer detected a light coming from the prominences
of the sun which was not given by any substance known on earth, and
attributed this to an unknown gas which he called helium, from the Greek
_helios_, the sun. _In 1895 Sir William Ramsay discovered in certain
minerals the same gas identified by the spectroscope._ We can say,
therefore, that this gas was discovered in the sun nearly thirty years
before it was found on earth; this discovery of the long-lost heir is as
thrilling a chapter in the detective story of science as any in the
sensational stories of the day, and makes us feel quite certain that our
methods really tell us of what elements sun and stars are built up. The
light from the corona of the sun, as we have mentioned indicates a gas
still unknown on earth, which has been christened Coronium.

Measuring the Speed of Light

But this is not all; soon a new use was found for the spectroscope. We
found that we could measure with it the most difficult of all speeds
to measure, speed in the line of sight. Movement at right angles to the
direction in which one is looking is, if there is sufficient of it, easy
to detect, and, if the distance of the moving body is known, easy to
measure. But movement in the line of vision is both difficult to detect
and difficult to measure. Yet, even at the enormous distances with which
astronomers have to deal, the spectroscope can detect such movement and
furnish data for its measurement. If a luminous body containing, say,
sodium is moving rapidly towards the spectroscope, it will be found that
the sodium lines in the spectrum have moved slightly from their usual
definite positions towards the violet end of the spectrum, the amount of
the change of position increasing with the speed of the luminous body.
If the body is moving away from the spectroscope the shifting of the
spectral lines will be in the opposite direction, towards the red end of
the spectrum. In this way we have discovered and measured movements that
otherwise would probably not have revealed themselves unmistakably to us
for thousands of years. In the same way we have watched, and measured
the speed of, tremendous movements on the sun, and so gained proof that
the vast disturbances we should expect there actually do occur.


This pictorial diagram illustrates the principal of Spectrum Analysis,
showing how sunlight is decomposed into its primary colours. What we
call white light is composed of seven different colours. The diagram is
relieved of all detail which would unduly obscure the simple process by
which a ray of light is broken up by a prism into different
wave-lengths. The spectrum rays have been greatly magnified.]


§ 3

Now let us return to our consideration of the sun.

To us on the earth the most patent and most astonishing fact about the
sun is its tremendous energy. Heat and light in amazing quantities pour
from it without ceasing.

Where does this energy come from? Enormous jets of red glowing gases can
be seen shooting outwards from the sun, like flames from a fire, for
thousands of miles. Does this argue fire, as we know fire on the earth?
On this point the scientist is sure. The sun is not burning, and
combustion is not the source of its heat. Combustion is a chemical
reaction between atoms. The conditions that make it possible are known
and the results are predictable and measurable. But no chemical reaction
of the nature of combustion as we know it will explain the sun's energy,
nor indeed will any ordinary chemical reaction of any kind. If the sun
were composed of combustible material throughout and the conditions of
combustion as we understand them were always present, the sun would burn
itself out in some thousands of years, with marked changes in its heat
and light production as the process advanced. There is no evidence of
such changes. There is, instead, strong evidence that the sun has been
emitting light and heat in prodigious quantities, not for thousands, but
for millions of years. Every addition to our knowledge that throws light
on the sun's age seems to make for increase rather than decrease of its
years. This makes the wonder of its energy greater.

And we cannot avoid the issue of the source of the energy by saying
merely that the sun is gradually radiating away an energy that
originated in some unknown manner, away back at the beginning of things.
Reliable calculations show that the years required for the mere cooling
of a globe like the sun could not possibly run to millions. In other
words, the sun's energy must be subject to continuous and more or less
steady renewal. However it may have acquired its enormous energy in the
past, it must have some source of energy in the present.

The best explanation that we have to-day of this continuous accretion of
energy is that it is due to shrinkage of the sun's bulk under the force
of gravity. Gravity is one of the most mysterious forces of nature, but
it is an obvious fact that bodies behave as if they attracted one
another, and Newton worked out the law of this attraction. We may say,
without trying to go too deeply into things, that every particle of
matter attracts every other throughout the universe. If the diameter of
the sun were to shrink by one mile all round, this would mean that all
the millions of tons in the outer one-mile thickness would have a
straight drop of one mile towards the centre. And that is not all,
because obviously the layers below this outer mile would also drop
inwards, each to a less degree than the one above it. What a tremendous
movement of matter, however slowly it might take place! And what a
tremendous energy would be involved! Astronomers calculate that the
above shrinkage of one mile all round would require fifty years for its
completion, assuming, reasonably, that there is close and continuous
relationship between loss of heat by radiation and shrinkage. Even if
this were true we need not feel over-anxious on this theory; before the
sun became too cold to support life many millions of years would be

It was suggested at one time that falls of meteoric matter into the sun
would account for the sun's heat. This position is hardly tenable now.
The mere bulk of the meteoric matter required by the hypothesis, apart
from other reasons, is against it. There is undoubtedly an enormous
amount of meteoric matter moving about within the bounds of the solar
system, but most of it seems to be following definite routes round the
sun like the planets. The stray erratic quantities destined to meet
their doom by collision with the sun can hardly be sufficient to account
for the sun's heat.

Recent study of radio-active bodies has suggested another factor that
may be working powerfully along with the force of gravitation to
maintain the sun's store of heat. In radio-active bodies certain atoms
seem to be undergoing disintegration. These atoms appear to be splitting
up into very minute and primitive constituents. But since matter may be
split up into such constituents, may it not be built up from them?

The question is whether these "radio-active" elements are undergoing
disintegration, or formation, in the sun. If they are undergoing
disintegration--and the sun itself is undoubtedly radio-active--then we
have another source of heat for the sun that will last indefinitely.



§ 1

It is quite clear that there cannot be life on the stars. Nothing solid
or even liquid can exist in such furnaces as they are. Life exists only
on planets, and even on these its possibilities are limited. Whether all
the stars, or how many of them, have planetary families like our sun, we
cannot positively say. If they have, such planets would be too faint and
small to be visible tens of trillions of miles away. Some astronomers
think that our sun may be exceptional in having planets, but their
reasons are speculative and unconvincing. Probably a large proportion at
least of the stars have planets, and we may therefore survey the globes
of our own solar system and in a general way extend the results to the
rest of the universe.

In considering the possibility of life as we know it we may at once rule
out the most distant planets from the sun, Uranus and Neptune. They are
probably intrinsically too hot. We may also pass over the nearest planet
to the sun, Mercury. We have reason to believe that it turns on its axis
in the same period as it revolves round the sun, and it must therefore
always present the same side to the sun. This means that the heat on the
sunlit side of Mercury is above boiling-point, while the cold on the
other side must be between two and three hundred degrees below

The Planet Venus

The planet Venus, the bright globe which is known to all as the morning
and evening "star," seems at first sight more promising as regards the
possibility of life. It is of nearly the same size as the earth, and it
has a good atmosphere, but there are many astronomers who believe that,
like Mercury, it always presents the same face to the sun, and it would
therefore have the same disadvantage--a broiling heat on the sunny side
and the cold of space on the opposite side. We are not sure. The
surface of Venus is so bright--the light of the sun is reflected to us
by such dense masses of cloud and dust--that it is difficult to trace
any permanent markings on it, and thus ascertain how long it takes to
rotate on its axis. Many astronomers believe that they have succeeded,
and that the planet always turns the same face to the sun. If it does,
we can hardly conceive of life on its surface, in spite of the

[Illustration: FIG. 14.--THE MOON

Showing a great plain and some typical craters. There are thousands of
these craters, and some theories of their origin are explained on page

[Illustration: FIG. 15.--MARS

1} Drawings by Prof. Lowell to accompany actual photographs of Mars
showing many of the
2} canals. Taken in 1907 by Mr. E. C. Slipher of the Lowell Observatory.
3 Drawing by Prof. Lowell made January 6, 1914.
4 Drawing by Prof. Lowell made January 21, 1914.

Nos. 1 and 2 show the effect of the planet's rotation. Nos. 3 and 4
depict quite different sections. Note the change in the polar snow-caps
in the last two.]


Note the mysterious "rays" diverging from the almost perfectly circular
craters indicated by the arrows (Tycho, upper; Copernicus, lower), and
also the mountains to the right with the lunar dawn breaking on them.]

We turn to Mars; and we must first make it clear why there is so much
speculation about life on Mars, and why it is supposed that, if there
_is_ life on Mars, it must be more advanced than life on the earth.

Is there Life on Mars?

The basis of this belief is that if, as we saw, all the globes in our
solar system are masses of metal that are cooling down, the smaller will
have cooled down before the larger, and will be further ahead in their
development. Now Mars is very much smaller than the earth, and must have
cooled at its surface millions of years before the earth did. Hence, if
a story of life began on Mars at all, it began long before the story of
life on the earth. We cannot guess what sort of life-forms would be
evolved in a different world, but we can confidently say that they would
tend toward increasing intelligence; and thus we are disposed to look
for highly intelligent beings on Mars.

But this argument supposes that the conditions of life, namely air and
water, are found on Mars, and it is disputed whether they are found
there in sufficient quantity. The late Professor Percival Lowell, who
made a lifelong study of Mars, maintained that there are hundreds of
straight lines drawn across the surface of the planet, and he claimed
that they are beds of vegetation marking the sites of great channels or
pipes by means of which the "Martians" draw water from their polar
ocean. Professor W. H. Pickering, another high authority, thinks that
the lines are long, narrow marshes fed by moist winds from the poles.
There are certainly white polar caps on Mars. They seem to melt in the
spring, and the dark fringe round them grows broader.

Other astronomers, however, say that they find no trace of water-vapour
in the atmosphere of Mars, and they think that the polar caps may be
simply thin sheets of hoar-frost or frozen gas. They point out that, as
the atmosphere of Mars is certainly scanty, and the distance from the
sun is so great, it may be too cold for the fluid water to exist on the

If one asks why our wonderful instruments cannot settle these points,
one must be reminded that Mars is never nearer than 34,000,000 miles
from the earth, and only approaches to this distance once in fifteen or
seventeen years. The image of Mars on the photographic negative taken in
a big telescope is very small. Astronomers rely to a great extent on the
eye, which is more sensitive than the photographic plate. But it is easy
to have differences of opinion as to what the eye sees, and so there is
a good deal of controversy.

In August, 1924, the planet will again be well placed for observation,
and we may learn more about it. Already a few of the much-disputed
lines, which people wrongly call "canals," have been traced on
photographs. Astronomers who are sceptical about life on Mars are often
not fully aware of the extraordinary adaptability of life. There was a
time when the climate of the whole earth, from pole to pole, was
semi-tropical for millions of years. No animal could then endure the
least cold, yet now we have plenty of Arctic plants and animals. If the
cold came slowly on Mars, as we have reason to suppose, the population
could be gradually adapted to it. On the whole, it is possible that
there is advanced life on Mars, and it is not impossible, in spite of
the very great difficulties of a code of communication, that our "elder
brothers" may yet flash across space the solution of many of our

§ 2

Jupiter and Saturn

Next to Mars, going outward from the sun, is Jupiter. Between Mars and
Jupiter, however, there are more than three hundred million miles of
space, and the older astronomers wondered why this was not occupied by a
planet. We now know that it contains about nine hundred "planetoids," or
small globes of from five to five hundred miles in diameter. It was at
one time thought that a planet might have burst into these fragments (a
theory which is not mathematically satisfactory), or it may be that the
material which is scattered in them was prevented by the nearness of the
great bulk of Jupiter from uniting into one globe.

For Jupiter is a giant planet, and its gravitational influence must
extend far over space. It is 1,300 times as large as the earth, and has
nine moons, four of which are large, in attendance on it. It is
interesting to note that the outermost moons of Jupiter and Saturn
revolve round these planets in a direction contrary to the usual
direction taken by moons round planets, and by planets round the sun.
But there is no life on Jupiter.

The surface which we see in photographs (Fig. 12) is a mass of cloud or
steam which always envelops the body of the planet. It is apparently
red-hot. A red tinge is seen sometimes at the edges of its cloud-belts,
and a large red region (the "red spot"), 23,000 miles in length, has
been visible on it for half a century. There may be a liquid or solid
core to the planet, but as a whole it is a mass of seething vapours
whirling round on its axis once in every ten hours. As in the case of
the sun, however, different latitudes appear to rotate at different
rates. The interior of Jupiter is very hot, but the planet is not
self-luminous. The planets Venus and Jupiter shine very brightly, but
they have no light of their own; they reflect the sunlight.

Saturn is in the same interesting condition. The surface in the
photograph (Fig. 13) is steam, and Saturn is so far away from the sun
that the vaporisation of its oceans must necessarily be due to its own
internal heat. It is too hot for water to settle on its surface. Like
Jupiter, the great globe turns on its axis once in ten hours--a
prodigious speed--and must be a swirling, seething mass of metallic
vapours and gases. It is instructive to compare Jupiter and Saturn in
this respect with the sun. They are smaller globes and have cooled down
more than the central fire.

Saturn is a beautiful object in the telescope because it has ten moons
(to include one which is disputed) and a wonderful system of "rings"
round it. The so-called rings are a mighty swarm of meteorites--pieces
of iron and stone of all sorts and sizes, which reflect the light of the
sun to us. This ocean of matter is some miles deep, and stretches from a
few thousand miles from the surface of the planet to 172,000 miles out
in space. Some astronomers think that this is volcanic material which
has been shot out of the planet. Others regard it as stuff which would
have combined to form an eleventh moon but was prevented by the nearness
of Saturn itself. There is no evidence of life on Saturn.


Mars and Venus are therefore the only planets, besides the earth, on
which we may look for life; and in the case of Venus, the possibility is
very faint. But what about the moons which attend the planets? They
range in size from the little ten-miles-wide moons of Mars, to Titan, a
moon of Saturn, and Ganymede, a satellite of Jupiter, which are about
3,000 miles in diameter. May there not be life on some of the larger of
these moons? We will take our own moon as a type of the class.

A Dead World

The moon is so very much nearer to us than any other heavenly body that
we have a remarkable knowledge of it. In Fig. 14 you have a photograph,
taken in one of our largest telescopes, of part of its surface. In a
sense such a telescope brings the moon to within about fifty miles of
us. We should see a city like London as a dark, sprawling blotch on the
globe. We could just detect a Zeppelin or a Diplodocus as a moving speck
against the surface. But we find none of these things. It is true that a
few astronomers believe that they see signs of some sort of feeble life
or movement on the moon. Professor Pickering thinks that he can trace
some volcanic activity. He believes that there are areas of vegetation,
probably of a low order, and that the soil of the moon may retain a
certain amount of water in it. He speaks of a very thin atmosphere, and
of occasional light falls of snow. He has succeeded in persuading some
careful observers that there probably are slight changes of some kind
taking place on the moon.


The plains were originally supposed to be seas: hence the name "Mare."]

EARTH PASSING THROUGH THEM] [Illustration: _Photo: Royal Observatory,

FIG. 19.--COMET, September 29, 1908

Notice the tendency to form a number of tails. (See photograph below.)]

[Illustration: _Photo: Royal Observatory, Greenwich._

FIG. 20.--COMET, October 3, 1908

The process has gone further and a number of distinct tails can now be

But there are many things that point to absence of air on the moon. Even
the photographs we reproduce tell the same story. The edges of the
shadows are all hard and black. If there had been an appreciable
atmosphere it would have scattered the sun's light on to the edges and
produced a gradual shading off such as we see on the earth. This
relative absence of air must give rise to some surprising effects. There
will be no sounds on the moon, because sounds are merely air waves. Even
a meteor shattering itself to a violent end against the surface of the
moon would make no noise. Nor would it herald its coming by glowing into
a "shooting star," as it would on entering the earth's atmosphere. There
will be no floating dust, no scent, no twilight, no blue sky, no
twinkling of the stars. The sky will be always black and the stars will
be clearly visible by day as by night. The sun's wonderful corona, which
no man on earth, even by seizing every opportunity during eclipses, can
hope to see for more than two hours in all in a long lifetime, will be
visible all day. So will the great red flames of the sun. Of course,
there will be no life, and no landscape effects and scenery effects due
to vegetation.

The moon takes approximately twenty-seven of our days to turn once on
its axis. So for fourteen days there is continuous night, when the
temperature must sink away down towards the absolute cold of space. This
will be followed without an instant of twilight by full daylight. For
another fourteen days the sun's rays will bear straight down, with no
diffusion or absorption of their heat, or light, on the way. It does not
follow, however, that the temperature of the moon's surface must rise
enormously. It may not even rise to the temperature of melting ice.
Seeing there is no air there can be no check on radiation. The heat that
the moon gets will radiate away immediately. We know that amongst the
coldest places on the earth are the tops of very high mountains, the
points that have reared themselves nearest to the sun but farthest out
of the sheltering blanket of the earth's atmosphere. The actual
temperature of the moon's surface by day is a moot point. It may be
below the freezing-point or above the boiling-point of water.

The Mountains of the Moon

The lack of air is considered by many astronomers to furnish the
explanation of the enormous number of "craters" which pit the moon's
surface. There are about a hundred thousand of these strange rings, and
it is now believed by many that they are spots where very large
meteorites, or even planetoids, splashed into the moon when its surface
was still soft. Other astronomers think that they are the remains of
gigantic bubbles which were raised in the moon's "skin," when the globe
was still molten, by volcanic gases from below. A few astronomers think
that they are, as is popularly supposed, the craters of extinct
volcanoes. Our craters, on the earth, are generally deep cups, whereas
these ring-formations on the moon are more like very shallow and broad
saucers. Clavius, the largest of them, is 123 miles across the interior,
yet its encircling rampart is not a mile high.

The mountains on the moon (Fig. 16) rise to a great height, and are
extraordinarily gaunt and rugged. They are like fountains of lava,
rising in places to 26,000 and 27,000 feet. The lunar Apennines have
three thousand steep and weird peaks. Our terrestrial mountains are
continually worn down by frost acting on moisture and by ice and water,
but there are none of these agencies operating on the moon. Its
mountains are comparatively "everlasting hills."

The moon is interesting to us precisely because it is a dead world. It
seems to show how the earth, or any cooling metal globe, will evolve in
the remote future. We do not know if there was ever life on the moon,
but in any case it cannot have proceeded far in development. At the most
we can imagine some strange lowly forms of vegetation lingering here and
there in pools of heavy gas, expanding during the blaze of the sun's
long day, and frozen rigid during the long night.


We may conclude our survey of the solar system with a word about
"shooting stars," or meteors, and comets. There are few now who do not
know that the streak of fire which suddenly lights the sky overhead at
night means that a piece of stone or iron has entered our atmosphere
from outer space, and has been burned up by friction. It was travelling
at, perhaps, twenty or thirty miles a second. At seventy or eighty miles
above our heads it began to glow, as at that height the air is thick
enough to offer serious friction and raise it to a white heat. By the
time the meteor reached about twenty miles or so from the earth's
surface it was entirely dissipated, as a rule in fiery vapour.

Millions of Meteorites

It is estimated that between ten and a hundred million meteorites enter
our atmosphere and are cremated, every day. Most of them weigh only an
ounce or two, and are invisible. Some of them weigh a ton or more, but
even against these large masses the air acts as a kind of "torpedo-net."
They generally burst into fragments and fall without doing damage.

It is clear that "empty space" is, at least within the limits of our
solar system, full of these things. They swarm like fishes in the seas.
Like the fishes, moreover, they may be either solitary or gregarious.
The solitary bit of cosmic rubbish is the meteorite, which we have just
examined. A "social" group of meteorites is the essential part of a
comet. The nucleus, or bright central part, of the head of a comet (Fig.
19) consists of a swarm, sometimes thousands of miles wide, of these
pieces of iron or stone. This swarm has come under the sun's
gravitational influence, and is forced to travel round it. From some
dark region of space it has moved slowly into our system. It is not then
a comet, for it has no tail. But as the crowded meteors approach the
sun, the speed increases. They give off fine vapour-like matter and the
fierce flood of light from the sun sweeps this vapour out in an
ever-lengthening tail. Whatever way the comet is travelling, the tail
always points away from the sun.

A Great Comet

The vapoury tail often grows to an enormous length as the comet
approaches the sun. The great comet of 1843 had a tail two hundred
million miles long. It is, however, composed of the thinnest vapours
imaginable. Twice during the nineteenth century the earth passed through
the tail of a comet, and nothing was felt. The vapours of the tail are,
in fact, so attenuated that we can hardly imagine them to be white-hot.
They may be lit by some electrical force. However that may be, the comet
dashes round the sun, often at three or four hundred miles a second,
then may pass gradually out of our system once more. It may be a
thousand years, or it may be fifty years, before the monarch of the
system will summon it again to make its fiery journey round his throne.

[Illustration: _Photo: Harvard College Observatory._


Six main types of stellar spectra. Notice the lines they have in common,
showing what elements are met with in different types of stars. Each of
these spectra corresponds to a different set of physical and chemical
conditions.] [Illustration: _Photo: Mount Wilson Observatory._


Showing a great projection of "dark matter" cutting off the light from

[Illustration: _Photo: Astrophysical Observatory, Victoria, British


A wonderful cluster of stars. It has been estimated that the distance of
this cluster is such that it would take light more than 100,000 years to
reach us.]


§ 1

The immensity of the Stellar Universe, as we have seen, is beyond our
apprehension. The sun is nothing more than a very ordinary star, perhaps
an insignificant one. There are stars enormously greater than the sun.
One such, Betelgeux, has recently been measured, and its diameter is
more than 300 times that of the sun.

The Evolution of Stars

The proof of the similarity between our sun and the stars has come to us
through the spectroscope. The elements that we find by its means in the
sun are also found in the same way in the stars. Matter, says the
spectroscope, is essentially the same everywhere, in the earth and the
sun, in the comet that visits us once in a thousand years, in the star
whose distance is incalculable, and in the great clouds of "fire-mist"
that we call nebulæ.

In considering the evolution of the stars let us keep two points clearly
in mind. The starting-point, the nebula, is no figment of the scientific
imagination. Hundreds of thousands of nebulæ, besides even vaster
irregular stretches of nebulous matter, exist in the heavens. But the
stages of the evolution of this stuff into stars are very largely a
matter of speculation. Possibly there is more than one line of
evolution, and the various theories may be reconciled. And this applies
also to the theories of the various stages through which the stars
themselves pass on their way to extinction.

The light of about a quarter of a million stars has been analysed in the
spectroscope, and it is found that they fall into about a dozen classes
which generally correspond to stages in their evolution (Fig. 21).

The Age of Stars

In its main lines the spectrum of a star corresponds to its colour, and
we may roughly group the stars into red, yellow, and white. This is also
the order of increasing temperature, the red stars being the coolest and
the white stars the hottest. We might therefore imagine that the white
stars are the youngest, and that as they grow older and cooler they
become yellowish, then red, and finally become invisible--just as a
cooling white-hot iron would do. But a very interesting recent research
shows that there are two kinds of red stars; some of them are amongst
the oldest stars and some are amongst the youngest. The facts appear to
be that when a star is first formed it is not very hot. It is an immense
mass of diffuse gas glowing with a dull-red heat. It contracts under the
mutual gravitation of its particles, and as it does so it grows hotter.
It acquires a yellowish tinge. As it continues to contract it grows
hotter and hotter until its temperature reaches a maximum as a white
star. At this point the contraction process does not stop, but the
heating process does. Further contraction is now accompanied by cooling,
and the star goes through its colour changes again, but this time in the
inverse order. It contracts and cools to yellow and finally to red. But
when it again becomes a red star it is enormously denser and smaller
than when it began as a red star. Consequently the red stars are divided
into two classes called, appropriately, Giants and Dwarfs. This theory,
which we owe to an American astronomer, H. N. Russell, has been
successful in explaining a variety of phenomena, and there is
consequently good reason to suppose it to be true. But the question as
to how the red giant stars were formed has received less satisfactory
and precise answers.

The most commonly accepted theory is the nebular theory.


§ 2

Nebulæ are dim luminous cloud-like patches in the heavens, more like
wisps of smoke in some cases than anything else. Both photography and
the telescope show that they are very numerous, hundreds of thousands
being already known and the number being continually added to. They are
not small. Most of them are immensely large. Actual dimensions cannot be
given, because to estimate these we must first know definitely the
distance of the nebulæ from the earth. The distances of some nebulæ are
known approximately, and we can therefore form some idea of size in
these cases. The results are staggering. The mere visible surface of
some nebulæ is so large that the whole stretch of the solar system would
be too small to form a convenient unit for measuring it. A ray of light
would require to travel for years to cross from side to side of such a
nebula. Its immensity is inconceivable to the human mind.

There appear to be two types of nebulæ, and there is evidence suggesting
that the one type is only an earlier form of the other; but this again
we do not know.

The more primitive nebulæ would seem to be composed of gas in an
extremely rarified form. It is difficult to convey an adequate idea of
the rarity of nebular gases. The residual gases in a vacuum tube are
dense by comparison. A cubic inch of air at ordinary pressure would
contain more matter than is contained in millions of cubic inches of the
gases of nebulæ. The light of even the faintest stars does not seem to
be dimmed by passing through a gaseous nebula, although we cannot be
sure on this point. The most remarkable physical fact about these gases
is that they are luminous. Whence they derive their luminosity we do not
know. It hardly seems possible to believe that extremely thin gases
exposed to the terrific cold of space can be so hot as to be luminous
and can retain their heat and their luminosity indefinitely. A cold
luminosity due to electrification, like that of the aurora borealis,
would seem to fit the case better.

Now the nebular theory is that out of great "fire-mists," such as we
have described, stars are born. We do not know whether gravitation is
the only or even the main force at work in a nebula, but it is supposed
that under the action of gravity the far-flung "fire-mists" would begin
to condense round centres of greatest density, heat being evolved in the
process. Of course the condensation would be enormously slow, although
the sudden irruption of a swarm of meteors or some solid body might
hasten matters greatly by providing large, ready-made centres of

Spiral Nebulæ

It is then supposed that the contracting mass of gas would begin to
rotate and to throw off gigantic streamers, which would in their turn
form centres of condensation. The whole structure would thus form a
spiral, having a dense region at its centre and knots or lumps of
condensed matter along its spiral arms. Besides the formless gaseous
nebulæ there are hundreds of thousands of "spiral" nebulæ such as we
have just mentioned in the heavens. They are at all stages of
development, and they are visible to us at all angles--that is to say,
some of them face directly towards us, others are edge on, and some are
in intermediate positions. It appears, therefore, that we have here a
striking confirmation of the nebular hypothesis. But we must not go so
fast. There is much controversy as to the nature of these spiral nebulæ.
Some eminent astronomers think they are other stellar universes,
comparable in size with our own. In any case they are vast structures,
and if they represent stars in process of condensation, they must be
giving birth to huge agglomerations of stars--to star clusters at least.
These vast and enigmatic objects do not throw much light on the origin
of our own solar system. The nebular hypothesis, which was invented
by Laplace to explain the origin of our solar system, has not yet met
with universal acceptance. The explanation offers grave difficulties,
and it is best while the subject is still being closely investigated, to
hold all opinions with reserve. It may be taken as probable, however,
that the universe has developed from masses of incandescent gas.

[Illustration: _Photo: Yerkes Observatory._


The most impressive nebula in the heavens. It is inconceivably greater
in dimensions than the whole solar system.]

[Illustration: _Photo: Lick Observatory._

FIG. 25--GIANT SPIRAL NEBULA, March 23, 1914

This spiral nebula is seen full on. Notice the central nucleus and the
two spiral arms emerging from its opposite directions. Is matter flowing
out of the nucleus into the arms or along the arms into the nucleus? In
either case we should get two streams in opposite directions within the


§ 3

Variable, New, and Dark Stars: Dying Suns

Many astronomers believe that in "variable stars" we have another star,
following that of the dullest red star, in the dying of suns. The light
of these stars varies periodically in so many days, weeks, or years. It
is interesting to speculate that they are slowly dying suns, in which
the molten interior periodically bursts through the shell of thick
vapours that is gathering round them. What we saw about our sun seems to
point to some such stage in the future. That is, however, not the
received opinion about variable stars. It may be that they are stars
which periodically pass through a great swarm of meteors or a region of
space that is rich in cosmic dust of some sort, when, of course, a great
illumination would take place.

One class of these variable stars, which takes its name from the star
Algol, is of special interest. Every third night Algol has its light
reduced for several hours. Modern astronomy has discovered that in this
case there are really two stars, circulating round a common centre, and
that every third night the fainter of the two comes directly between us
and its companion and causes an "eclipse." This was until recently
regarded as a most interesting case in which a dead star revealed itself
to us by passing before the light of another star. But astronomers have
in recent years invented something, the "selenium-cell," which is even
more sensitive than the photographic plate, and on this the supposed
dead star registers itself as very much alive. Algol is, however,
interesting in another way. The pair of stars which we have discovered
in it are hundreds of trillions of miles away from the earth, yet we
know their masses and their distances from each other.

The Death and Birth of Stars

We have no positive knowledge of dead stars; which is not surprising
when we reflect that a dead star means an invisible star! But when we
see so many individual stars tending toward death, when we behold a vast
population of all conceivable ages, we presume that there are many
already dead. On the other hand, there is no reason to suppose that the
universe as a whole is "running down." Some writers have maintained
this, but their argument implies that we know a great deal more about
the universe than we actually do. The scientific man does not know
whether the universe is finite or infinite, temporal or eternal; and he
declines to speculate where there are no facts to guide him. He knows
only that the great gaseous nebulæ promise myriads of worlds in the
future, and he concedes the possibility that new nebulæ may be forming
in the ether of space.

The last, and not the least interesting, subject we have to notice is
the birth of a "new star." This is an event which astronomers now
announce every few years; and it is a far more portentous event than the
reader imagines when it is reported in his daily paper. The story is
much the same in all cases. We say that the star appeared in 1901, but
you begin to realise the magnitude of the event when you learn that the
distant "blaze" had really occurred about the time of the death of
Luther! The light of the conflagration had been speeding toward us
across space at 186,000 miles a second, yet it has taken nearly three
centuries to reach us. To be visible at all to us at that distance the
fiery outbreak must have been stupendous. If a mass of petroleum ten
times the size of the earth were suddenly fired it would not be seen at
such a distance. The new star had increased its light many hundredfold
in a few days.

There is a considerable fascination about the speculation that in such
cases we see the resurrection of a dead world, a means of renewing the
population of the universe. What happens is that in some region of the
sky where no star, or only a very faint star, had been registered on our
charts, we almost suddenly perceive a bright star. In a few days it may
rise to the highest brilliancy. By the spectroscope we learn that this
distant blaze means a prodigious outpour of white-hot hydrogen at
hundreds of miles a second. But the star sinks again after a few months,
and we then find a nebula round it on every side. It is natural to
suppose that a dead or dying sun has somehow been reconverted in whole
or in part into a nebula. A few astronomers think that it may have
partially collided with another star, or approached too closely to
another, with the result we described on an earlier page. The general
opinion now is that a faint or dead star had rushed into one of those
regions of space in which there are immense stretches of nebulous
matter, and been (at least in part) vaporised by the friction.

But the difficulties are considerable, and some astronomers prefer to
think that the blazing star may merely have lit up a dark nebula which
already existed. It is one of those problems on which speculation is
most tempting but positive knowledge is still very incomplete. We may be
content, even proud, that already we can take a conflagration that has
occurred more than a thousand trillion miles away and analyse it
positively into an outflame of glowing hydrogen gas at so many miles a


§ 4

Our Universe a Spiral Nebula

What is the shape of our universe, and what are its dimensions? This is
a tremendous question to ask. It is like asking an intelligent insect,
living on a single leaf in the midst of a great Brazilian forest, to say
what is the shape and size of the forest. Yet man's ingenuity has proved
equal to giving an answer even to this question, and by a method exactly
similar to that which would be adopted by the insect. Suppose, for
instance, that the forest was shaped as an elongated oval, and the
insect lived on a tree near the centre of the oval. If the trees were
approximately equally spaced from one another they would appear much
denser along the length of the oval than across its width. This is the
simple consideration that has guided astronomers in determining the
shape of our stellar universe. There is one direction in the heavens
along which the stars appear denser than in the directions at right
angles to it. That direction is the direction in which we look towards
the Milky Way. If we count the number of stars visible all over the
heavens, we find they become more and more numerous as we approach the
Milky Way. As we go farther and farther from the Milky Way the stars
thin out until they reach a maximum sparseness in directions at right
angles to the plane of the Milky Way. We may consider the Milky Way to
form, as it were, the equator of our system, and the line at right
angles to point to the north and south poles.

Our system, in fact, is shaped something like a lens, and our sun is
situated near the centre of this lens. In the remoter part of this lens,
near its edge, or possibly outside it altogether, lies the great series
of star clouds which make up the Milky Way. All the stars are in motion
within this system, but the very remarkable discovery has been made that
these motions are not entirely random. The great majority of the stars
whose motions can be measured fall into two groups drifting past one
another in opposite directions. The velocity of one stream relative to
the other is about twenty-five miles per second. The stars forming these
two groups are thoroughly well mixed; it is not a case of an inner
stream going one way and an outer stream the other. But there are not
quite as many stars going one way as the other. For every two stars in
one stream there are three in the other. Now, as we have said, some
eminent astronomers hold that the spiral nebulæ are universes like our
own, and if we look at the two photographs (Figs. 25 and 26) we see that
these spirals present features which, in the light of what we have just
said about our system, are very remarkable. The nebula in Coma Berenices
is a spiral edge-on to us, and we see that it has precisely the
lens-shaped middle and the general flattened shape that we have found in
our own system. The nebula in Canes Venatici is a spiral facing towards
us, and its shape irresistibly suggests motions along the spiral arms.
This motion, whether it is towards or away from the central, lens-shaped
portion, would cause a double streaming motion in that central portion
of the kind we have found in our own system. Again, and altogether apart
from these considerations, there are good reasons for supposing our
Milky Way to possess a double-armed spiral structure. And the great
patches of dark absorbing matter which are known to exist in the Milky
Way (see Fig. 22) would give very much the mottled appearance we notice
in the arms (which we see edge-on) of the nebula in Coma Berenices. The
hypothesis, therefore, that our universe is a spiral nebula has much to
be said for it. If it be accepted it greatly increases our estimate of
the size of the material universe. For our central, lens-shaped system
is calculated to extend towards the Milky Way for more than twenty
thousand times a million million miles, and about a third of this
distance towards what we have called the poles. If, as we suppose, each
spiral nebula is an independent stellar universe comparable in size with
our own, then, since there are hundreds of thousands of spiral nebulæ,
we see that the size of the whole material universe is indeed beyond our

[Illustration: _Photo: Mount Wilson Observatory._


Notice the lens-shaped formation of the nucleus and the arm stretching
as a band across it. See reference in the text to the resemblance
between this and our stellar universe.]

[Illustration: _Photo: H. J. Shepstone._


A reflecting telescope: the largest in the world. The mirror is situated
at the base of the telescope.]


| |
| | | | | |
| | MILES) | (IN YEARS) | | |
| | | | | |
| MERCURY | 36.0 | 0.24 | 3030 | 0 |
| VENUS | 67.2 | 0.62 | 7700 | 0 |
| EARTH | 92.9 | 1.00 | 7918 | 1 |
| MARS | 141.5 | 1.88 | 4230 | 2 |
| JUPITER | 483.3 | 11.86 | 86500 | 9 |
| SATURN | 886.0 | 29.46 | 73000 | 10 |
| URANUS | 1781.9 | 84.02 | 31900 | 4 |
| NEPTUNE | 2971.6 | 164.78 | 34800 | 1 |
| SUN | ------ | ------ | 866400 | -- |
| MOON | ------ | ------ | 2163 | -- |

FIG. 27]


| |
| |
| |
| POLARIS 76 |
| CAPELLA 49.4 |
| RIGEL 466 |
| SIRIUS 8.7 |
| PROCYON 10.5 |
| REGULUS 98.8 |
| ARCTURUS 43.4 |
| VEGA 34.7 |
| |


FIG. 28

The above distances are merely approximate and are subject to further
revision. A "light-year" is the distance that light, travelling at the
rate of 186,000 miles per second, would cover in one year.]

In this simple outline we have not touched on some of the more debatable
questions that engage the attention of modern astronomers. Many of these
questions have not yet passed the controversial stage; out of these will
emerge the astronomy of the future. But we have seen enough to convince
us that, whatever advances the future holds in store, the science of the
heavens constitutes one of the most important stones in the wonderful
fabric of human knowledge.


§ 1

The Telescope

The instruments used in modern astronomy are amongst the finest triumphs
of mechanical skill in the world. In a great modern observatory the
different instruments are to be counted by the score, but there are two
which stand out pre-eminent as the fundamental instruments of modern
astronomy. These instruments are the telescope and the spectroscope, and
without them astronomy, as we know it, could not exist.

There is still some dispute as to where and when the first telescope was
constructed; as an astronomical instrument, however, it dates from the
time of the great Italian scientist Galileo, who, with a very small and
imperfect telescope of his own invention, first observed the spots on
the sun, the mountains of the moon, and the chief four satellites of
Jupiter. A good pair of modern binoculars is superior to this early
instrument of Galileo's, and the history of telescope construction, from
that primitive instrument to the modern giant recently erected on Mount
Wilson, California, is an exciting chapter in human progress. But the
early instruments have only an historic interest: the era of modern
telescopes begins in the nineteenth century.

During the last century telescope construction underwent an
unprecedented development. An immense amount of interest was taken in
the construction of large telescopes, and the different countries of the
world entered on an exciting race to produce the most powerful possible
instruments. Besides this rivalry of different countries there was a
rivalry of methods. The telescope developed along two different lines,
and each of these two types has its partisans at the present day. These
types are known as _refractors_ and _reflectors_, and it is necessary to
mention, briefly, the principles employed in each. The _refractor_ is
the ordinary, familiar type of telescope. It consists, essentially, of a
large lens at one end of a tube, and a small lens, called the eye-piece,
at the other. The function of the large lens is to act as a sort of
gigantic eye. It collects a large amount of light, an amount
proportional to its size, and brings this light to a focus within the
tube of the telescope. It thus produces a small but bright image, and
the eye-piece magnifies this image. In the _reflector_, instead of a
large lens at the top of the tube, a large mirror is placed at the
bottom. This mirror is so shaped as to reflect the light that falls on
it to a focus, whence the light is again led to an eye-piece. Thus the
refractor and the reflector differ chiefly in their manner of gathering
light. The powerfulness of the telescope depends on the size of the
light-gatherer. A telescope with a lens four inches in diameter is four
times as powerful as the one with a lens two inches in diameter, for the
amount of light gathered obviously depends on the _area_ of the lens,
and the area varies as the _square_ of the diameter.

The largest telescopes at present in existence are _reflectors_. It is
much easier to construct a very large mirror than to construct a very
large lens; it is also cheaper. A mirror is more likely to get out of
order than is a lens, however, and any irregularity in the shape of a
mirror produces a greater distorting effect than in a lens. A refractor
is also more convenient to handle than is a reflector. For these reasons
great refractors are still made, but the largest of them, the great
Yerkes' refractor, is much smaller than the greatest reflector, the one
on Mount Wilson, California. The lens of the Yerkes' refractor measures
three feet four inches in diameter, whereas the Mount Wilson reflector
has a diameter of no less than eight feet four inches.


(The largest _refracting_ telescope in the world. Its big lens weighs
1,000 pounds, and its mammoth tube, which is 62 feet long, weighs about
12,000 pounds. The parts to be moved weigh approximately 22 tons.

The great _100-inch reflector_ of the Mount Wilson reflecting
telescope--the largest _reflecting_ instrument in the world--weighs
nearly 9,000 pounds and the moving parts of the telescope weigh about
100 tons.

The new _72-inch reflector_ at the Dominion Astrophysical Observatory,
near Victoria, B. C., weighs nearly 4,500 pounds, and the moving parts
about 35 tons.)]

[Illustration: _Photo: H. J. Shepstone._


The smaller telescope at the top of the picture acts as a "finder"; the
field of view of the large telescope is so restricted that it is
difficult to recognise, as it were, the part of the heavens being
surveyed. The smaller telescope takes in a larger area and enables the
precise object to be examined to be easily selected.]


(_By A. Hilger, Ltd._)

The light is brought through one telescope, is split up by the prism,
and the resulting spectrum is observed through the other telescope.]

But there is a device whereby the power of these giant instruments,
great as it is, can be still further heightened. That device is the
simple one of allowing the photographic plate to take the place of the
human eye. Nowadays an astronomer seldom spends the night with his eye
glued to the great telescope. He puts a photographic plate there. The
photographic plate has this advantage over the eye, that it builds up
impressions. However long we stare at an object too faint to be seen, we
shall never see it. With the photographic plate, however, faint
impressions go on accumulating. As hour after hour passes, the star
which was too faint to make a perceptible impression on the plate goes
on affecting it until finally it makes an impression which can be made
visible. In this way the photographic plate reveals to us phenomena in
the heavens which cannot be seen even through the most powerful

Telescopes of the kind we have been discussing, telescopes for exploring
the heavens, are mounted _equatorially_; that is to say, they are
mounted on an inclined pillar parallel to the axis of the earth so that,
by rotating round this pillar, the telescope is enabled to follow the
apparent motion of a star due to the rotation of the earth. This motion
is effected by clock-work, so that, once adjusted on a star, and the
clock-work started, the telescope remains adjusted on that star for any
length of time that is desired. But a great official observatory, such
as Greenwich Observatory or the Observatory at Paris, also has _transit_
instruments, or telescopes smaller than the equatorials and without the
same facility of movement, but which, by a number of exquisite
refinements, are more adapted to accurate measurements. It is these
instruments which are chiefly used in the compilation of the _Nautical
Almanac_. They do not follow the apparent motions of the stars. Stars
are allowed to drift across the field of vision, and as each star
crosses a small group of parallel wires in the eye-piece its precise
time of passage is recorded. Owing to their relative fixity of position
these instruments can be constructed to record the _positions_ of stars
with much greater accuracy than is possible to the more general and
flexible mounting of equatorials. The recording of transit is
comparatively dry work; the spectacular element is entirely absent;
stars are treated merely as mathematical points. But these observations
furnish the very basis of modern mathematical astronomy, and without
them such publications as the _Nautical Almanac_ and the _Connaissance
du Temps_ would be robbed of the greater part of their importance.

§ 2

The Spectroscope

We have already learnt something of the principles of the spectroscope,
the instrument which, by making it possible to learn the actual
constitution of the stars, has added a vast new domain to astronomy. In
the simplest form of this instrument the analysing portion consists of a
single prism. Unless the prism is very large, however, only a small
degree of dispersion is obtained. It is obviously desirable, for
accurate analytical work, that the dispersion--that is, the separation
of the different parts of the spectrum--should be as great as possible.
The dispersion can be increased by using a large number of prisms, the
light emerging from the first prism, entering the second, and so on. In
this way each prism produces its own dispersive effect and, when a
number of prisms are employed, the final dispersion is considerable. A
considerable amount of light is absorbed in this way, however, so that
unless our primary source of light is very strong, the final spectrum
will be very feeble and hard to decipher.

Another way of obtaining considerable dispersion is by using a
_diffraction grating_ instead of a prism. This consists essentially of a
piece of glass on which lines are ruled by a diamond point. When the
lines are sufficiently close together they split up light falling on
them into its constituents and produce a spectrum. The modern
diffraction grating is a truly wonderful piece of work. It contains
several thousands of lines to the inch, and these lines have to be
spaced with the greatest accuracy. But in this instrument, again, there
is a considerable loss of light.

We have said that every substance has its own distinctive spectrum, and
it might be thought that, when a list of the spectra of different
substances has been prepared, spectrum analysis would become perfectly
straightforward. In practice, however, things are not quite so simple.
The spectrum emitted by a substance is influenced by a variety of
conditions. The pressure, the temperature, the state of motion of the
object we are observing, all make a difference, and one of the most
laborious tasks of the modern spectroscopist is to disentangle these
effects from one another. Simple as it is in its broad outlines,
spectroscopy is, in reality, one of the most intricate branches of
modern science.


(The following list of books may be useful to readers wishing to pursue
further the study of Astronomy.)

BALL, _The Story of the Heavens_.
BALL, _The Story of the Sun_.
FORBES, _History of Astronomy_.
HINCKS, _Astronomy_.
KIPPAX, _Call of the Stars_.
LOWELL, _Mars and Its Canals_.
LOWELL, _Evolution of Worlds_.
MCKREADY, _A Beginner's Star-Book_.
NEWCOMB, _Popular Astronomy_.
NEWCOMB, _The Stars: A Study of the Universe_.
OLCOTT, _Field Book of the Stars_.
PRICE, _Essence of Astronomy_.
SERVISS, _Curiosities of the Skies_.
WEBB, _Celestial Objects for Common Telescopes_.
YOUNG, _Text-Book of General Astronomy_.

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