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

J >> J. Arthur Thomson >> The Outline of Science, Vol. 1 (of 4)

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[Illustration: _Photo: Harvard College Observatory._

FIG. 21.--TYPICAL SPECTRA

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._

FIG. 22.--A NEBULAR REGION SOUTH OF ZETA ORIONIS

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

[Illustration: _Photo: Astrophysical Observatory, Victoria, British
Columbia._

FIG. 23.--STAR CLUSTER IN HERCULES

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.]

THE STELLAR UNIVERSE

Sec. 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 nebulae.

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 nebulae, 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.

THE NEBULAR THEORY

Sec. 2

Nebulae 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 nebulae from the earth. The distances of some nebulae 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 nebulae 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 nebulae, 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 nebulae 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 nebulae. 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
condensation.

Spiral Nebulae

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
nebulae there are hundreds of thousands of "spiral" nebulae 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 nebulae.
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._

FIG. 24.--THE GREAT NEBULA IN ORION

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
nucleus.]

THE BIRTH AND DEATH OF STARS

Sec. 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 nebulae promise myriads of worlds in the
future, and he concedes the possibility that new nebulae 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
second.

THE SHAPE OF OUR UNIVERSE

Sec. 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 nebulae 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 nebulae,
we see that the size of the whole material universe is indeed beyond our
comprehension.

[Illustration: _Photo: Mount Wilson Observatory._

FIG. 26.--A SPIRAL NEBULA SEEN EDGE-ON

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._

100-INCH TELESCOPE, MOUNT WILSON

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

[Illustration:

________________________________________________________________
| |
| THE SOLAR SYSTEM |
|________________________________________________________________|
| | | | | |
| | MEAN DISTANCE | PERIOD OF | | |
| NAME | FROM SUN (IN | REVOLUTION | DIAMETER | NUMBER OF |
| | MILLIONS OF | AROUND SUN | (IN MILES) | SATELLITES |
| | 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]

[Illustration:

______________________________________
| |
| STAR DISTANCES |
|______________________________________|
| |
| DISTANCE IN |
| STAR LIGHT-YEARS |
| |
| POLARIS 76 |
| CAPELLA 49.4 |
| RIGEL 466 |
| SIRIUS 8.7 |
| PROCYON 10.5 |
| REGULUS 98.8 |
| ARCTURUS 43.4 |
| [ALPHA] CENTAURI 4.29 |
| VEGA 34.7 |
|______________________________________|
| |
| SMALLER MAGELLANIC CLOUD 32,600[A] |
| GREAT CLUSTER IN HERCULES 108,600[A] |
|______________________________________|

[A] ESTIMATED

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.

ASTRONOMICAL INSTRUMENTS

Sec. 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.

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