TRANSCRIBER'S NOTE
Italic text is denoted by _underscores_.
Obvious typographical errors and punctuation errors have been
corrected after careful comparison with other occurrences within
the text and consultation of external sources.
More detail can be found at the end of the book.
PRACTICAL TALKS BY AN ASTRONOMER
[Illustration: The Moon. First Quarter.
Photographed by Loewy and Puiseux, February 13, 1894.]
PRACTICAL TALKS BY AN ASTRONOMER
BY
HAROLD JACOBY
ADJUNCT PROFESSOR OF ASTRONOMY IN
COLUMBIA UNIVERSITY
ILLUSTRATED
NEW YORK
CHARLES SCRIBNER'S SONS
1902
COPYRIGHT, 1902, BY
CHARLES SCRIBNER'S SONS
Published, April, 1902
TROW DIRECTORY
PRINTING AND BOOKBINDING COMPANY
NEW YORK
PREFACE
The present volume has not been designed as a systematic treatise
on astronomy. There are many excellent books of that kind, suitable
for serious students as well as the general reader; but they are
necessarily somewhat dry and unattractive, because they must aim at
completeness. Completeness means detail, and detail means dryness.
But the science of astronomy contains subjects that admit of
detached treatment; and as many of these are precisely the ones of
greatest general interest, it has seemed well to select several,
and describe them in language free from technicalities. It is hoped
that the book will thus prove useful to persons who do not wish to
give the time required for a study of astronomy as a whole, but
who may take pleasure in devoting a half-hour now and then to a
detached essay on some special topic.
Preparation of the book in this form has made it suitable for prior
publication in periodicals; and the several essays have in fact
all been printed before. But the intention of collecting them into
a book was kept in mind from the first; and while no attempt has
been made at consecutiveness, it is hoped that nothing of merely
ephemeral value has been included.
CONTENTS
PAGE
NAVIGATION AT SEA 1
THE PLEIADES 10
THE POLE-STAR 18
NEBULÆ 27
TEMPORARY STARS 37
GALILEO 47
THE PLANET OF 1898 58
HOW TO MAKE A SUN-DIAL 69
PHOTOGRAPHY IN ASTRONOMY 81
TIME STANDARDS OF THE WORLD 111
MOTIONS OF THE EARTH'S POLE 131
SATURN'S RINGS 140
THE HELIOMETER 152
OCCULTATIONS 161
MOUNTING GREAT TELESCOPES 170
THE ASTRONOMER'S POLE 184
THE MOON HOAX 199
THE SUN'S DESTINATION 210
ILLUSTRATIONS
THE MOON. FIRST QUARTER _Frontispiece_
_Photographed by Loewy and Puiseux,
February 13, 1894._
FACING
PAGE
SPIRAL NEBULA IN CONSTELLATION LEO 26
_Photographed by Keeler,
February 24, 1900._
NEBULA IN ANDROMEDA 28
_Photographed by Barnard,
November 21, 1892._
THE "DUMB-BELL" NEBULA 34
_Photographed by Keeler,
July 31, 1899._
STAR-FIELD IN CONSTELLATION MONOCEROS 84
_Photographed by Barnard,
February 1, 1894._
SOLAR CORONA. TOTAL ECLIPSE 108
_Photographed by Campbell,
January 22, 1898; Jeur, India._
FORTY-INCH TELESCOPE, YERKES OBSERVATORY 170
YERKES OBSERVATORY, UNIVERSITY OF CHICAGO 176
PRACTICAL TALKS BY AN ASTRONOMER
NAVIGATION AT SEA
A short time ago the writer had occasion to rummage among the
archives of the Royal Astronomical Society in London, to consult,
if possible, the original manuscripts left by one Stephen
Groombridge, an English astronomer of the good old days, who
died in 1832. It was known that they had been filed away about a
generation ago, by the late Sir George Airy, who was Astronomer
Royal of England between the years 1835 and 1881. After a long
search, a large and dusty box was found and opened. It was filled
with documents, of which the topmost was in Sir George's own
handwriting, and began substantially as follows:
"List of articles within this box.
"No. 1, This list,
"No. 2, etc., etc."
Astronomical precision can no further go: he had listed even the
list itself. Truly, Airy was rightly styled "prince of precisians."
A worthy Astronomer Royal was he, to act under the royal warrant
of Charles II., who established the office in 1675. Down to this
present day that warrant still makes it the duty of His Majesty's
Astronomer "to apply himself with the most exact care and diligence
to the rectifying of the tables of the motions of the heavens and
the places of the fixed stars, in order to find out the so much
desired longitude at sea, for the perfecting the art of navigation."
The "so much desired longitude at sea" is, indeed, a vastly
important thing to a maritime nation like England. And only in
comparatively recent years has it become possible and easy for
vessels to be navigated with safety and convenience upon long
voyages. The writer was well acquainted with an old sea-captain
of New York, who had commanded one of the earliest transatlantic
steamers, and who died only a few years ago. He had a goodly store
of ocean yarn, fit and ready for the spinning, if he could but
find someone who, like himself, had known and loved the ocean. In
his early sea-going days, only the wealthiest of captains owned
chronometers. This instrument is now considered indispensable in
navigation, but at that time it was a new invention, very rare and
costly. Upon a certain voyage from England to Rio Janeiro, in South
America, the old captain could remember the following odd method of
navigation: The ship was steered by compass to the southward and
westward, more or less, until the skipper's antique quadrant showed
that they had about reached the latitude of Rio. Then they swung
her on a course due west by compass, and away she went for Rio,
relying on the lookout man forward to keep the ship from running
ashore. For after a certain lapse of time, being ignorant of the
longitude, they could not know whether they would "raise" the land
within an hour or in six weeks. We are glad of an opportunity to
put this story on record, for the time is not far distant when
there will be no man left among the living who can remember how
ships were taken across the seas in the good old days before
chronometers.
Anyone who has ever been a passenger on a great transatlantic
liner of to-day knows what an important, imposing personage is
the brass-bound skipper. A very different creature is he on the
deck of his ship from the modest seafaring man we meet on land,
clad for the time being in his shore-going togs. But the captain's
dignity is not all brass buttons and gold braid. He has behind
him the powerful support of a deep, delightful mystery. He it is
who "takes the sun" at noon, and finds out the ship's path at
sea. And in truth, regarded merely as a scientific experiment,
the guiding of a vessel across the unmarked trackless ocean has
few equals within the whole range of human knowledge. It is our
purpose here to explain quite briefly the manner in which this
seeming impossibility is accomplished. We shall not be able to
go sufficiently into details to enable him who reads to run and
navigate a magnificent steamer. But we hope to diminish somewhat
that small part of the captain's vast dignity which depends upon
his mysterious operations with the sextant.
To begin, then, with the sextant itself. It is nothing but an
instrument with which we can measure how high up the sun is in the
sky. Now, everyone knows that the sun slowly climbs the sky in
the morning, reaches its greatest height at noon, and then slowly
sinks again in the afternoon. The captain simply begins to watch
the sun through the sextant shortly before noon, and keeps at it
until he discovers that the sun is just beginning to descend. That
is the instant of noon on the ship. The captain quickly glances
at the chronometer, or calls out "noon" to an officer who is near
that instrument. And so the error of the chronometer becomes known
then and there without any further astronomical calculations
whatever. Navigators can also find the chronometer error by sextant
observations when the sun is a long way from noon. The methods of
doing this are somewhat less simple than for the noon observation;
they belong to the details of navigation, into which we cannot
enter here.
Incidentally, the captain also notes with the sextant how high
the sun was in the sky at the noon observation. He has in his
mysterious "chart-room" some printed astronomical tables,
which tell him in what terrestrial latitude the sun will have
precisely that height on that particular day of the year. Thus the
terrestrial latitude becomes known easily enough, and if only the
captain could get his longitude too, he would know just where his
ship was that day at noon.
We have seen that the sextant observations furnish the error of
the chronometer according to ship's time. In other words, the
captain is in possession of the correct local time in the place
where the ship actually is. Now, if he also had the correct time
at that moment of some well-known place on shore, he would know
the difference in time between that place on shore and the ship.
But every traveller by land or sea is aware that there are always
differences of time between different places on the earth. If a
watch be right on leaving New York, for instance, it will be much
too fast on arriving at Chicago or San Francisco; the farther you
go the larger becomes the error of your watch. In fact, if you
could find out how much your watch had gone into error, you would
in a sense know how far east or west you had travelled.
Now the captain's chronometer is set to correct "Greenwich time"
on shore before the ship leaves port. His observations having then
told him how much this is wrong on that particular day, and in
that particular spot where the ship is, he knows at once just how
far he has travelled east or west from Greenwich. In other words,
he knows his "longitude from Greenwich," for longitude is nothing
more than distance from Greenwich in an east-and-west direction,
just as latitude is only distance from the equator measured in
a north-and-south direction. Greenwich observatory is usually
selected as the beginning of things for measuring longitudes,
because it is almost the oldest of existing astronomical
establishments, and belongs to the most prominent maritime nation,
England.
One of the most interesting bits of astronomical history was
enacted in connection with this matter of longitude. From what has
been said, it is clear that the ship's longitude will be obtained
correctly only if the chronometer has kept exact time since the
departure of the ship from port. Even a very small error of the
chronometer will throw out the longitude a good many miles, and we
can understand readily that it must be difficult in the extreme to
construct a mechanical contrivance capable of keeping exact time
when subjected to the rolling and pitching of a vessel at sea.
It was as recently as the year 1736 that the first instrument
capable of keeping anything like accurate time at sea was
successfully completed. It was the work of an English watchmaker
named John Harrison, and is one of the few great improvements in
matters scientific which the world owes to a desire for winning a
money prize. It appears that in 1714 a committee was appointed by
the House of Commons, with no less a person than Sir Isaac Newton
himself as one of its members, to consider the desirability of
offering governmental encouragement for the invention of some means
of finding the longitude at sea. Finally, the British Government
offered a reward of $50,000 for an instrument which would find
the longitude within sixty miles; $75,000, if within forty miles,
and $100,000, if within thirty miles. Harrison's chronometer was
finished in 1736, but he did not receive the final payment of his
prize until 1764.
We shall not enter into a detailed account of the vexatious delays
and official procedures to which he was forced to submit during
those twenty-eight long years. It is a matter of satisfaction that
Harrison lived to receive the money which he had earned. He had the
genius to plan and master intricate mechanical details, but perhaps
he lacked in some degree the ability of tongue and pen to bring
them home to others. This may be the reason he is so little known,
though it was his fortune to contribute so large and essential
a part to the perfection of modern navigation. Let us hope this
brief mention may serve to recall his memory from oblivion even for
a fleeting moment; that we may not have written in vain of that
longitude to which his life was given.
THE PLEIADES
Famed in legend; sung by early minstrels of Persia and Hindustan;
"--like a swarm of fire-flies tangled in a silver braid";
yonder distant misty little cloud of Pleiades has always won
and held the imagination of men. But it was not only for the
inspiration of poets, for quickening fancy into song, that the
seven daughters of Atlas were fixed upon the firmament. The
problems presented by this group of stars to the unobtrusive
scientific investigator are among the most interesting known to
astronomy. Their solution is still very incomplete, but what we
have already learned may be counted justly among the richest spoils
brought back by science from the stored treasure-house of Nature's
secrets.
The true student of astronomy is animated by no mere vulgar
curiosity to pry into things hidden. If he seeks the concealed
springs that move the complex visible mechanism of the heavens, he
does so because his imagination is roused by the grandeur of what
he sees; and deep down within him stirs the true love of the artist
for his art. For it is indeed a fine art, that science of astronomy.
It can have been no mere chance that has massed the Pleiades
from among their fellow stars. Men of ordinary eyesight see but
a half-dozen distinct objects in the cluster; those of acuter
vision can count fourteen; but it is not until we apply the
space-penetrating power of the telescope that we realize the
extraordinary scale upon which the system of the Pleiades is
constructed. With the Paris instrument Wolf in 1876 catalogued 625
stars in the group; and the searching photographic survey of Henry
in 1887 revealed no less than 2,326 distinct stars within and near
the filmy gauze of nebulous matter always so conspicuous a feature
of the Pleiades.
The means at our disposal for the study of stellar distances are
but feeble. Only in the case of a very small number of stars have
we been able to obtain even so much as an approximate estimate
of distance. The most powerful observational machinery, though
directed by the tried skill of experience, has not sufficed to
sound the profounder depths of space. The Pleiad stars are among
those for which no measurement of distance has yet been made, so
that we do not know whether they are all equally far away from
us. We see them projected on the dark background of the celestial
vault; but we cannot tell from actual measurement whether they are
all situated near the same point in space. It may be that some
are immeasurably closer to us than are the great mass of their
companions; possibly we look through the cluster at others far
behind it, clinging, as it were, to the very fringe of the visible
universe.
Farther on we shall find evidence that something like this really
is the case. But under no circumstances is it reasonable to suppose
that the whole body of stars can be strung out at all sorts of
distances near a straight line pointing in the direction of the
visible cluster. Such a distribution may perhaps remain among the
possibilities, so long as we cannot measure directly the actual
distances of the individual stars. But science never accepts a mere
possibility against which we can marshal strong circumstantial
evidence. We may conclude on general principles that the gathering
of these many objects into a single close assemblage denotes
community of origin and interests.
The Pleiades then really belong to one another. What is the nature
of their mutual tie? What is their mystery, and can we solve it?
The most obvious theory is, of course, suggested by what we know
to be true within our own solar system. We owe to Newton the
beautiful conception of gravitation, that unique law by means of
which astronomers have been enabled to reduce to perfect order the
seeming tangle of planetary evolutions. The law really amounts, in
effect, to this: All objects suspended within the vacancy of space
attract or pull one another. How they can do this without a visible
connecting link between them is a mystery which may always remain
unsolved. But mystery as it is, we must accept it as an ascertained
fact. It is this pull of gravitation that holds together the sun
and planets, forcing them all to follow out their due and proper
paths, and so to continue throughout an unbroken cycle until the
great survivor, Time, shall be no more.
This same gravitational attraction must be at work among the
Pleiades. They, too, like ourselves, must have bounds and orbits
set and interwoven, revolutions and gyrations far more complex
than the solar system knows. The visual discovery of such motion
of rotation among the Pleiades may be called one of the pressing
problems of astronomy to-day. We feel sure that the time is ripe,
and that the discovery is actually being made at the present
moment: for a generation of men is not too great a period to call a
moment, when we have to deal with cosmic time.
It is indeed the lack of observations extending through sufficient
centuries that stays our hand from grasping the coveted result. The
Pleiades are so far from us that we cannot be sure of changes among
them. Magnitudes are always relative. It matters not how large the
actual movements may be; if they are extremely small in comparison
with our distance, they must shrink to nothingness in our eyes.
Trembling on the verge of invisibility, elusive, they are in that
borderland where science as yet but feels her way, though certain
that the way is there.
The foundations of exact modern knowledge of the group were laid
by Bessel about 1840. With the modesty characteristic of the
great, he says quite simply that he has made a number of measures
of the Pleiades, thinking that the time may come when astronomers
will be able to find some evidence of motion. In this unassuming
way he prefaces what is still the classic model of precision and
thoroughness in work of this kind. Bessel cleared the ground for a
study of inter-stellar motion within the close star-clusters; and
it is probable that only by such study may we hope to demonstrate
the universality of the law of gravitation in cosmic space.
Bessel's acuteness in forecasting the direction of coming research
was amply verified by the work of Elkin in 1885 at Yale College.
Provided with a more modern instrument, but similar to Bessel's,
Elkin was able to repeat his observations with a slight increase
of precision. Motions in the interval of forty-five years,
sufficiently great to hint at coming possibilities, were shown
conclusively to exist. Six stars at all events have been fairly
excluded from the group on account of their peculiar motions shown
by Elkin's research. It is possible that they are merely seen in
the background through the interstices of the cluster itself,
or they may be suspended between us and the Pleiades, in either
case having no real connection with the group. Finally, these
observations make it reasonably certain that many of the remaining
mass of stars really constitute a unit aggregation in space.
Astronomers of a coming generation will again repeat the Besselian
work. At present we have been able to use his method only for the
separation from the true Pleiades of chance stars that happen to
lie in the same direction. Let us hope that man shall exist long
enough upon this earth to see the clustered stars themselves begin
and carry out such gyrations as gravitation imposes.
These will doubtless be of a kind not even suggested by the
lesser complexities of our solar system. For the most wonderful
thing of all about the Pleiades seems to point to an intricacy of
structure whose details may be destined to shake the confidence of
the profoundest mathematician. There is an extraordinary nebulous
condensation that seems to pervade the entire space occupied by
the stellar constituents of the group. The stars are swimming in a
veritable sea of luminous cloud. There are filmy tenuous places,
and again condensing whirls of material doubtless still in the
gaseous or plastic stage. Most noticeable of all are certain almost
straight lines of nebula that connect series of stars. In one case,
shown upon a photograph made by Henry at Paris, six stars are
strung out upon such a hazy line. We might give play to fancy, and
see in this the result of some vast eruption of gaseous matter that
has already begun to solidify here and there into stellar nuclei.
But sound science gives not too great freedom to mere speculative
theories. Her duty has been found in quiet research, and her
greatest rewards have flowed from imaginative speculation, only
when tempered by pure reason.
THE POLE-STAR
One of the most brilliant observations of the last few years
is Campbell's recent discovery of the triple character of this
star. Centuries and centuries ago, when astronomy, that venerable
ancient among the sciences, was but an infant, the pole-star must
have been considered the very oldest of observed heavenly bodies.
In the beginning it was the only sure guide of the navigator at
night, just as to this day it is the foundation-stone for all
observational stellar astronomy of precision. There has never been
a time in the history of astronomy when the pole-star might not
have been called the most frequently measured object in the sky of
night. So it is indeed strange that we should find out something
altogether new about it after all these ages of study.
But the importance of the discovery rests upon a surer foundation
than this. The method by which it has been made is almost a new
one in the science. A generation ago, men thought the "perfect
science," for so we love to call astronomy, could advance only by
increasing a little the exact precision of observation. The citadel
of perfect truth might be more closely invested; the forces of
science might push forward step by step; the machinery of research
might be strengthened, but that a new engine of investigation
would be discovered capable of penetrating where no telescope can
ever reach, this, indeed, seemed far beyond the liveliest hope
of science. Even the discoverer of the spectroscope could never
have dreamed of its possibilities, could never have foreseen its
successes, its triumphs.
The very name of this instrument suggests mystery to the popular
mind. It is set down at once among the things too difficult, too
intricate, too abstruse to understand. Yet in its essentials there
is nothing about the spectroscope that cannot be made clear in a
few words. Even the modern "undulatory theory" of light itself is
terrible only in the length of its name. Anyone who has seen the
waves of ocean roll, roll, and ever again roll in upon the shore,
can form a very good notion of how light moves. 'Tis just such
a series of rolling waves; started perhaps from some brilliant
constellation far out upon the confining bounds of the visible
universe, or perhaps coming from a humble light upon the student's
table; yet it is never anything but a succession of rolling waves.
Only, unlike the waves of the sea, light waves are all excessively
small. We should call one whose length was a twenty-thousandth of
an inch a big one!
Now the human eye possesses the property of receiving and
understanding these little waves. The process is an unconscious
one. Let but a set of these tiny waves roll up, as it were, out
of the vast ocean of space and impinge upon the eye, and all the
phenomena of light and color become what we call "visible." We see
the light.
And how does all this find an application in astronomy? Not
to enter too much into technical details, we may say that the
spectroscope is an instrument which enables us to measure the
length of these light waves, though their length is so exceedingly
small. The day has indeed gone by when that which poets love to
call the Book of Nature was printed in type that could be read
by the eye unaided. Telescope, microscope, and spectroscope are
essential now to him who would penetrate any of Nature's secrets.
But measurements with a telescope, like eye observations, are
limited strictly to determining the directions in which we see
the heavenly bodies. Ever since the beginning of things, when
old Hipparchus and Ulugh Beg made the first rude but successful
attempts to catalogue the stars, the eye and telescope have been
able to measure only such directions. We aim the telescope at a
star, and record the direction in which it was pointed. Distances
in astronomy can never be measured directly. All that we know of
them has been obtained by calculations based upon the Newtonian law
of gravitation and observations of directions.
Now the spectroscope seems to offer a sort of exception to this
rule. Suppose we can measure the wave-lengths of the light sent us
from a star. Suppose again that the star is itself moving swiftly
toward us through space, while continually setting in motion the
waves of light that are ultimately to reach the waiting astronomer.
Evidently the light waves will be crowded together somewhat on
account of the star's motion. More waves per second will reach us
than would be received from a star at rest. It is as though the
light waves were compressed or shortened a little. And if the star
is leaving us, instead of coming nearer, opposite effects will
occur. We have then but to compare spectroscopically starlight with
some artificial source of light in the observatory in order to find
out whether the star is approaching us or receding from us. And by
a simple process of calculation this stellar motion can be obtained
in miles per second. Thus we can now actually measure directly, in
a certain sense, linear speed in stellar space, though we are still
without the means of getting directly at stellar distances.
But the most wonderful thing of all about these spectroscopic
measures is the fact that it makes no difference whatever how far
away is the star under observation. What we learn through the
spectroscope comes from a study of the waves themselves, and it
is of no consequence how far they have travelled, or how long
they have been a-coming. For it must not be supposed that these
waves consume no time in passing from a distant star to our own
solar system. It is true that they move exceeding fast; certainly
180,000 miles per second may be called rapid motion. But if
this cosmic velocity of light is tremendous, so also are cosmic
distances correspondingly vast. Light needs to move quickly coming
from a star, for even at the rate of motion we have mentioned it
requires many years to reach us from some of the more distant
constellations. It has been well said that an observer on some
far-away star, if endowed with the power to see at any distance,
however great, might at this moment be looking on the Crusaders
proceeding from Europe against the Saracen at Jerusalem. For it is
quite possible that not until now has the light which would make
the earth visible had time to reach him. Yet distant as such an
observer might be, light from the star on which he stood could be
measured in the spectroscope, and would infallibly tell us whether
the earth and star are approaching in space or gradually drawing
farther asunder.
The pole-star is not one of the more distant stellar systems. We do
not know how far it is from us very exactly, but certainly not less
than forty or fifty years are necessary for its light to reach us.
The star might have gone out of existence twenty years ago, and we
not yet know of it, for we would still be receiving the light which
began its long journey to us about 1850 or 1860. But no matter
what may be its distance, Campbell found by careful observations,
made in the latter part of 1896, that the pole-star was then
approaching the earth at the rate of about twelve miles per second.
So far there was nothing especially remarkable. But in August and
September of the present year twenty-six careful determinations
were made, and these showed that now the rate of approach varied
between about five and nine miles per second. More astonishing
still, there was a uniform period in the changes of velocity. In
about four days the rate of motion changed from about five to
nine miles and back again. And this variation kept on with great
regularity. Every successive period of four days saw a complete
cycle of velocity change forward and back between the same limits.
There can be but one reasonable explanation. This star must be a
double, or "binary" star. The two components, under the influence
of powerful mutual gravitational attraction, must be revolving
in a mighty orbit. Yet this vast orbit, as a whole, with the two
great stars in it, must be approaching our part of the universe
all the time. For the spectroscope shows the velocity of approach
to increase and diminish, indeed, but it is always present. Here,
then, is this great stellar system, having a four-day revolution of
its own, and yet swinging rapidly through space in our direction.
Nor is this all. One of the component stars must be nearly or
quite dark; else its presence would infallibly be detected by our
instruments.
And now we come to the most astonishing thing of all. How comes it
that the average rate of approach of the "four-day system," as a
whole, changed between 1896 and 1899? In 1896 only this velocity
of the whole system was determined, the four-day period remaining
undiscovered until the more numerous observations of 1899.
But even without considering the four-day period, the changing
velocity of the entire system offers one of those problems that
exact science can treat only by the help of the imagination. There
must be some other great centre of attraction, some cosmic giant,
holding the visible double pole-star under its control. Thus, that
which we see, and call the pole-star, is in reality threading its
path about the third and greatest member of the system, itself
situated in space, we know not where.
[Illustration: Spiral Nebula in Constellation Leo.
Photographed by Keeler, February 24, 1900.
Exposure, three hours, fifty minutes.]
NEBULÆ
Scattered about here and there among the stars are certain patches
of faint luminosity called by astronomers Nebulæ. These "little
clouds" of filmy light are among the most fascinating of all the
kaleidoscopic phenomena of the heavens; for it needs but a glance
at one of them to give the impression that here before us is the
stuff of which worlds are made. All our knowledge of Nature leads
us to expect in her finished work the result of a series of gradual
processes of development. Highly organized phenomena such as those
existing in our solar system did not spring into perfection in
an instant. Influential forces, easy to imagine, but difficult
to define, must have directed the slow, sure transformation of
elemental matter into sun and planets, things and men. Therefore
a study of those forces and of their probable action upon nebular
material has always exerted a strong attraction upon the acutest
thinkers among men of exact science.
Our knowledge of the nebulæ is of two kinds--that which has
been ascertained from observation as to their appearance, size,
distribution, and distance; and that which is based upon hypotheses
and theoretical reasoning about the condensation of stellar systems
out of nebular masses. It so happens that our observational
material has received a very important addition quite recently
through the application of photography to the delineation of
nebulæ, and this we shall describe farther on.
Two nebulæ only are visible to the unaided eye. The brighter
of these is in the constellation Andromeda; it is of oval or
elliptical shape, and has a distinct central condensation or
nucleus. Upon a photograph by Roberts it appears to have several
concentric rings surrounding the nebula proper, and gives the
general impression of a flat round disk foreshortened into an oval
shape on account of the observer's position not being square to
the surface of the disk. Very recent photographs of this nebula,
made with the three-foot reflecting telescope of the Lick
Observatory, bring out the fact that it is really spiral in form,
and that the outlying nebulous rings are only parts of the spires
in a great cosmic whorl.
[Illustration: Nebula in Andromeda.
Lower object in the photograph is a Comet.
Photographed by Barnard, November 21, 1892.]
This Andromeda nebula is the one in which the temporary star of
1885 appeared. It blazed up quite suddenly near the apparent
centre of the nebula, and continued in view for six months, fading
finally beyond the reach of our most powerful telescopes. There
can be little doubt that the star was actually in the nebula,
and not merely seen through it, though in reality situated in
the extreme outlying part of space at a distance immeasurably
greater than that separating us from the nebula itself. Such an
accidental superposition of nebula and star might even be due to
sudden incandescence of a new star between us and the nebula. In
such a case we should see the star projected upon the surface of
the nebula, so that the superposition would be identical with that
actually observed. Therefore, while it is, indeed, possible that
the star may have been either far behind the nebula or in front of
it, we must accept as more probable the supposition that there was
a real connection between the two. In that case there is little
doubt that we have actually observed one of those cataclysms that
mark successive steps of cosmic evolution. We have no thoroughly
satisfactory theory to account for such an explosive catastrophe
within the body of the nebula itself.
The other naked-eye nebula is in the constellation Orion. In
the telescope it is a more striking object, perhaps, than the
Andromeda nebula; for it has no well-defined geometrical form,
but consists of an immense odd-shaped mass of light enclosing and
surrounding a number of stars. It is unquestionably of a very
complicated structure, and is, therefore, less easily studied and
explained than the nebulæ of simpler form. There is no doubt that
the Orion nebula is composed of luminous gas, and is not merely a
cluster of small stars too numerous and too near together to be
separated from each other, even in our most powerful telescopes.
It was, indeed, supposed, until about forty years ago, that all
the nebulæ are simply irresolvable star-clusters; but we now have
indisputable evidence, derived from the spectroscope, that many
nebulæ are composed of true gases, similar to those with which
we experiment in chemical laboratories. This spectroscopic proof
of the gaseous character of nebulæ is one of the most important
discoveries contributed by that instrument to our small stock of
facts concerning the structure of the sidereal universe.
Coming now to the smaller nebulæ, we find a great diversity of
form and appearance. Some are ring-shaped, perhaps having a
less brilliant nebulosity within the ring. Many show a central
condensation of disk-like appearance (planetary nebulæ), or have
simply a star at the centre (nebulous stars). Altogether about
ten thousand such objects have been catalogued by successive
generations of astronomers since the invention of the telescope,
and most of these have been reported as oval in form. Now we have
already referred to the important addition to our knowledge of
the nebulæ obtained by recent photographic observations; and this
addition consists in the discovery that most of these oval nebulæ
are in reality spirals. Indeed, it appears that the spiral type is
the normal type, and that nebulæ of irregular or other forms are
exceptions to the general rule. Even the great Andromeda nebula, as
we have seen, is now recognized as a spiral.
The instrument with which its convolute structure was discovered
is a three-foot reflecting telescope, made by Common of England,
and now mounted at the Lick Observatory, in California. The late
Professor Keeler devoted much of his time to photographing nebulæ
during the last year or two. He was able to establish the important
fact just mentioned, that most nebulæ formerly thought to be mere
ovals, turn out to be spiral when brought under the more searching
scrutiny of the photographic plate applied at the focus of a
telescope of great size, and with an exposure to the feeble nebular
light extending through three or four consecutive hours.
Many of the spirals have more than a single volute. It is as though
one were to attach a number of very flexible rods to an axle,
like spokes of a wheel without a rim and then revolve the axle
rapidly. The flexible rods would bend under the rapid rotation, and
form a series of spiral curves not unlike many of these nebulæ.
Indeed, it is impossible to escape the conviction that these great
celestial whorls are whirling around an axis. And it is most
important in the study of the growth of worlds, to recognize that
the type specimen is a revolving spiral. Therefore, the rotating
flattened globe of incandescent matter postulated by Laplace's
nebular hypothesis would make of our solar system an exceptional
world, and not a type of stellar evolution in general.
Keeler's photographs have taught us one thing more. Scarcely is
there a single one of his negatives that does not show nebulæ
previously uncatalogued. It is estimated that if this process of
photography could be extended so as to cover the entire sky, the
whole number of nebulæ would add up to the stupendous total of
120,000; and of these the great majority would be spiral.
When we approach the question of the distribution of nebulæ in
different parts of the sky, as shown by their catalogued positions,
we are met by a curious fact. It appears that the region in the
neighborhood of the Milky Way is especially poor in nebulæ,
whereas these objects seem to cluster in much larger numbers about
those points in the sky that are farthest from the Milky Way.
But we know that the Milky Way is richer in stars than any other
part of the sky, since it is, in fact, made up of stellar bodies
clustered so closely that it is wellnigh impossible to see between
them in the denser portions. Now, it cannot be the result of chance
that the stars should tend to congregate in the Milky Way, while
the nebulæ tend to seek a position as far from it as possible.
Whatever may be the cause, we must conclude that the sidereal
system, as we see it, is in general constructed upon a single plan,
and does not consist of a series of universes scattered at random
throughout space. If we are to suppose that nebulæ turn into stars
as a result of condensation or any other change, then it is not
astonishing to find a minimum of nebulæ where there is a maximum of
stars, since the nebulæ will have been consumed, as it were, in the
formation of the stars.
[Illustration: The "Dumb-Bell" Nebula.
Photographed by Keeler, July 31, 1899.
Exposure, three hours.]
It is never advisable to push philosophical speculation very
far when supported by too slender a basis of fact. But if we
are to regard the visible universe as made up on the whole of a
single system of bodies, we may well ask one or two questions to
be answered by speculative theory. We have said the stars are not
uniformly distributed in space. Their concentration in the Milky
Way, forming a narrow band dividing the sky into two very nearly
equal parts, must be due to their being actually massed in a
thin disk or ring of space within which our solar system is also
situated. This thin disk projected upon the sky would then appear
as the narrow star-band of the Milky Way. Now, suppose this disk
has an axis perpendicular to itself, and let us imagine a rotation
of the whole sidereal system about that axis. Then the fact that
the visible nebulæ are congregated far from the Milky Way means
that they are actually near the imaginary axis.
Possibly the diminished velocity of motion near the axis may have
something to do with the presence of the nebulæ there. Possibly
the nebulæ themselves have axes perpendicular to the plane of
the Milky Way. If so, we should see the spiral nebulæ near the
Milky Way edgewise, and those far from it without foreshortening.
Thus, the paucity of nebulæ near the Milky Way may be due in
part to the increased difficulty of seeing them when looked at
edgewise. Indeed, there is no limit to the possibilities of
hypothetical reasoning about the nebular structure of our universe;
unfortunately, the whole question must be placed for the present
among those intensely interesting cosmic problems awaiting
elucidation, let us hope, in this new century.
TEMPORARY STARS
Nothing can be more erroneous than to suppose that the stellar
multitude has continued unchanged throughout all generations of
men. "Eternal fires" poets have called the stars; yet they burn
like any little conflagration on the earth; now flashing with
energy, brilliant, incandescent, and again sinking into the dull
glow of smouldering half-burned ashes. It is even probable that
space contains many darkened orbs, stars that may have risen in
constellations to adorn the skies of prehistoric time--now cold,
unseen, unknown. So far from dealing with an unvarying universe,
it is safe to say that sidereal astronomy can advance only by the
discovery of change. Observational science watches with untiring
industry, and night hides few celestial events from the ardent
scrutiny of astronomers. Old theories are tested and newer ones
often perfected by the detection of some slight and previously
unsuspected alteration upon the face of the sky. The interpretation
of such changes is the most difficult task of science; it has taxed
the acutest intellects among men throughout all time.
If, then, changes can be seen among the stars, what are we to think
of the most important change of all, the blazing into life of a
new stellar system? Fifteen times since men began to write their
records of the skies has the birth of a star been seen. Surely
we may use this term when we speak of the sudden appearance of a
brilliant luminary where nothing visible existed before. But we
shall see further on that scientific considerations make it highly
probable that the phenomenon in question does not really involve
the creation of new matter. It is old material becoming suddenly
luminous for some hidden reason. In fact, whenever a new object of
great brilliancy has been discovered, it has been found to lose its
light again quite soon, ending either in total extinction or at
least in comparative darkness. It is for this reason that the name
"temporary star" has been applied to cases of this kind.
The first authenticated instance dates from the year 134 B.C.,
when a new star appeared in the constellation Scorpio. It was this
star that led Hipparchus to construct his stellar catalogue, the
first ever made. It occurred to him, of course, that there could
be but one way to make sure in the future that any given object
discovered in the sky was new; it was necessary to make a complete
list of everything visible in his day. Later astronomers need then
only compare Hipparchus's catalogue with the heavens from time to
time in order to find out whether anything unknown had appeared.
This work of Hipparchus became the foundation of sidereal study,
and led to most important discoveries of various kinds.
But no records remain concerning his new star except the bare fact
of its appearance in Scorpio. Hipparchus's published works are all
lost. We do not even know the exact place of his birth, and as for
those two dates of entry and exit that history attaches to great
names--we have them not. Yet he was easily the first astronomer of
antiquity, one of the first of all time; and we know of him only
from the writings of Ptolemy, who lived three hundred years after
him.
More than five centuries elapsed before another temporary star was
entered in the records of astronomy. This happened in the year 389
A.D., when a star appeared in Aquila; and of this one also we know
nothing further. But about twelve centuries later, in November,
1572, a new and brilliant object was found in the constellation
Cassiopeia. It is known as Tycho's star, since it was the means
of winning for astronomy a man who will always take high rank in
her annals, Tycho Brahe, of Denmark. When he first saw this star,
it was already very bright, equalling even Venus at her best; and
he continued a careful series of observations for sixteen months,
when it faded finally from his view. The position of the new star
was measured with reference to other stars in the constellation
Cassiopeia, and the results of Tycho's observations were finally
published by him in the year 1573. It appears that much urging on
the part of friends was necessary to induce him to consent to this
publication, not because of a modest reluctance to rush into print,
but for the reason that he considered it undignified for a nobleman
of Denmark to be the author of a book!
An important question in cosmic astronomy is opened by Tycho's
star. Did it really disappear from the heavens when he saw it
no more, or had its lustre simply been reduced below the visual
power of the unaided eye? Unfortunately, Tycho's observations of
the star's position in the constellation were necessarily crude.
He possessed no instruments of precision such as we now have
at our disposal, and so his work gives us only a rather rough
approximation of the true place of the star. A small circle might
be imagined on the sky of a size comparable with the possible
errors of Tycho's observations. We could then say with certainty
that his star must have been situated somewhere within that little
circle, but it is impossible to know exactly where.
It happens that our modern telescopes reveal the existence of
several faint stars within the space covered by such a circle.
Any one of these would have been too small for Tycho to see, and,
therefore, any one of them may be his once brilliant luminary
reduced to a state of permanent or temporary semi-darkness. These
considerations are, indeed, of great importance in explaining the
phenomena of temporary stars. If Tycho had been able to leave us a
more exact determination of his star's place in the sky, and even
if our most powerful instruments could not show anything in that
place to-day, we might nevertheless theorize on the supposition
that the object still exists, but has reached a condition almost
entirely dark.
Indeed, the latest theory classes temporary stars among those known
as variable. For many stars are known to undergo quite decided
changes in brilliancy; possibly inconstancy of light is the rule
rather than the exception. But while such changes, when they
exist, are too small to be perceptible in most cases, there is
certainly a large number of observable variables, subject to easily
measurable alterations of light. Astronomers prefer to see in the
phenomena of temporary stars simple cases of variation in which the
increase of light is sudden, and followed by a gradual diminution.
Possibly there is then a long period of comparative or even
complete darkness, to be followed as before by a sudden blazing up
and extinction. No temporary star, however, has been observed to
reappear in the same celestial place where once had glowed its
sudden outburst. But cases are not wanting where incandescence has
been both preceded and followed by a continued existence, visible
though not brilliant.
For such cases as these it is necessary to come down to modern
records. We cannot be sure that some faint star has been
temporarily brilliant, unless we actually see the conflagration
itself, or are able to make the identity of the object's precise
location in the sky before and after the event perfectly certain
by the aid of modern instruments of precision. But no one has
ever seen the smouldering fires break out. Temporary stars have
always been first noticed only after having been active for hours
if not for days. So we must perforce fall back on instrumental
identification by determinations of the star's exact position upon
the celestial vault.