The Outline of Science, Vol. 1 (of 4)

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Sec. 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 nebulae" (Fig 4). We shall see at a
later stage what a nebula is, and we shall see that some astronomers
regard these spiral nebulae 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
nebulae are "other universes."

[Illustration: _Photo: Harvard College Observatory._

FIG. 2.--THE MILKY WAY

Note the cloud-like effect.]

[Illustration: FIG. 3--THE MOON ENTERING THE SHADOW CAST BY THE EARTH

The diagram shows the Moon partially eclipsed.]

[Illustration: _From a photograph taken at the Yerkes Observatory_

FIG. 4.--THE GREAT NEBULA IN ANDROMEDA, MESSIER 31]

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.

THE SOLAR SYSTEM

THE SUN

Sec. 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 aeons 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.

[Illustration: FIG. 5.--DIAGRAM SHOWING THE MAIN LAYERS OF THE SUN

Compare with frontispiece.]

[Illustration: _Photo: Royal Observatory, Greenwich._

FIG. 6.--SOLAR PROMINENCES SEEN AT TOTAL SOLAR ECLIPSE, May 29, 1919.
TAKEN AT SOBRAL, BRAZIL.

The small Corona is also visible.]

[Illustration: FIG. 7.--THE VISIBLE SURFACE OF THE SUN

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.

Sec. 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 deg. C. to 7,000 deg. 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.

Sun-spots

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.
Macmillan_)

THE AURORA BOREALIS

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.

A WONDERFUL INSTRUMENT AND WHAT IT REVEALS

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._

FIG. 9.--THE GREAT SUN-SPOT OF JULY 17, 1905]

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

FIG. 10.--SOLAR PROMINENCES

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
nebulae 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.

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