Transcriber’s Notes:
Underscores “_” before and after a word or phrase indicate _italics_
in the original text.
Equal signs “=” before and after a word or phrase indicate =bold=
in the original text.
Small capitals have been converted to SOLID capitals.
Illustrations and footnotes have been moved so they do not break up
paragraphs.
Deprecated spellings have been preserved.
Typographical and punctuation errors have been silently corrected.
THE UNIVERSE AROUND US
Cambridge University Press
Fetter Lane, London
_Bombay_, _Calcutta_, _Madras_
_Toronto_
Macmillan
_Tokyo_
Maruzen Company, Ltd
Copyrighted in the United States of
America by the Macmillan Company
All rights reserved
[Illustration: _Lowell Observatory_
The Discovery of Pluto
The two photographs were taken at the Lowell Observatory on the nights
of March 2 and 5, 1930. The object indicated by arrows was seen to have
moved considerably in the interval, and this proved that it was of
planetary character.
The bright object in the bottom left-hand corner is the star δ
Geminorum (see p. 260).]
THE
UNIVERSE
AROUND US
_By_
SIR JAMES JEANS
M.A., D.Sc., Sc.D.
LL.D., F.R.S.
[Illustration]
CAMBRIDGE
AT THE UNIVERSITY PRESS
1930
_First Edition, September 1929_
_Reprinted November 1929_
” _January 1930_
_Second Edition, October 1930_
PRINTED IN GREAT BRITAIN
PREFACE
The present book contains a brief account, written in simple language,
of the methods and results of modern astronomical research, both
observational and theoretical. Special attention has been given to
problems of cosmogony and evolution, and to the general structure of
the universe. My ideal, perhaps never wholly attainable, has been that
of making the entire book intelligible to readers with no special
scientific knowledge.
Parts of the book cover the same ground as various lectures I have
recently delivered to University and other audiences, including
a course of wireless talks I gave last autumn. It has been found
necessary to re-write these almost in their entirety, so that very few
sentences remain in their original form, but those who have asked me to
publish my lectures and wireless talks will find the substance of them
in the present book.
J. H. JEANS
DORKING
_1 May 1929_
PREFACE TO SECOND EDITION
In preparing a second edition, I have taken advantage of a great
number of suggestions made by correspondents and reviewers, to whom I
offer my sincerest thanks. I have also inserted discussions of the new
planet Pluto, the rotation of the galaxy, the apparent expansion of
the universe, and other subjects which have become important since the
first edition was published, and in general have tried to bring the
book up to date.
J. H. JEANS
DORKING
_2 August 1930_
CONTENTS
PAGE
_Introduction_—The Study of Astronomy 1
_Chapter_ I Exploring the Sky 16
II Exploring the Atom 92
III Exploring in Time 150
IV Carving out the Universe 194
V Stars 253
VI Beginnings and Endings 325
_Index_ 355
PLATES
The Discovery of Pluto FRONTISPIECE
PLATE FACING PAGE
I The Milky Way in the neighbourhood
of the Southern Cross 23
II Planetary Nebulae:
1. N.G.C. 2022
2. N.G.C. 6720 (The Ring Nebula)
3. N.G.C. 1501
4. N.G.C. 7662 28
III The Nebula in Cygnus 29
IV The Great Nebula _M_ 31 in Andromeda 30
V The Nebula N.G.C. 891 in Andromeda
seen edge-on 31
VI The “Horse’s Head” in the Great
Nebula in Orion 37
VII The Trifid Nebula _M_ 20 in Sagittarius 44
VIII Stellar Spectra 51
IX The Globular Cluster _M_ 13 in Hercules 63
X The Region of ρ Ophiuchi 65
XI Magnification of part of the outer
regions of the Great Nebula _M_ 31
in Andromeda 70
XII Magnification of the central region of
the Great Nebula _M_ 31 in Andromeda 71
XIII The tracks of α- and β-particles 113
XIV Collisions of α-particles with Atoms 117
XV The Nebula N.G.C. 4594 in Virgo
The Nebula N.G.C. 7217 204
XVI A sequence of Nebular Configurations:
1. N.G.C. 3379
2. N.G.C. 4621
3. N.G.C. 3115
4. N.G.C. 4594 in Virgo
5. N.G.C. 4565 in Berenice’s hair 207
XVII The “Whirlpool” Nebula _M_ 51 in
Canes Venatici 210
XVIII The Nebula _M_ 81 in Ursa Major 211
XIX The Nebula _M_ 101 in Ursa Major 212
XX The Nebula _M_ 33 in Triangulum 213
XXI The Lesser and Greater Magellanic Clouds 214
XXII Two Nebulae (N.G.C. 4395, 4401) suggestive
of Tidal Action 236
XXIII The twin Nebulae N.G.C. 4567-8
The Nebula N.G.C. 7479 237
XXIV Saturn and its System of Rings 250
INTRODUCTION
_The Study of Astronomy_
On the evening of January 7, 1610, a fateful day for the human race,
Galileo Galilei, Professor of Mathematics in the University of Padua,
sat in front of a telescope he had made with his own hands.
More than three centuries previously, Roger Bacon, the inventor of
spectacles, had explained how a telescope could be constructed so as
“to make the stars appear as near as we please.” He had shewn how a
lens could be so shaped that it would collect all the rays of light
falling on it from a distant object, bend them until they met in a
focus and then pass them on through the pupil of the eye on to the
retina. Such an instrument would increase the power of the human
eye, just as an ear trumpet increases the power of the human ear by
collecting all the waves of sound which fall on a large aperture,
bending them, and passing them through the orifice of the ear on to the
ear drum.
Yet it was not until 1608 that the first telescope had been constructed
by Lippershey, a Flemish spectacle-maker. On hearing of this
instrument, Galileo had set to work to discover the principles of its
construction and had soon made himself a telescope far better than the
original. His instrument had created no small sensation in Italy. Such
extraordinary stories had been told of its powers that he had been
commanded to take it to Venice and exhibit it to the Doge and Senate.
The citizens of Venice had then seen the most aged of their Senators
climbing the highest bell-towers to spy through the telescope at ships
which were too far out at sea to be seen at all without its help. The
telescope admitted about a hundred times as much light as the unaided
human eye, and, according to Galileo, it shewed an object at fifty
miles as clearly as if it were only five miles away.
Perhaps it need hardly be said that this is quite insignificant in
comparison with the power of modern instruments. The telescope of
100-inch aperture at Mount Wilson, California, the largest at present
in existence, admits 2500 times as much light as Galileo’s tiny
instrument, and so 250,000 times as much light as the unaided eye. It
is hoped that a 200-inch telescope will shortly be built in California;
this will admit four times as much light as the 100-inch instrument, or
about a million times as much light as the unaided eye.
The absorbing interest of his new instrument had almost driven from
Galileo’s mind a problem to which he had at one time given much
thought. Over two thousand years previously, Pythagoras and Philolaus
had taught that the earth is not fixed in space but rotates on its
axis every twenty-four hours, thus causing the alternation of day and
night. Aristarchus of Samos, perhaps the greatest of all the Greek
mathematicians, had further maintained that the earth not only turned
on its axis, but also described a yearly journey round the sun, this
being the cause of the cycle of the seasons.
Then these doctrines had fallen into disfavour. Aristotle had
pronounced against them, asserting that the earth formed a fixed
centre to the universe. Later Ptolemy had explained the tracks of the
planets across the sky in terms of a complicated system of cycles and
epicycles; the planets moved in circular paths around moving points,
which themselves moved in circles around an immoveable earth. The
Church had given its sanction and active support to these doctrines.
Indeed, it is difficult to see what else it could have done, for it
seemed almost impious to suppose that the great drama of man’s fall and
redemption, in which the Son of God had Himself taken part, could have
been enacted on any lesser stage than the very centre of the Universe.
Yet, even in the Church, the doctrine had not gained universal
acceptance. Oresme, Bishop of Lisieux, and Cardinal Nicholas of Cusa,
had both declared against it, the latter writing in 1440:
I have long considered that this earth is not fixed, but
moves as do the other stars. To my mind the earth turns upon
its axis once every day and night.
At a later date those who held these views incurred the active
hostility of the Church, and in 1600 Giordano Bruno was burned at the
stake. He had written:
It has seemed to me unworthy of the divine goodness and
power to create a finite world, when able to produce beside
it another and others infinite; so that I have declared that
there are endless particular worlds similar to this of the
earth; with Pythagoras I regard it as a star, and similar
to it are the moon, the planets and other stars, which are
infinite in number, and all these bodies are worlds.
The most weighty attack on orthodox doctrine had, however, been
delivered neither by theologians nor philosophers, but by the Polish
astronomer, Nicolaus Copernicus (1473-1543). In his great work _De
revolutionibus orbium coelestium_ Copernicus had shewn that Ptolemy’s
elaborate structure of cycles and epicycles was unnecessary, because
the tracks of the planets across the sky could be explained quite
simply by supposing that the earth and the planets all moved round a
fixed central sun. The sixty-six years which had elapsed since this
book was published had seen these theories hotly debated, but they were
still neither proved nor disproved.
Galileo had already found that his new telescope provided a means
of testing astronomical theories. As soon as he had turned it on to
the Milky Way, a whole crowd of legends and fables as to its nature
and structure had vanished into thin air; it proved to be nothing
more than a swarm of faint stars scattered like golden dust on the
black background of the sky. Another glance through the telescope had
disclosed the true nature of the moon. It had on it mountains which
cast shadows, and so proved, as Giordano Bruno had maintained, to be a
world like our own. What if the telescope should now in some way prove
able to decide between the orthodox doctrine that the earth formed the
hub of the universe, and the new doctrine that the earth was only one
of a number of bodies, all circling round the sun like moths round a
candle-flame?
And now Galileo catches Jupiter in the field of his telescope and sees
four small bodies circling around the great mass of the planet—like
moths round a candle-flame. What he sees is an exact replica of the
solar system as imagined by Copernicus, and it provides direct visual
proof that such systems are at least not alien to the architectural
plan of the universe. On January 30th he writes to Belisario Vinta that
these small bodies move round the far greater mass of Jupiter “just as
Venus and Mercury, and perhaps the other planets, move round the sun.”
Any lingering doubts that Galileo may have felt as to the significance
of his discovery are removed nine months later when he observes the
phases of Venus. Venus might have been self-luminous, in which case
she would always appear as a full circle of light. If she were not
self-luminous but moved in a Ptolemaic epicycle, then, as Ptolemy had
himself pointed out, she could never shew more than half her surface
illuminated. On the other hand, the Copernican view of the solar system
required that both Venus and Mercury should exhibit “phases” like
those of the moon, their shining surfaces ranging in appearance from
crescent-shape through half moon to full moon, and then back through
half moon to crescent-shape. That such phases were not shewn by Venus
had indeed been urged as an objection to the Copernican theory.
Galileo’s telescope now shews that, as Copernicus had foretold, Venus
passes through the full cycle of phases, so that, in Galileo’s own
words, we “are now supplied with a determination most conclusive,
and appealing to the evidence of our senses, of two very important
problems, which up to this day have been discussed by the greatest
intellects with different conclusions. One is that the planets are not
self-luminous. The other is that we are absolutely compelled to say
that Venus, and Mercury also, revolve around the sun, as do also all
the rest of the planets, a truth believed indeed by the Pythagorean
school, by Copernicus, and by Kepler, but never proved by the evidence
of our senses, as is now proved in the case of Venus and Mercury.”
These discoveries of Galileo made it clear that Aristotle, Ptolemy
and the majority of those who had thought about these things in the
last 2000 years had been utterly and hopelessly wrong. In estimating
his position in the universe, man had up to now been guided mainly by
his own desires, and his self-esteem; long fed on boundless hopes, he
had spurned the simpler fare offered by patient scientific thought.
Inexorable facts now dethroned him from his self-arrogated station at
the centre of the universe; henceforth he must reconcile himself to the
humble position of the inhabitant of a speck of dust, and adjust his
views on the meaning of human life accordingly.
The adjustment was not made at once. Human vanity, reinforced by the
authority of the Church, contrived to make a rough road for those
who dared draw attention to the earth’s insignificant position in
the universe. Galileo was forced to abjure his beliefs. Well on into
the eighteenth century the ancient University of Paris taught that
the motion of the earth round the sun was a convenient _but false_
hypothesis, while the newer American Universities of Harvard and Yale
taught the Ptolemaic and Copernican systems of astronomy side by side
as though they were equally tenable. Yet men could not keep their heads
buried in the sand for ever, and when at last its full implications
were accepted, the revolution of thought initiated by Galileo’s
observations of January 7, 1610, proved to be the most catastrophic in
the history of the race. The cataclysm was not confined to the realms
of abstract thought; henceforth human existence itself was to appear
in a new light, and human aims and aspirations would be judged from a
different standpoint.
This oft-told story has been told once again, in the hope that it may
serve to explain some of the interest taken in astronomy to-day. The
more mundane sciences prove their worth by adding to the amenities and
pleasures of life, or by alleviating pain or distress, but it may well
be asked what reward astronomy has to offer. Why does the astronomer
devote arduous nights, and still more arduous days, to studying the
structure, motions and changes of bodies so remote that they can have
no conceivable influence on human life?
In part at least the answer would seem to be that many have begun to
suspect that the astronomy of to-day, like that of Galileo, may have
something to say on the enthralling question of the relation of human
life to the universe in which it is placed, and on the beginnings,
meaning and destiny of the human race. Bede records how, some twelve
centuries ago, human life was compared in poetic simile to the flight
of a bird through a warm hall in which men sit feasting, while the
winter storms rage without.
The bird is safe from the tempest for a brief moment, but
immediately passes from winter to winter again. So man’s
life appears for a little while, but of what is to follow,
or of what went before, we know nothing. If, therefore, a
new doctrine tells us something certain, it seems to deserve
to be followed.
These words, originally spoken in advocacy of the Christian religion,
describe what is perhaps the main interest of astronomy to-day. Man
only knowing
Life’s little lantern between dark and dark
wishes to probe further into the past and future than his brief span
of life permits. He wishes to see the universe as it existed before
man was, as it will be after the last man has passed again into the
darkness from which he came. The wish does not originate solely in
mere intellectual curiosity, in the desire to see over the next range
of mountains, the desire to attain a summit commanding a wide view,
even if it be only of a promised land which he may never hope himself
to enter; it has deeper roots and a more personal interest. Before he
can understand himself, man must first understand the universe from
which all his sense perceptions are drawn. He wishes to explore the
universe, both in space and time, because he himself forms part of it,
and it forms part of him.
We may well admit that science cannot at present hope to say anything
final on the questions of human existence and human destiny, but this
is no justification for not becoming acquainted with the best that it
has to offer. It is rare indeed for science to give a final “Yes” or
“No” answer to any question propounded to her. When we are able to put
a question in such a definite form that either of these answers could
be given in reply, we are generally already in a position to supply the
answer ourselves. Science advances rather by providing a succession of
approximations to the truth, each more accurate than the last, but each
capable of endless degrees of higher accuracy. To the question, “where
does man stand in the universe?” the first attempt at an answer, at any
rate in recent times, was provided by the astronomy of Ptolemy: “at
the centre.” Galileo’s telescope provided the next, and incomparably
better, approximation: “man’s home in space is only one of a number of
small bodies revolving round a huge central sun.” Nineteenth-century
astronomy swung the pendulum still further in the same direction,
saying: “there are millions of stars in the sky, each similar to our
sun, each doubtless surrounded, like our sun, by a family of planets
on which life may be kept in being by the light and heat received from
its sun.” Twentieth-century astronomy suggests, as we shall see, that
the nineteenth century had swung the pendulum too far; life now seems
to be more of a rarity than our fathers thought, or would have thought
if they had given free play to their intellects.
We are setting out to explain the approximation to the truth provided
by twentieth-century astronomy. No doubt it is not the final
truth, but it is a step on towards it, and unless we are greatly
in error it is very much nearer to the truth than was the teaching
of nineteenth-century astronomy. It claims to be nearer the truth,
not because the twentieth-century astronomer claims to be better at
guessing than his predecessors of the nineteenth-century, but because
he has incomparably more facts at his disposal. Guessing has gone out
of fashion in science; it was at best a poor substitute for knowledge,
and modern science, eschewing guessing severely, confines itself,
except on very rare occasions, to ascertained facts and the inferences
which, so far as can be seen, follow unequivocally from them.
It would of course be futile to pretend that the whole interest of
astronomy centres round the questions just mentioned. Astronomy offers
at least three other groups of interest which may be described as
utilitarian, scientific and aesthetic.
At first astronomy, like other sciences, was studied for mainly
utilitarian reasons. It provided measures of time, and enabled mankind
to keep a tally on the flight of the seasons; it taught him to find
his way across the trackless desert, and later, across the trackless
ocean. In the guise of astrology, it held out hopes of telling him
his future. There was nothing intrinsically absurd in this, for even
to-day the astronomer is largely occupied with foretelling the future
movements of the heavenly bodies, although not of human affairs—a
considerable part of the present book will consist of an attempt
to foretell the future, and predict the final end, of the material
universe. Where the astrologers went wrong was in supposing that
terrestrial empires, kings and individuals formed such important items
in the scheme of the universe that the motions of the heavenly bodies
could be intimately bound up with their fates. As soon as man began
to realise, even faintly, his own insignificance in the universe,
astrology died a natural and inevitable death.
The utilitarian aspect of astronomy has by now shrunk to very modest
proportions. The national observatories still broadcast the time of
day, and help to guide ships across the ocean, but the centre of
astronomical interest has shifted so completely that the remotest of
nebulae arouse incomparably more enthusiasm than “clock-stars,” and the
average astronomer totally neglects our nearest neighbours in space,
the planets, for stars so distant that their light takes hundreds,
thousands, or even millions, of years to reach us.
Recently, astronomy has acquired a new scientific interest through
establishing its position as an integral part of the general body of
science. The various sciences can no longer be treated as distinct;
scientific discovery advances along a continuous front which extends
unbroken from electrons of a fraction of a millionth of a millionth
of an inch in diameter, to nebulae whose diameters are measured
in hundreds of thousands of millions of millions of miles. A gain
of astronomical knowledge may add to our knowledge of physics and
chemistry, and _vice versa_. The stars have long ago ceased to be
treated as mere points of light. Each is now regarded as an experiment
on a heroic scale, a high temperature crucible in which nature herself
operates with ranges of temperature and pressure far beyond those
available in our laboratories, and permits us to watch the results. In
so doing, we may happen upon properties of matter which have eluded the
terrestrial physicist, owing to the small range of physical conditions
at his command. For instance matter exists in nebulae with a density
at least a million times lower than anything we can approach on earth,
and in certain stars at a density nearly a million times greater. How
can we expect to understand the whole nature of matter from laboratory
experiments in which we can command only one part in a million million
of the whole range of density known to nature?
Yet for each one who feels the purely scientific appeal of astronomy,
there are probably a dozen who are attracted by its aesthetic appeal.
Many even of those who seek after knowledge for its own sake, driven by
that intellectual curiosity which provides the fundamental distinction
between themselves and the beasts, find their main interest in
astronomy, as the most poetical and the most aesthetically gratifying
of the sciences. They want to exercise their faculties and imaginations
on something remote from everyday trivialities, to find an occasional
respite from “the long littleness of life,” and they satisfy their
desires in contemplating the serene immensities of the outer universe.
To many, astronomy provides something of the vision without which the
people perish.
Before proceeding to describe the results of the modern astronomer’s
survey of the sky, let us try to envisage in its proper perspective the
platform from which his observations are made.
Later on, we shall see how the earth was born out of the sun, something
like two thousand millions of years ago. It was born in a form in which
we should find it hard to recognise the solid earth of to-day with its
seas and rivers, its rich vegetation and overflowing life. Our home in
space came into being as a globe of intensely hot gas on which no life
of any kind could either gain or retain a foothold.
Gradually this globe of gas cools down, becoming first liquid, then
plastic. Finally its outer crust solidifies, rocks and mountains
forming a permanent record of the irregularities of its earlier plastic
form. Vapours condense into liquids, and rivers and oceans come into
being, while the “permanent” gases form an atmosphere. Gradually the
earth assumes a condition suited to the advent of life, which finally
appears, we know not how, whence or why.
It is not easy to estimate the time since life first appeared on
earth, but it can hardly have been more than a small fraction of the
whole 2000 million years of the earth’s existence. Still, there was
probably life on earth at least 300 million years ago. The first life
appears to have been wholly aquatic, but gradually fishes changed into
reptiles, reptiles into mammals, and finally man emerged from mammals.
The evidence favours a period of about 300,000 years ago for this last
event. Thus life has inhabited the earth for only a fraction of its
existence, and man for only a tiny fraction of this fraction. To put
it in another way, the astronomical time-scale is incomparably longer
than the human time-scale—the generations of man, and even the whole
of human existence, are only ticks of the astronomer’s clock.
Most of the 10,000 or so of generations of men who connect us up with
our ape-like ancestry must have lived lives which did not differ
greatly from those of their animal predecessors. Hunting, fishing and
warfare filled their lives, leaving but little time or opportunity for
intellectual contemplation. Then, at last, man began to awake from his
long intellectual slumber, and, as civilisation slowly dawned, to feel
the need for occupations other than the mere feeding and clothing of
his body. He began to discover revelations of infinite beauty in the
grace of the human form or the play of light on the myriad-smiling
sea, which he tried to perpetuate in carefully chiselled marble or
exquisitely chosen words. He began to experiment with metals and herbs,
and with the effects of fire and water. He began to notice, and try
to understand, the motions of the heavenly bodies, for to those who
could read the writing in the sky, the nightly rising and setting of
the stars and planets provided evidence that beyond the confines of the
earth lay an unknown universe built on a far grander scale.
In this way the arts and sciences came to earth, bringing astronomy
with them. We cannot quite say when, but compared even with the age of
the human race, they came but yesterday, while in comparison with the
whole age of the earth, their age is but a twinkling of the eye.
Scientific astronomy, as distinguished from mere star-gazing, can
hardly claim an age of more than 3000 years. It is less than this since
Pythagoras, Aristarchus and others explained that the earth moved
around a fixed sun. Yet the really significant figure for our present
purpose is not so much the time since men began to make conjectures
about the structure of the universe, as the time since they began
to unravel its true structure by the help of ascertained fact. The
important length of time is that which has elapsed since that evening
in 1610 when Galileo first turned his telescope on to Jupiter—a mere
three centuries or so.
We begin to grasp the true significance of these round-number estimates
when we re-write them in tabular form. We have:
Age of earth about 2,000,000,000 years
Age of life on earth ” 300,000,000 ”
Age of man on earth ” 300,000 ”
Age of astronomical science ” 3,000 ”
Age of telescopic astronomy ” 300 ”
When the various figures are displayed in this form we see what a very
recent phenomenon astronomy is. Its total age is only a hundredth part
of the age of man, only a hundred-thousandth part of the time that life
has inhabited the earth. During 99,999 parts out of the 100,000 of its
existence, life on earth was hardly concerned about anything beyond
the earth. But whereas the past of astronomy is to be measured on the
human time-scale, a hundred generations or so of men, there is every
reason to expect that its future will be measured on the astronomical
time-scale. We shall discuss the probable future stretching before the
human race in a later chapter. For the moment it is not unreasonable to
suppose that this future will probably be terminated by astronomical
causes, so that its length is to be measured on the astronomical
time-scale. As the earth has already existed for 2000 million years,
it is _à priori_ reasonable to suppose that it will exist for at
least something of the order of 2000 million years yet to come, and
humanity and astronomy with it. Actually we shall find reasons for
expecting it to last far longer than this. But if once it is conceded
that its future life is to be estimated on the astronomical time-scale,
no matter in what exact way, we see that astronomy is still at the
very opening of its existence. This is why its message can claim no
finality—we are not describing the mature convictions of a man, so much
as the first impressions of a new-born babe which is just opening its
eyes. Even so they are better than the idle introspective dreamings in
which it indulged before it had learned to look around itself and away
from itself.
And so we set out to learn what astronomy has to tell us about the
universe in which we live our lives. Our inquiry will not be entirely
limited to this one science. We shall call upon other sciences,
physics, chemistry and geology, as well as the more closely allied
sciences of astrophysics and cosmogony, to give help, when they can, in
interpreting the message of observational astronomy. The information we
shall obtain will be fragmentary. If it must be compared to anything,
let it be to the pieces of a jig-saw puzzle. Could we get hold of
all the pieces, they would, we are confident, form a single complete
consistent picture, but many of them are still missing. It is too much
to hope that the incomplete series of pieces we have already found will
disclose the whole picture, but we may at least collect them together,
arrange them in some sort of methodical order, fit together pieces
which are obviously contiguous, and perhaps hazard a guess as to what
the finished picture will prove to be when all its pieces have been
found and finally fitted together.
CHAPTER I
_Exploring the Sky_
We have seen how man, after inhabiting the earth for 300,000 years,
has within the last 300 years—the last one-thousandth part of his
life on earth—become possessed of an optical means of studying the
outer universe. In the present chapter we shall try to describe the
impressions he has formed with his newly-awakened eyes. The description
will be arranged in a very rough chronological order. This is also an
order of increasing telescopic power, or again of seeing further and
further into space, so that our order of arrangement might equally be
described as one of increasing distance from the sun. We shall not
attempt any sort of continuous record, but shall merely mention a few
landmarks so as to shew in broad outline the order in which territory
was won and consolidated in man’s survey of the universe.
THE SOLAR SYSTEM
We may conveniently start with the solar system, the structure of which
was unravelled by Galileo and his successors.
The sun’s family of planets falls naturally into distinct groups. Near
to the sun are the four small planets, Mercury, Venus, the Earth and
Mars. At much greater distances are the four great planets, Jupiter,
Saturn, Uranus and Neptune. Beyond all these is the newly discovered
planet Pluto, the outermost member of our system so far known.
Mercury is nearest of all to the sun; next comes Venus. The orbits
of these two planets lie between the earth’s orbit and the sun. As
seen from the earth, these planets appear to describe relatively small
circles round the sun, and so must necessarily appear near to the sun
in the sky. As a consequence, they can only be seen either in the early
morning, if they happen to rise just before the sun, or in the evening
if they set after the sun. The ancients not altogether recognising that
the same planets could appear both as morning and evening stars, gave
them different names according as they figured as the one or the other.
As a morning star Venus was called Phosphoros by the Greeks and Lucifer
by the Romans; as an evening star it was called Hesperus by both.
Next beyond the earth, proceeding outward from the sun into space,
comes Mars, completing the group of small planets. Mars, Venus and
Mercury are all smaller than the earth in size, although Venus is only
slightly so.
There is a wide gap between the orbit of Mars, the last of the small
planets, and that of Jupiter, the first of the great planets. This is
not empty; it is occupied by the orbits of thousands of tiny planets
known as asteroids. None of these approaches the earth in size; Ceres,
the largest, is only 480 miles in diameter, and only four are known
with diameters of more than 100 miles. The planets Mercury, Venus and
Mars have all been known from remote antiquity, but the asteroids only
entered astronomy with the nineteenth-century, Ceres, the first and
largest, having been discovered by Piazzi on January 1, 1801.
Beyond the asteroids come the four great planets Jupiter, Saturn,
Uranus and Neptune, all of which are far larger than the earth.
Jupiter, the largest, has, according to Sampson, a diameter of 88,640
miles, or more than eleven times the diameter of the earth; fourteen
hundred bodies of the size of the earth could be packed inside Jupiter,
and leave room to spare. Saturn, which comes next in order, is second
only to Jupiter in size, having a diameter of about 70,000 miles. These
two are by far the largest of the planets.
Uranus and Neptune have each about four times the diameter, and so
about sixty-four times the volume, of the earth. The size of Pluto is
not yet known with accuracy, but it can hardly be larger than the earth
and is probably considerably smaller.
Jupiter and Saturn form such conspicuous objects in the sky that they
have necessarily been known from the earliest times, but Uranus and
Neptune are comparatively recent discoveries. Sir William Herschel
discovered Uranus quite accidentally in 1781, while looking through
his telescope with no motive other than the hope of finding something
interesting in the sky. By contrast, Neptune was discovered in 1846 as
the result of intricate mathematical calculations, which many at the
time regarded as the greatest triumph of the human mind, at any rate
since the time of Newton. It was a triumph of youth. The honour must
be apportioned in approximately equal shares between an Englishman,
John Couch Adams, then only 27 years old, who was afterwards Professor
of Astronomy at Cambridge, and a young French astronomer, Urbain J. J.
Leverrier, who was only eight years his senior. Both attributed certain
vagaries in the observed motion of Uranus to the gravitational pull
of an exterior planet, and both set to work to calculate the orbit in
which this supposed outer planet must move to explain these vagaries.
Adams finished his calculations first, and informed observers at
Cambridge as to the part of the sky in which the new planet ought to
lie. As a result, Neptune was observed twice, although without being
immediately identified as the wanted planet. Before this identification
had been established at Cambridge, Leverrier had finished his
computations and communicated his results to Galle, an assistant at
Berlin, who was able to identify the planet at once, Berlin possessing
better star-charts of the region of the sky in question than were
accessible at Cambridge.
Gradually it emerged that the gravitational pull of Neptune was
inadequate to account for all the vagaries in the motions of Uranus,
while similar vagaries began to appear in Neptune’s own motion. This
pointed to the existence of yet another planet, further out even than
Neptune. Just as Adams and Leverrier had done on the former occasion,
so Dr Percival Lowell, of Flagstaff Observatory, Arizona, computed the
orbit in which the conjectured new planet, “Planet X,” ought to move,
but it was only recently (March 1930), after many years of careful
search, that the Flagstaff observers discovered the planet Pluto,
moving in almost precisely the orbit which Lowell had predicted fifteen
years previously.
As far back as 1772, Bode had pointed out a simple numerical relation
connecting the distances of the various planets from the sun. This is
obtained as follows: Write first the series of numbers
0 1 2 4 8 16 32 64 128 256
in which each number after the first two is double the preceding.
Multiply each by three, thus obtaining
0 3 6 12 24 48 96 192 384 768
and add four to each, giving
4 7 10 16 28 52 100 196 388 772
These numbers are very approximately proportional to the actual
distances of the planets from the sun, which are (taking the earth’s
distance to be 10):
Mercury 3·9
Venus 7·2
Earth 10·0
Mars 15·2
Asteroids 26·5
Jupiter 52·0
Saturn 95·4
Uranus 191·9
Neptune 300·7
Pluto 400
The law was enunciated before Uranus and the asteroids had been
discovered, so that it is somewhat remarkable that these fit so
well into their predicted places. On the other hand, the law fails
completely for Neptune and the newly discovered Pluto, so that it seems
more than likely that it is a mere coincidence with no underlying
rational explanation.
The outermost planets are at enormous distances from the sun.
An inhabitant of Pluto, if such existed, would receive only a
sixteen-hundredth part as much light and heat from the sun as an
inhabitant of the earth receives. It can be calculated that if Pluto’s
surface were warmed only by the heat of the sun, it would be at a
very low temperature indeed, somewhere in the neighbourhood of -230°
Centigrade, or more than 400 degrees of frost on the Fahrenheit scale.
A telescope collects heat as well as light. Not only is the
heat-gathering power of a large telescope tremendous, but extremely
sensitive instruments have been designed to measure this heat.
The 100-inch telescope at Mount Wilson is said to be capable of
detecting the heat received from a single candle on the banks of the
Mississippi, 2000 miles away. This great sensitiveness has made
it possible to measure the infinitesimal amounts of heat received
from single stars and planets, and so to estimate the temperatures
of their surfaces. Recent measurements indicate that the surface of
Jupiter is at a temperature of about -150° Centigrade, which is just
about that at which it would be maintained by the sun’s heat alone.
On the other hand similar measurements assign temperatures of -150°
and -170° respectively to Saturn and Uranus, both of which are rather
higher than would be expected if these planets had no source of heat
beyond the sun’s radiation. But it seems clear that any sources of
internal heat must be quite small, and that all the major planets
are very cold indeed. There can be neither seas nor rivers on their
surfaces, since all water must be frozen into ice, neither can there
be rain or water-vapour in their atmospheres. It has been suggested
that the clouds which obscure our view of Jupiter’s surface may be
condensed particles of carbon-dioxide, or some other gas which boils at
temperatures far below the freezing point of water.
The physical conditions of the smaller planets are much more like those
with which we are familiar on earth. Owing to its greater distance from
the sun, Mars is somewhat, but not enormously, colder than the earth.
Its day of 24 hours 37 minutes is only slightly longer than our own,
so that its surface must experience alternations of warmth by day and
cold by night similar to those we find on earth. In the equatorial
regions the temperature rises well above the freezing point at noon,
occasionally reaching 50° Fahrenheit or even more. But even here it
falls below freezing some time before sunset, and from then until well
on in the next day, the climate must be very cold. The polar regions
are of course colder still, the temperature of the snowcap which covers
the poles being somewhere about -70° Centigrade or -94° Fahrenheit—126
degrees of frost!
Venus, being nearer the sun, must have a higher average temperature
than the earth. But as each of its days and nights is several days of
our terrestrial time, the difference between the temperatures of day
and night must be far greater than with us, so that its surface must
experience great extremes of heat by day and of cold by night. The
night temperature appears to be fairly uniformly equal to about -25°
Centigrade or -13° Fahrenheit. At any point on the planet’s surface
weeks of this bitterly cold night temperature must alternate with weeks
of a roasting day temperature.
Mercury is so near the sun that its average temperature is necessarily
far higher than that of the earth. It reflects only a tiny
fraction—about a fourteenth—of the light and heat it receives from
the sun. All the rest goes to heating up its surface. A number of
considerations make it likely that the planet always turns the same
face to the sun, just as the moon always turns the same face to the
earth. If so the unwarmed half of its surface must be intensely cold,
and the warmed half intensely hot. It can be calculated that in this
case the warmed hemisphere ought to have a temperature of about 357°
Centigrade; if however the planet was in fairly rapid rotation, its
whole surface would have a temperature of only about 170° Centigrade.
Quite recently Pettit and Nicholson have measured the amount of heat
received on earth from the warmed hemisphere, and find that its
temperature must be about 350° Centigrade or 662° Fahrenheit, thus
confirming that the planet always turns the same face to the sun.
Its warm hemisphere is at a temperature which melts lead; the other
hemisphere, eternally dark and unwarmed, is probably colder than
anything we can imagine.
[Illustration: PLATE I _Franklin-Adams Chart_
The Milky Way in the neighbourhood of the Southern Cross]
Galileo’s discovery of the four satellites of Jupiter was followed in
time by the discovery that every planet was attended by satellites,
except the two whose orbits lay inside the earth’s. In 1655 Huyghens
discovered Titan, the largest of Saturn’s satellites, and by 1684
Cassini had discovered four more. Then, after the lapse of a full
century, Sir William Herschel discovered two satellites of Uranus in
1787 and two more satellites of Saturn in 1789. We shall discuss the
full system of planetary satellites and also the smaller bodies of the
solar system—comets, meteors and shooting-stars—in a later chapter,
when we come to deal with the way they came into being.
THE GALACTIC SYSTEM
Our next landmark is the survey of the stars by the two Herschels, Sir
William Herschel, the father (1738-1822) and Sir John Herschel, the
son (1792-1871). What Galileo had done for the solar system, the two
Herschels set out to do for the huge family of stars—the “galactic”
system, bounded by the Milky Way—of which our sun is a member.
On a clear moonless night the Milky Way is seen to stretch, like a
great arch of faint light, from horizon to horizon. It is found to be
only part of a full circle of light—the galactic circle—which stretches
completely round the earth and divides the sky into two equal halves,
forming a sort of celestial “equator,” with reference to which
astronomers are accustomed to measure latitude and longitude in the
sky. Galileo’s telescope had shewn that it consists of a crowd of faint
stars, each too dim to be seen individually without telescopic aid (see
Plate I). And, as might be expected, the proper interpretation of this
great belt of faint stars has proved to be fundamental in understanding
the architecture of the universe.
If stars were scattered uniformly through infinite space, we should at
last come to a star in whatever direction we looked, so that the sky
would appear as a uniform blaze of intolerable light. It is true that
this would not be the case if light were dimmed or blotted out after
travelling a certain distance, but even then, the sky would appear the
same in all directions, for there would be no reason why one part of
the sky should be more lavishly spangled with stars than another. Thus
the existence of the Milky Way shews that the system of the stars does
not extend uniformly to infinity. It must have a definite structure,
and it was the architecture of this that Sir William Herschel set
himself to unravel. The work he did for the northern half of the sky
was subsequently extended to the southern hemisphere by his son, Sir
John Herschel.
We shall best understand the method employed by the Herschels if we
first imagine all the stars in the sky to be intrinsically similar
objects. Each would then emit the same amount of light, so that
the nearer stars would appear bright, and the further stars faint,
merely as an effect of distance. The way in which apparent brightness
decreases with distance is of course well known; the law is that of
the “inverse square of the distance,” which means that the apparent
brightness decreases just as rapidly as the square of its distance
increases; a star which is twice as distant as a second similar star
appears only a quarter as bright, and so on. Thus if all stars emitted
the same amount of light, we could estimate the relative distances of
any two stars in the sky from their relative brightnesses. By cutting
wires of lengths proportional to the distances of various stars, and
pointing these in the directions of the stars to which they referred,
we could form a model of the arrangement of the stars in the sky. We
should, in fact, know the whole structure of the system of stars except
for its scale. To represent the faint stars of the Milky Way, a great
number of very long wires would be needed. In the model these would
all point towards different parts of the Milky Way, forming a flat
wheel-like structure.
The problem which confronted Sir William Herschel was more intricate
because he knew that the stars were of different intrinsic brightness
as well as at different distances, and both factors combined to produce
differences of apparent brightness. One of the main difficulties of
astronomy, both to the Herschels and to the astronomer of to-day, is
that these two factors have to be disentangled before any definite
conclusions are reached.
Herschel found that the number of stars visible in his telescope-field
varied enormously with different directions in space. It was of course
greatest when the telescope was pointed at the Milky Way, and fell
off, steadily and rapidly, as the telescope was moved away from the
Milky Way. Generally speaking, two telescope-fields which were at equal
distances from the Milky Way contained about the same number of stars.
In the technical language of astronomy, the richness of the star-field
depended mainly on the galactic latitude, just as the earth’s climate
depends mainly on the geographic latitude, and not to any great extent
on the longitude.
Fields at different distances from the Milky Way were found to differ
in quality as well as in number of stars. The brightest stars of all
occurred about equally in all fields, the difference in the fields
resulting mainly from faint stars, and particularly the faintest
stars of all, becoming enormously more abundant as the Milky Way was
approached.
Sir William Herschel rightly interpreted this as shewing that the
system of stars surrounding the sun began to thin out within distances
reached by his telescope, and that they began to thin out soonest in
directions furthest away from the Milky Way. He supposed the general
shape of the galactic system of stars to be that of a bun or a biscuit
or a watch, the stars being most thickly scattered near the centre, and
occurring more sparsely in the outer regions. The plane of the Milky
Way of course formed the central plane of the structure. The fact that
the Milky Way divides the sky into two almost exactly equal halves
suggested to him that the sun must be very nearly in this central
plane, and this is confirmed by the recent very refined investigations
of Seares and van Rhijn, and others. From the fact that parts of the
sky which were equidistant from the Milky Way appeared about equally
bright, Herschel inferred that the sun not only lay in the central
plane of the system, but was very near to its actual centre. This view
has prevailed until quite recently, but the researches of Shapley and
others now shew it to be untenable (see p. 65 below).
Fig. 1 shews a cross-section of the general kind of structure which
Sir William Herschel assigned to the galactic system, although the
detailed distribution of stars shewn in the diagram is that given at
a much later date (1922) by Kapteyn. It is easy to see how a structure
of this type would account for the general appearance of the sky.
Those stars which appear brightest of all are, generally speaking,
the nearest; they are so near that no appreciable thinning out of
stars occurs within this distance. For this reason the very bright
stars occur in about equal numbers in all directions. The stars which
appear very faint are mostly very distant, so distant that the great
depth of the system in directions in or near to the galactic plane is
brought into play. In such directions, layer after layer of stars,
ranged almost endlessly one behind the other, give rise to the apparent
concentration of faint stars which we call the Milky Way.
[Illustration: Fig. 1. The Structure of the Galactic System according
to Herschel and Kapteyn.]
The final acceptance of the Copernican view of the structure of the
solar system was in a large measure due to Galileo’s discovery of
the similar system of Jupiter, which was so situated in space that a
terrestrial observer could obtain a bird’s-eye view of it as a whole.
We can never obtain a bird’s-eye view of the solar system as a whole
because we can only see it from inside, so that optical proof that
such systems could exist, could come only from the discovery of other
similar systems, which we could see from outside.
Sir William Herschel believed he had confirmed his own view of the
structure of the galactic system in the same way, by discovering
similar systems, of which he could obtain a bird’s-eye view because
they were entirely extraneous to the galaxy. He spoke of these objects
as “island universes” and believed them to be clouds of stars. They
were of hazy nebular appearance, and although it was impossible to
distinguish the separate stars in them, he believed that sufficient
telescopic power would make this possible, just as it had enabled
Galileo to see the separate stars in the Milky Way. These objects,
which we shall describe almost immediately, are generally known as
“extra-galactic nebulae“ from their position, although we shall
frequently find it convenient to use the briefer term “great nebulae,”
to which their immense size fully entitles them.
NEBULAE
A telescope exhibits a planet as a disc of appreciable size, and an
eye-piece which magnifies 60 times will make Jupiter look as large as
the moon. Yet an eye-piece which magnifies 60 times, or any greater
number of times, can never make a star look as large as the moon. No
magnification within our command causes any star to appear as anything
other than a mere point of light. The stars are of course enormously
larger than Jupiter, but they are also enormously more distant, and it
is the distance that wins.
[Illustration: PLATE II N.G.C. 2022 N.G.C. 6720 N.G.C. 1501 N.G.C. 7662
_Mt Wilson Observatory_ Planetary Nebulae]
[Illustration: PLATE III _Mt Wilson Observatory_ The Nebula in Cygnus]
The telescope nevertheless shews a number of objects which appear
bigger than mere points of light. They are generally of a faint,
hazy appearance, and so have received the general name of “nebulae.”
Detailed investigation has shewn that these fall into three distinct
classes.
PLANETARY NEBULAE. The first class are generally described as
“Planetary Nebulae.” There is nothing of a planetary nature about them
beyond the fact that, like the planets, they shew as finite discs in
a telescope. A few hundreds only of these objects are known, four
typical examples being illustrated in Plate II. They all lie within
the galactic system. We shall discuss their physical structure below
(p. 321). For the moment, it is enough to say that they are probably
of the nature of stars which have in some way become surrounded by
luminous atmospheres of enormous extent. If so they of course disprove
our general statement that no star ever appears as anything but a point
of light in a telescope; we must make an exception in favour of the
planetary nebulae.
GALACTIC NEBULAE. The second class are generally described as “Galactic
Nebulae,” examples being shewn in Plates III, VI (p. 37) and VII (p.
44). These are completely irregular in shape. Their general appearance
is that of huge glowing wisps of gas stretching from star to star,
and in effect this is pretty much what they are. Like the planetary
nebulae, they lie entirely within the galactic system. Even a cursory
glance shews that each irregular nebula contains several stars enmeshed
with it; minute telescopic examination often extends the dimensions of
the nebula almost indefinitely, so that we may have almost the whole of
a constellation wrapped up in a single nebula.
There is but little doubt as to the physical nature of these nebulae.
The space between the stars is not utterly void of matter, but is
occupied by a thin cloud of gas of a tenuity which is generally almost
beyond description. Here and there this cloud may be denser than usual;
here and there again it may be lighted up and made to incandesce by the
radiation of the stars within it. In other places it may be entirely
opaque to light, lying like a black curtain across the sky. The
variations of density, opacity and luminosity in combination produce
all the fantastic shapes and varied degrees of light and shade we see
in the galactic nebulae.
This same opacity is responsible for the dark patches which occur in
the general arrangement of the stars. A conspicuous example occurs in
the part of the Milky Way shewn in Plate I (p. 23). The dark patch,
which looks at first like a hole in the system of stars, is graphically
described as “The Coal Sack.” These black patches in the sky cannot
represent actual holes, because it is inconceivable that there should
be so many empty tunnels through the stars all pointing exactly
earthward, so that we are compelled to interpret them as veils of
obscuring matter which dim or extinguish the light of the stars behind
them.
EXTRA-GALACTIC NEBULAE. The third class of nebula is of an altogether
different nature. Its members are for the most part of definite and
regular shape, and shew various other characteristics which make them
easy of identification. They used to be called “white nebulae” from
the quality of the light they emitted. Later Lord Rosse’s giant 6-foot
telescope revealed that many of them had a spiral structure; these were
called “spiral nebulae.” The most conspicuous of all the spiral nebulae
is the Great Nebula _M_ 31 in Andromeda, shewn in Plate IV, which is
just, and only just, visible to the naked eye. Marius, observing it
telescopically in 1612, described it as looking “like a candle-light
seen through horn.” Plate V shews a second example, probably of very
similar structure, which is viewed from another angle, so as to appear
almost exactly edge-on.
[Illustration: PLATE IV _Yerkes Observatory_
The Great Nebula _M_ 31 in Andromeda]
[Illustration: PLATE V _Mt Wilson Observatory_
The Nebula N.G.C. 891 in Andromeda seen edge-on]
It is now abundantly proved that nebulae of this type all lie outside
the galactic system, so that the term “extra-galactic nebulae”
adequately describes them. Their size is colossal. Either of the
photographs shewn in Plates IV and V would have to be enlarged to the
size of the whole of Europe before a body of the size of the earth
became visible in it, even under a powerful microscope. Their general
shape is similar to that which Sir William Herschel assigned to the
galactic system, and it was this that originally led him to regard them
as “island universes” similar to the galactic system. We shall see
later how far his conjecture has been confirmed by recent research.
THE DISTANCES OF THE STARS
The year 1838 provides our next landmark; it is the year in which the
distance of a star was first measured.
In the second century after Christ, Ptolemy had argued that if the
earth moved in space, its position relative to the surrounding stars
must continually change. As the earth swung round the sun, its
inhabitants would be in the position of a child in a swing. And,
just as the swinging child sees the nearer trees, persons and houses
oscillating rhythmically against a remote background of distant hills
and clouds, so the inhabitants of the earth ought to see the nearer
stars continually changing their position against their background of
more distant stars. Yet night after night the constellations remained
the same, or so Ptolemy argued; the same stars circled eternally in
the same relative positions around the pole, and conspicuous groups of
stars such as the seven stars of the Great Bear, the Pleiades or the
constellation of Orion shewed no signs of change. For aught the unaided
human eye could tell, the stars might be spots of luminous paint on a
canvas background, with the earth as the unmoving pivot around which
the whole structure swung.
In opposition to this, the Copernican theory of course required that
the nearer stars should be seen to move against the background of the
more distant stars, as the earth performed its yearly journey round
the sun. Yet year after year, and even century after century, passed
without any such motion being detected. The old Ptolemaic contention
that the earth formed the fixed centre of the universe might almost
have regained its former position, had it not been that various
lines of evidence had begun to shew that even the nearest stars were
necessarily very distant, so distant, indeed, that their apparent want
of motion need cause no surprise. The child in a swing cannot expect to
have optical evidence of its motion if the nearest object it can see is
twenty miles away.
Very few stars appear brighter than Saturn at its brightest; it looks
about as bright as Altair, the eleventh brightest star in the sky.
Yet Saturn shines only by the light it reflects from the sun, and its
distance from the sun is such that it receives only about one part in
2500 million of the total light emitted by the sun. And, as the surface
of Saturn only reflects back about two-fifths of the light it receives,
it follows that Saturn shines with only a 6000 millionth part of the
light of the sun. If, as Kepler and others had maintained, Altair was
essentially similar to the sun, it would probably be of about the same
candle-power as the sun, and so would give out about 6000 million times
as much light as Saturn. The fact that Altair and Saturn appear about
equally bright in the sky can only mean that Altair is 80,000 times as
distant as Saturn[1]. This argument is essentially identical with one
which Newton gave in his _System of the World_ to shew that even the
brightest stars, such as Altair, must be very distant indeed.
[1] For the apparent brightness of an object falls off as the inverse
square of its distance, and the square of 80,000 is approximately equal
to 6000 million.
And such has proved to be the case. All efforts to discover the
apparent swinging motion of the stars—“parallactic motion,” as it is
technically called—which results from the earth’s orbital motion failed
until 1838, when three astronomers, Bessel, Henderson, and Struve,
almost simultaneously detected the parallactic motions of the three
stars, 61 Cygni, α Centauri and α Lyrae respectively. The amount of
their parallactic motion made it possible to calculate the distances of
the stars, so that the inhabitants of the earth were not only placed
in possession of definite ocular proof that they were swinging round
the sun, but from the visible effects of this swing they were able to
compute the distances of the nearer stars. The calculated values were
not accurate when judged by modern standards, but they provided the
first definite estimates of the scale on which the universe is built.
Let us pause for a minute to consider how this scale is built up. The
first step is to select a convenient base-line a few miles in length
on the surface of the earth, and to measure this in terms of standard
yards or metres. Starting out from this base-line, a geodetic survey
maps out a long narrow strip of the earth’s surface, preferably running
due north and south. The difference of latitude at the two ends is
then measured by astronomical methods, as for instance by noticing the
difference in the altitude of the pole-star at the two places. As the
length of the strip is already known in miles, this immediately gives
the dimensions of the earth. According to Hayford (1909), the earth’s
equatorial radius is 6378·388 kilometres, or 3963·34 miles, its polar
radius being 6356·909 kilometres or 3949·99 miles.
The next step is to determine the size of the solar system in terms
of that of the earth. When the sun is eclipsed by the moon, the time
at which the moon first begins to cover the sun’s disc is different
for different stations on the earth’s surface, and the observed
differences of time enable us to measure the moon’s distance in terms
of known distances on the surface of the earth. In this way the mean
distance of the moon is found to be 384,403 kilometres or 238,857
miles. In the same way the transit of the planet Venus across the disc
of the sun provides an opportunity for determining the scale of the
solar system in terms of the dimensions of the earth. The asteroid
Eros provides still better opportunities. The Paris Conference (1911)
adopted 149,450,000 kilometres, or 92,870,000 miles, as the most likely
value for the mean distance of the earth from the sun. The next and
final step, which the year 1838 saw accomplished, is that of using
the diameter of the earth’s orbit as base-line, and determining the
distances of the stars.
The first step, from the standard yard or metre to the measured
base-line on the earth’s surface, involves an increase of several
thousand-fold in length. The increase involved in the next step, from
the base-line to the earth’s diameter, is again one of thousands.
And again the next step, from the diameter of the earth to that of
the earth’s orbit, involves an increase of thousands. But the last
step of all, from the earth’s orbit to stellar distances, involves a
million-fold increase.
Recent measurements shew that the nearest stars are at almost exactly
a million times the distances of the nearest planets. At its nearest
approach to the earth, Venus is 26 million miles distant, while the
nearest star, Proxima Centauri, is 25,000,000 million miles away;
this latter star is a faint companion of the well-known bright star α
Centauri in the southern hemisphere. The distances of the planets when
at their nearest, and of the nearest stars, are shewn in the following
table:
------------------+------------------------------------------------+
PLANETS | STARS |
-------+----------+-------------------+-------------------+--------+
| | | |Distance|
Name | Distance | Name | Distance |(light- |
| (miles) | | (miles) | years)|
-------+----------+-------------------+-------------------+--------+
Venus |26,000,000| { Proxima Centauri| 25,000,000 million| 4·27 |
| | { α Centauri | | 4·31 |
| | | | |
Mars |35,000,000| Munich 15040 | 36,000,000 ” | 6·06 |
| | | | |
| | { Wolf 359 | 47,000,000 ” | 8·07 |
Mercury|47,000,000| { Lalande 21185 | 49,000,000 ” | 8·33 |
| | { Sirius | 51,000,000 ” | 8·65 |
-------+----------+-------------------+-------------------+--------+
As it is almost impossible to visualise a million, the mere statement
that the stars are a million times as remote as the planets gives only
a feeble indication of the immensity of the gap that divides the solar
system from its nearest neighbours in space. Perhaps the apparent
fixity of the stars gives a more vivid impression.
The earth performs its yearly journey round the sun at a speed of about
18½ miles a second, which is about 1200 times the speed of an express
train. The sun moves through the stars at nearly the same rate—to be
precise, at about 800 times the speed of an express train. And, broadly
speaking, the nearer planets and the majority of the stars move with
similar speeds. We shall not obtain a bad approximation to the truth if
we imagine that all astronomical bodies move with exactly equal speeds,
let us say, to fix our thoughts, a speed equal to 1000 times the speed
of an express train. The distances of astronomical objects are now
betrayed by the speed with which they appear to move across the sky—the
slower their apparent motion the greater their distances, and _vice
versa_. Now the planets move across the sky so rapidly that it is quite
easy to detect their motion from night to night and even from hour to
hour; the stars move so slowly that, except with telescopic aid, no
motion can be detected from generation to generation, or even from
age to age. Even the conspicuous constellations in the sky, which on
the whole are formed of the nearer stars, have retained their present
appearance throughout the whole of historic times. The contrast between
the planets which change their positions every hour, and the stars
which fail to shew any appreciable change in a century, gives a vivid
impression of the extent to which the stars are more distant than the
planets.
It is far more difficult to visualise the actual distances of the
stars. The statement that even the nearest of them is 25,000,000
million miles away hardly conveys a definite picture to the mind, but
we may fare better with the alternative statement that the distance is
4·27 light-years—that is to say the distance that light, travelling at
186,000 miles a second, takes in 4·27 years to traverse.
[Illustration: PLATE VI _Mt Wilson Observatory_
The “Horse’s Head” in the Great Nebula in Orion]
Light travels with the same speed as wireless signals because both are
waves of electric disturbance. Incidentally this speed is just about
a million times that of sound. The enormous disparity in the speeds
of sound and of electric waves is vividly brought out in the ordinary
process of broadcasting. When a speaker broadcasts from London his
voice takes longer to travel 3 feet from his mouth to the microphone
as a sound wave, than it does to travel a further 560 miles to Berlin
or Milan as an electric wave. Wireless listeners in Australia hear the
music of a broadcast concert sooner than an ordinary listener at the
back of the concert hall who relies on sound alone; they hear it a
fifteenth of a second after it is played. Yet light, or wireless waves
travelling with the same speed as light, takes 4·27 years to reach the
nearest star, so that the inhabitants of Proxima Centauri would be over
four and a quarter years late in hearing a terrestrial concert. And
in time we shall have to consider other and even more distant stars
which terrestrial music would not yet have reached had it started on
its journey before the Norman Conquest, before the Pyramids were built,
even before man appeared on earth.
THE PHOTOGRAPHIC EPOCH
If we were only allowed to select one more landmark in the progress
of astronomy, we might well choose the application of photography
to astronomy in the closing years of the nineteenth-century; this
opened the floodgates of progress more thoroughly than anything
else had done since the invention of the telescope. Hitherto the
telescope, after collecting and bending rays of light from the sky,
had projected the concentrated beam of light through the pupil of the
human eye on to the retina; in future it was to project it on to the
incomparably more sensitive photographic plate. The eye can retain an
impression only for a fraction of a second; the photographic plate
adds up all the impressions it receives for hours or even days, and
records them practically for ever. The eye can only measure distances
between astronomical objects by the help of an intricate machinery
of cross-wires, screws and verniers; the photographic plate records
distances automatically. The eye, betrayed by preconceived ideas,
impatience or hope, can and does make every conceivable type of error;
the camera cannot lie.
And so it comes about that if we try to pick out landmarks in
twentieth-century astronomy we find that, in a sense, it consists of
nothing but landmarks; the slow, arduous methods of conquest of the
nineteenth century have given place to a sort of gold-rush in which
claims are staked out, the surface scratched, the more conspicuous
nuggets collected, and the excavation abandoned for something more
promising, all with such rapidity that any attempt to describe the
position is out of date almost before it can be printed. We can only
attempt a general impression of the new territory, and with this will
be inextricably mixed a discussion of old territory seen in the light
of new knowledge.
GROUPS OF STARS AND BINARY SYSTEMS
A glance at the sky, or, better, at a photograph of a fragment of the
sky, suggests that, in the main, the stars are scattered at random over
the sky, except for the concentration of faint stars in and towards the
Milky Way, which we have already considered. Any small bit of the sky
does not look very different from what it would if bright and faint
stars had been sprinkled haphazard out of a celestial pepperpot.
Yet this is not quite the whole story. Here and there groups of
conspicuous stars are to be seen, which can hardly have come together
purely by accident. Orion’s belt, the Pleiades, Berenice’s hair, even
the Great Bear itself, do not look like accidents, and in point of fact
are not. It is the existence of these natural groups of stars that
lies at the root of, and justifies, the division of the stars into
constellations. We shall explain later how the physical properties
of the stars are studied; for the present it is enough to remark
that physical study confirms the suspicion that groups such as those
just mentioned are, generally speaking, true families, and not mere
accidental concourses, of stars. The stars of any one group, such as
the Pleiades, not only shew the same physical properties, but also have
identical motions through space, thus journeying perpetually through
the sky in one another’s society. As such a group of stars are both
physically similar, and travel in company, they might appropriately be
described as a family of stars. The astronomer, however, prefers to
call them a “moving cluster.”
These families are of almost all sizes, the smallest and commonest
type consisting of only two members. After this the next commonest
type consists of three members; our nearest three neighbours in space,
Proxima Centauri and the two stars of α Centauri, form such a triple
system. Then come systems of four, five and six members, and so on
indefinitely.
Let us first turn our attention to families consisting of only two
members—“binary systems,” as they are generally called. Even if the
stars had been sprinkled on to the sky at random out of a pepperpot,
the laws of chance would require that in a certain number of cases,
pairs of stars should appear very close together. And a study of a
photograph of any star-field shews that a large number of such close
pairs actually exist. The number is, however, greater than can be
explained by the laws of chance alone. Some pairs of stars may be close
together by accident, but a physical cause is needed to account for the
remainder. We can unravel the mystery by photographing the field at
intervals of a few years and comparing the various results obtained.
Some of the stars which originally appeared as close pairs will be
found to move steadily apart. These are the pairs of stars which,
although they appeared close together in the sky, were not so in space;
one star merely happened to be almost exactly in line with the other
as seen from the earth. Other pairs do not break up with the passage
of time; the two components change their relative positions but never
become completely separated. In the simplest case of all one star may
be found to describe an approximately circular orbit about the other,
just as the earth does round the sun, and the moon round the earth, and
for precisely the same reason: gravitation keeps them together.
THE LAW OF GRAVITATION. Drop a cricket ball from your hand and it falls
to the ground. We say that the cause of its fall is the gravitational
pull of the earth. In the same way, a cricket ball thrown into the air
does not move on for ever in the direction in which it is thrown; if it
did it would leave the earth for good, and voyage off into space. It
is saved from this fate by the earth’s gravitational pull which drags
it gradually down, so that it falls back to earth. The faster we throw
it, the further it travels before this occurs; a similar ball projected
from a gun would travel for many miles before being pulled back to
earth.
[Illustration: Fig. 2.]
The law governing all these phenomena is quite simple. It is that the
earth’s gravitational pull causes all bodies to fall 16 feet earthward
in a second. This is true of all bodies which are free to fall, no
matter how they are moving; every body which is not in some way held up
against gravitation is 16 feet lower at the end of any second than it
would have been if gravitation had not acted through that second.
To illustrate what this means, let the big circular curve _BʹAʹCʹ_ in
fig. 2 represent the earth’s surface, and imagine that a shot is fired
horizontally from _A_, the top of an elevation _AAʹ_. If the shot were
not pulled earthwards by gravitation, it would travel indefinitely
along the line _AB_ out into space. If _AB_ is the distance it would
travel in a second under these imaginary conditions, the end of a
second’s actual flight does not find it at _B_, but at a point 16
feet nearer the earth, gravitation having pulled it down this 16 feet
during its flight. For instance, if _BB′_ in fig. 2 should happen to
be 16 feet, the shot would strike the earth at _Bʹ_ after a flight of
precisely one second.
As another example, let us suppose that the 16-foot fall below _B_ does
not drag the shot down to earth but only to a point _b_, which is at
precisely the same height above the earth’s surface as the point _A_
at which the shot started. If gravitation were not acting, so that the
shot travelled along the line _AB_, its height above the earth would
continually increase. Actually in the case we are now considering,
gravitation pulls the shot down at just such a rate as to neutralise
the increase of height which would otherwise occur, so that the height
of the shot neither increases nor decreases; it neither flies off into
space nor drops to earth, but continues to describe circles round the
earth indefinitely.
A simple geometrical calculation shews that for the distance _Bb_ to
be 16 feet, the distance _AB_ travelled in one second must be 25,880
feet or 4·90 miles[2]. Thus if we could fire a shot horizontally with a
speed of 4·90 miles a second, it would describe endless circles round
the earth, the earth’s gravitational pull exactly neutralising the
natural tendency of the shot to fly away along the straight line _AB_.
[2] Let _C_ be the centre of the earth, and _bCD_ the diameter through
_b_. Then
_BA_² = _Bb_ × _BD_,
where _Bb_ = 16 feet, and _BD_, which is 16 feet more than the earth’s
diameter = 41,900,000 feet. From this we readily calculate that _BA_ =
25,880 feet. This calculation of course neglects the height of the hill
_AAʹ_ by comparison with the earth’s diameter.
In 1665 Newton began to suspect that this same gravitational pull
might be the cause of the moon describing a circular orbit around
the earth instead of running away at a tangent into space. The moon’s
distance from the earth’s centre is 238,857 miles, or 60·27 times the
radius of the earth. As the moon describes a circle of this size every
month (27 days, 4 hours, 43 minutes, 11·5 seconds), we can calculate
that its speed in its orbit is 2287 miles an hour. After one second it
will have travelled 3350 feet, and if it kept to a strictly rectilinear
course this would carry it 0·0044 feet further away from the earth.
Thus, to keep in an exact circular orbit around the earth, it must
fall 0·0044 feet in a second. This is far less than a body falls in a
second at the earth’s surface, but Newton conjectured that the force
of gravity must weaken as we recede from the earth’s surface. Actually
a body at the earth’s surface falls 3632 times as fast as the moon’s
earthward fall in its orbit. Now 3632 is the square of 60·27 (or 3632
= 60·27 × 60·27), whence Newton saw that the moon’s fall would be of
exactly the right amount if the force of gravity fell off as the
inverse square of the distance—that is to say, if it decreased just as
rapidly as the square of the distance increased. As we shall see later,
astronomical observation confirms the truth of this law in innumerable
ways. This led Newton to put forward his famous law of gravitation
according to which the gravitational pull of any body, such as the
earth, falls off inversely as the square of the distance from the body.
[Illustration: Fig. 3.]
Professor C. V. Boys and others have measured the gravitational pull
which a few tons of lead exert in the laboratory, and, with this
knowledge, it is easy to calculate how many tons the earth must contain
so as to exert its observed gravitational pull on bodies outside it.
It is found that the earth’s weight must be just under six thousand
million million million tons[3], or, as we shall write it, 6 × 10²¹
tons[4].
[3] Here, as throughout the book, we use the French or metric ton of a
million grammes or 2204·5 lbs. The English ton of 2240 lbs. is equal to
1·0160 French tons.
[4] The notation 6 × 10²¹ stands for the number formed by a 6 followed
by 21 zeros, this shorthand notation being essential, in the interests
of brevity, in discussing astronomical numbers. A million is 10⁶, a
million million is 10¹² and so on.
A similar notation is needed to express very small numbers. The
expression 10⁻²¹ is written for 1/10²¹ and so on. Thus 6 × 10⁻⁶ stands
for 6/1,000,000 or 0·000006.
[Illustration: PLATE VII _Mt Wilson Observatory_
The Trifid Nebula _M_ 20 in Sagittarius]
Just as the earth’s gravitational pull keeps the moon perpetually
describing circles around it, so the sun’s gravitational pull keeps
the earth and all the other planets describing circles around the sun.
Knowing the distance of any planet from the sun, and also its speed in
its orbit, we can calculate the distance this planet falls towards the
sun in a second. This tells us the amount of the sun’s gravitational
pull, and from this we can calculate that the sun’s weight must be
about 332,000 times the weight of the earth, or almost exactly 2 ×
10²⁷ tons. Whichever of the planets we use, we obtain exactly the same
weight for the sun. This not only gives us confidence in our result,
but incidentally it also provides striking confirmation of the truth of
Newton’s law of gravitation, for if this law were inexact or untrue,
the different planets would not all tell exactly the same story as
to the sun’s weight. Einstein has recently shewn that the law is not
absolutely exact, but the amount of inexactness is inappreciable except
for the nearest planet, Mercury, and even here it is so exceedingly
small that we need not trouble about it for our present purpose.
Just as we can weigh the sun and earth by studying the motion of a
body gripped by their gravitational pull—or “in their gravitational
fields,” as the mathematician would say—so we can weigh any other body
which keeps a second small body moving round it by its gravitational
attraction. The motions of Jupiter’s satellites make it possible to
weigh Jupiter; its weight is found to be about 1·92 × 10²⁴ tons, which
is 317 times that of the earth, although only ¹/₁₀₄₇ of that of the
sun. Similarly the weight of Saturn is found to be 5·71 × 10²³ tons or
about 94·9 times that of the earth.
WEIGHING THE STARS. And now we come to a striking application of
the principles just explained—when we observe two stars in the sky
describing orbits about one another, we can weigh the stars from a
study of their orbits. Generally the problem is not quite so simple as
those we have just discussed. For its adequate treatment, we must once
again levy toll on the mathematical work of Newton.
We have seen that a projectile fired horizontally with a speed of 4·90
miles a second, would describe endless circles round the earth. What
would happen if it were fired in some other direction and with some
other speed?
The answer was provided by Newton. He shewed that when a small body
is allowed to move freely under the gravitational pull of a big body,
it will run away altogether if its speed exceeds a certain critical
amount, in which case its orbit is the curve called a hyperbola. But
if its speed is less than this critical amount, its orbit will always
be an ellipse—a sort of pulled out circle or oval curve[5] (fig. 4, p.
47). Previous to this Kepler had found that the actual paths of the
planets round the sun were not exact circles but ellipses, although for
the most part ellipses which did not differ greatly from circles; they
are what the mathematician calls “ellipses of small eccentricity.” This
provides still further confirmation of Newton’s law of gravitation,
for it can be proved that if the force of gravitation falls off in any
way other than according to Newton’s law of the inverse square of the
distance, the orbits of the planets will not be elliptical.
[5] The simplest definition of an ellipse is that it is the curve drawn
by a moving point _P_ which moves in such a way that the sum of its
distances _PS_, _PT_ from two fixed points _S_, _T_ remains always
the same. In practice we can most easily draw an ellipse by slipping
an endless string _SPTS_ round two drawing pins _S_, _T_ stuck into
a drawing board. Stretch the string tight with a pencil at _P_, and
on letting the pencil move round, keeping the string always tight, we
shall draw an ellipse. If the pins _S_, _T_ in the drawing board are
placed near to one another the curve described by the pencil _P_ is
nearly circular. The ratio of the distance _ST_ to the length of the
remainder of the string _SP_ + _PT_ is called the “eccentricity” of
the ellipse; it is necessarily less than unity, because two sides of a
triangle are together greater than the third side.
In the limiting case in which the eccentricity is made zero, the
ellipse becomes a circle. If the eccentricity is nearly as large as
unity, the ellipse is very elongated. All the different shapes of
ellipses are obtained by letting the eccentricity change from 0 to 1,
and these represent all the different shapes of orbit that a small
body can describe around a heavy gravitating mass. The points _S_, _T_
are called the foci of the ellipse, and the big attracting body always
occupies one or other of the two foci of the ellipse.
[Illustration: Fig. 4. The oval curve is an ellipse; the points _S_,
_T_ are its “foci.”]
When the astronomer studies the motions of a binary star in the sky,
he generally finds that the two components do not move in circles
about one another but in ellipses[6]. Once again, Newton’s law is
confirmed, and we are entitled to assume that the forces which keep
binary stars together are the same gravitational forces as keep the
moon from running away from the earth, or the planets from the sun. By
a study of these ellipses it becomes possible to weigh the stars. If
one of the component masses were enormously heavier than the other,
the former would stand still while the lighter component described an
ellipse around it, the motion being essentially similar to that of a
planet around the sun. Such cases are not observed in actual binary
stars because the two components are generally comparable in weight,
and this brings new complications into the question. There is no need
to enter into mathematical details here. Suffice it to say that neither
star stands still; the two components describe ellipses of different
sizes, and from a study of these two ellipses the weights of both the
components can be determined.
[6] What he actually observes is the “projection” of the orbit on the
sky, but it is a well-known theorem of geometry that the projection of
an ellipse is always an ellipse.
The following table shews the result of weighing the four binary
systems nearest the sun in this way, the sun’s weight being taken as
unity:
_Stellar Weights_
Binary systems near the sun.
+----------------+-------------+-----------------------+-----------+
| | Distance in | Weights of components | |
| Star | light-years | in terms of sun’s |Luminosity |
| | from the sun| weight |(see p. 49)|
+----------------+-------------+-----------------------+-----------+
| α Centauri _A_ | 4·31 | 1·14 | 1·12 |
| ” _B_ | | 0·97 | 0·32 |
+----------------+-------------+-----------------------+-----------+
| Sirius _A_ | 8·65 | 2·45 | 26·3 |
| ” _B_ | | 0·85 | 0·0026 |
+----------------+-------------+-----------------------+-----------+
| Procyon _A_ | 10·5 | 1·24 | 5·5 |
| ” _B_ | | 0·39 | 0·00003|
+----------------+-------------+-----------------------+-----------+
| Kruger 60 _A_ | 12·7 | 0·25 | 0·0026 |
| ” _B_ | | 0·20 | 0·0007 |
+----------------+-------------+-----------------------+-----------+
We see that the weights of these stars do not differ greatly from that
of the sun, although naturally the whole of space provides a greater
range than the four stars of our table which happen to be near the sun.
But even in the whole of space, no star whose weight is known with any
accuracy has a weight less than Kruger 60 _B_, although at the other
end of the scale there are many stars with far greater weights than any
in our table. Of stars whose weights are known with fair accuracy, the
star H.D. 1337 (Pearce’s star) is the weightiest, its two components
being respectively 36·3 and 33·8 times as heavy as the sun. Plaskett’s
star B.D. 6° 1309 is certainly heavier still, its components weighing
at least 75 and 63 times as much as the sun, and probably more; the
exact weights are not known (see p. 55 below). The system 27 Canis
Majoris consists of four stars, whose combined weight, according to the
evidence at present available, appears to be at least 940 times that
of the sun, but we may properly exercise a certain amount of caution
before accepting a figure so far outside the usual run of stellar
weights.
The average constituent star in the above very short table has 0·94
times the weight of the sun, so that our sun appears to be of rather
more than average weight, and this is confirmed by a more extensive
study of stellar weights.
We might have expected _a priori_ that the stars would prove to have
all sorts of weights, for there is no obvious reason why stars should
not exist with weights millions of times that of the sun, or again with
weights only equal to that of the earth or less. Actually we find that
the weights of the stars are mostly fairly equal, very few stars having
weights greatly dissimilar from that of the sun. This seems to indicate
that a star is a definite species of astronomical product, not a mere
random chunk of luminous matter.
LUMINOSITY. The last column of the table on p. 48 gives the
“luminosities” of the stars, which means their candle-power as lights,
that of the sun being taken as unity. For instance the entry of 26·3
for Sirius means that Sirius, regarded as a lighthouse in space, has
26·3 times the candle-power of the sun. The luminosities of the stars
shew an enormously greater range than their weights. In a general way
the heaviest stars are found to be the most luminous, as we should
naturally expect, but their luminosity is out of all proportion to
their weight. The heavier component of Sirius has only 2·9 times the
weight of the lighter component, but 10,000 times its luminosity.
Again, in the system of Procyon the heavier component has 3·2 times the
weight, but 180,000 times the luminosity, of the lighter component. It
appears to be an almost universal law that the candle-power per ton is
far greater in heavy stars than in light. This is one of the central
and, at first sight, one of the most perplexing facts of physical
astronomy: it is so fundamental and so pervading that no view of
stellar mechanism can be accepted which fails to explain it.
SPECTROSCOPIC VELOCITIES. When a star’s distance is known, its motion
across the sky tells us its speed in a direction at right angles to
the line along which we look at it—i.e. across the line of sight—but
provides no means of discovering its speed along this line. We cannot
see the motion of a body which is coming straight towards us, and a
star moving at a million miles a second in a direction exactly along
the line of sight would yet appear to be standing still in the sky. To
evaluate velocities along the line of sight, the astronomer calls in
the aid of the spectroscope.
All light is a blend of lights of different colours, and just as
Newton, with his famous prism, analysed sunlight into all the colours
of the rainbow, so the spectroscope analyses the light from a star, or
indeed from any source whatever, into its various constituent colours.
The instrument spreads out the analysed light into a strip of light
of continuously graduated colour, which is described as a “spectrum.”
In this the colours are the same, and are found to be arranged in
the same order, as in the rainbow, running from violet through green
and orange to red. There is a physical reason for this. We shall see
later (p. 114) that light consists of trains of waves—like the ripples
which the wind blows up on a pond—and that the different colours of
light result from waves of different lengths, red light being produced
by the longest waves, and violet light by the shortest. The colours
in the spectrum occur in the order of their wave-lengths, from the
longest (red) to the shortest (violet). In the typical stellar spectrum
certain short ranges of colour or wave-length are generally missing,
for reasons we shall discuss later (p. 126), so that the spectrum
appears to be crossed by a number of dark lines or bands, thus forming
a pattern rather than a continuous gradation of colours. Examples of
stellar spectra are shewn in Plate VIII.
[Illustration: PLATE VIII
_B_ 0 ε Orionis
_A_ 0 Sirius
_F_ 0 δ Geminorum
_G_ 0 Capella
_K_ 0 Arcturus
_M_ 0 Betelgeux
Stellar Spectra
(The spectral types are indicated on the left)]
It is frequently convenient to classify stars by the type of spectra
they emit. It is now known that a star’s spectrum depends primarily
upon the temperature of its surface. As a consequence, stellar spectra
can, in the main, be arranged in a single continuous sequence, and
their usual classification is by a sequence of letters, _O_, _B_, _A_,
_F_, _G_, _K_, _M_ with decimal subdivisions, the temperature falling
as we pass along the sequence, so that _O_-type stars have the highest
surface-temperatures and _M_-type stars the lowest. Spectral types are
indicated on the left in Plate VIII.
When the light received from a star is analysed in a spectroscope, the
pattern of lines or bands may be found to be shifted bodily in one
direction or the other. If the shift is towards the red end of the
spectrum, the light emitted by the star is reaching us in a redder
state than that in which it ought normally to be, and as red light
has the longest wave-length, this means that every wave of light is
longer—more drawn out—than normal. We conclude that the star is
receding from us. In the same way, if the spectral pattern is shifted
toward the violet end of the spectrum, we know that the star must be
approaching us. The shift of a spectrum resulting from the motion of
the body which emits it is generally described as the “Doppler Effect.”
From its amount we can calculate a star’s actual speed along the line
of sight, and the calculation is surprisingly simple. If each line or
band in a spectrum is found to represent a wave-length a hundredth
of one per cent. longer than that usually associated with it, then
the star’s speed of recession is a hundredth of one per cent. of the
velocity of light, or 18·6 miles a second—and similarly for all other
displacements.