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Discovery of the Fixed Stars and Nebula

Posted on July 23rd, 2007 by Admin.
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Passing now from our solar system, which appears to be subject to the
action of the same forces as those we experience on our globe, there
remains an innumerable host of fixed stars, nebulas, and nebulous
clusters of stars. To these the attention of astronomers has been more
earnestly directed since telescopes have been so much enlarged.
Photography also has enabled a vast amount of work to be covered in a
comparatively short period, and the spectroscope has given them the
means, not only of studying the chemistry of the heavens, but also of
detecting any motion in the line of sight from less than a mile a
second and upwards in any star, however distant, provided it be bright
enough.

[Illustration: SIR WILLIAM HERSCHEL, F.R.S.–1738-1822. Painted by
Lemuel F. Abbott; National Portrait Gallery, Room XX.]

In the field of telescopic discovery beyond our solar system there is
no one who has enlarged our knowledge so much as Sir William Herschel,
to whom we owe the greatest discovery in dynamical astronomy among the
stars–viz., that the law of gravitation extends to the most distant
stars, and that many of them describe elliptic orbits about each
other. W. Herschel was born at Hanover in 1738, came to England in
1758 as a trained musician, and died in 1822. He studied science when
he could, and hired a telescope, until he learnt to make his own
specula and telescopes. He made 430 parabolic specula in twenty-one
years. He discovered 2,500 nebulae and 806 double stars, counted the
stars in 3,400 guage-fields, and compared the principal stars
photometrically.

Some of the things for which he is best known were results of those
accidents that happen only to the indefatigable enthusiast. Such was
the discovery of Uranus, which led to funds being provided for
constructing his 40-feet telescope, after which, in 1786, he settled
at Slough. In the same way, while trying to detect the annual parallax
of the stars, he failed in that quest, but discovered binary systems
of stars revolving in ellipses round each other; just as Bradley’s
attack on stellar parallax failed, but led to the discovery of
aberration, nutation, and the true velocity of light.

_Parallax_.–The absence of stellar parallax was the great
objection to any theory of the earth’s motion prior to Kepler’s
time. It is true that Kepler’s theory itself could have been
geometrically expressed equally well with the earth or any other point
fixed. But in Kepler’s case the obviously implied physical theory of
the planetary motions, even before Newton explained the simplicity of
conception involved, made astronomers quite ready to waive the claim
for a rigid proof of the earth’s motion by measurement of an annual
parallax of stars, which they had insisted on in respect of
Copernicus’s revival of the idea of the earth’s orbital motion.

Still, the desire to measure this parallax was only intensified by the
practical certainty of its existence, and by repeated failures. The
attempts of Bradley failed. The attempts of Piazzi and Brinkley,[1]
early in the nineteenth century, also failed. The first successes,
afterwards confirmed, were by Bessel and Henderson. Both used stars
whose proper motion had been found to be large, as this argued
proximity. Henderson, at the Cape of Good Hope, observed alpha
Centauri, whose annual proper motion he found to amount to 3″.6, in
1832-3; and a few years later deduced its parallax 1″.16. His
successor at the Cape, Maclear, reduced this to 0″.92.

In 1835 Struve assigned a doubtful parallax of 0″.261 to Vega (alpha
Lyrae). But Bessel’s observations, between 1837 and 1840, of 61 Cygni,
a star with the large proper motion of over 5″, established its annual
parallax to be 0″.3483; and this was confirmed by Peters, who found
the value 0″.349.

Later determinations for alpha2 Centauri, by Gill,[2] make its parallax
0″.75–This is the nearest known fixed star; and its light takes 4 1/3
years to reach us. The light year is taken as the unit of measurement
in the starry heavens, as the earth’s mean distance is “the
astronomical unit” for the solar system.[3] The proper motions and
parallaxes combined tell us the velocity of the motion of these stars
across the line of sight: alpha Centauri 14.4 miles a second=4.2
astronomical units a year; 61 Cygni 37.9 miles a second=11.2
astronomical units a year. These successes led to renewed zeal, and
now the distances of many stars are known more or less accurately.

Several of the brightest stars, which might be expected to be the
nearest, have not shown a parallax amounting to a twentieth of a
second of arc. Among these are Canopus, alpha Orionis, alpha Cygni, beta
Centauri, and gamma Cassiopeia. Oudemans has published a list of
parallaxes observed.[4]

_Proper Motion._–In 1718 Halley[5] detected the proper motions
of Arcturus and Sirius. In 1738 J. Cassinis[6] showed that the former
had moved five minutes of arc since Tycho Brahe fixed its position. In
1792 Piazzi noted the motion of 61 Cygni as given above. For a long
time the greatest observed proper motion was that of a small star 1830
Groombridge, nearly 7″ a year; but others have since been found
reaching as much as 10″.

Now the spectroscope enables the motion of stars to be detected at a
single observation, but only that part of the motion that is in the
line of sight. For a complete knowledge of a star’s motion the proper
motion and parallax must also be known.

When Huggins first applied the Doppler principle to measure velocities
in the line of sight,[7] the faintness of star spectra diminished the
accuracy; but Vogel, in 1888, overcame this to a great extent by long
exposures of photographic plates.

It has often been noticed that stars which seem to belong to a group
of nearly uniform magnitude have the same proper motion. The
spectroscope has shown that these have also often the same velocity in
the line of sight. Thus in the Great Bear, beta, gamma, delta,
epsilon, zeta, all agree as to angular proper motion. delta was too
faint for a spectroscopic measurement, but all the others have been
shown to be approaching us at a rate of twelve to twenty miles a
second. The same has been proved for proper motion, and line of sight
motion, in the case of Pleiades and other groups.

Maskelyne measured many proper motions of stars, from which W.
Herschel[8] came to the conclusion that these apparent motions are for
the most part due to a motion of the solar system in space towards a
point in the constellation Hercules, R.A. 257 degrees; N. Decl. 25
degrees. This grand discovery has been amply confirmed, and, though
opinions differ as to the exact direction, it happens that the point
first indicated by Herschel, from totally insufficient data, agrees
well with modern estimates.

Comparing the proper motions and parallaxes to get the actual velocity
of each star relative to our system, C.L. Struve found the probable
velocity of the solar system in space to be fifteen miles a second, or
five astronomical units a year.

The work of Herschel in this matter has been checked by comparing
spectroscopic velocities in the line of sight which, so far as the
sun’s motion is concerned, would give a maximum rate of approach for
stars near Hercules, a maximum rate of recession for stars in the
opposite part of the heavens, and no effect for stars half-way
between. In this way the spectroscope has confirmed generally
Herschel’s view of the direction, and makes the velocity eleven miles
a second, or nearly four astronomical units a year.

The average proper motion of a first magnitude star has been found to
be 0″.25 annually, and of a sixth magnitude star 0″.04. But that all
bright stars are nearer than all small stars, or that they show
greater proper motion for that reason, is found to be far from the
truth. Many statistical studies have been made in this connection, and
interesting results may be expected from this treatment in the hands
of Kapteyn of Groningen, and others.[9]

On analysis of the directions of proper motions of stars in all parts
of the heavens, Kapteyn has shown[10] that these indicate, besides the
solar motion towards Hercules, two general drifts of stars in nearly
opposite directions, which can be detected in any part of the
heavens. This result has been confirmed from independent data by
Eddington (_R.A.S., M.N._) and Dyson (_R.S.E. Proc._).

Photography promises to assist in the measurement of parallax and
proper motions. Herr Pulfrich, of the firm of Carl Zeiss, has vastly
extended the applications of stereoscopic vision to astronomy–a
subject which De la Rue took up in the early days of photography. He
has made a stereo-comparator of great beauty and convenience for
comparing stereoscopically two star photographs taken at different
dates. Wolf of Heidelberg has used this for many purposes. His
investigations depending on the solar motion in space are remarkable.
He photographs stars in a direction at right angles to the line of the
sun’s motion. He has taken photographs of the same region fourteen
years apart, the two positions of his camera being at the two ends of
a base-line over 5,000,000,000 miles apart, or fifty-six astronomical
units. On examining these stereoscopically, some of the stars rise out
of the general plane of the stars, and seem to be much nearer. Many of
the stars are thus seen to be suspended in space at different
distances corresponding exactly to their real distances from our solar
system, except when their proper motion interferes. The effect is most
striking; the accuracy of measurement exceeds that of any other method
of measuring such displacements, and it seems that with a long
interval of time the advantage of the method increases.

_Double Stars._–The large class of double stars has always been much
studied by amateurs, partly for their beauty and colour, and partly as
a test for telescopic definition. Among the many unexplained stellar
problems there is one noticed in double stars that is thought by some
to be likely to throw light on stellar evolution. It is this: There
are many instances where one star of the pair is comparatively faint,
and the two stars are contrasted in colour; and in every single case
the general colour of the faint companion is invariably to be classed
with colours more near to the blue end of the spectrum than that of
the principal star.

_Binary Stars._–Sir William Herschel began his observations of double
stars in the hope of discovering an annual parallax of the stars. In
this he was following a suggestion of Galileo’s. The presumption is
that, if there be no physical connection between the stars of a pair,
the largest is the nearest, and has the greatest parallax. So, by
noting the distance between the pair at different times of the year, a
delicate test of parallax is provided, unaffected by major
instrumental errors.

Herschel did, indeed, discover changes of distance, but not of the
character to indicate parallax. Following this by further observation,
he found that the motions were not uniform nor rectilinear, and by a
clear analysis of the movements he established the remarkable and
wholly unexpected fact that in all these cases the motion is due to a
revolution about their common centre of gravity.[11] He gave the
approximate period of revolution of some of these: Castor, 342 years;
delta Serpentis, 375 years; gamma Leonis, 1,200 years; epsilon Bootis,
1,681 years.

Twenty years later Sir John Herschel and Sir James South, after
re-examination of these stars, confirmed[12] and extended the results,
one pair of Coronae having in the interval completed more than a whole
revolution.

It is, then, to Sir William Herschel that we owe the extension of the
law of gravitation, beyond the limits of the solar system, to the
whole universe. His observations were confirmed by F.G.W. Struve (born
1793, died 1864), who carried on the work at Dorpat. But it was first
to Savary,[13] and later to Encke and Sir John Herschel, that we owe
the computation of the elliptic elements of these stars; also the
resulting identification of their law of force with Newton’s force of
gravitation applied to the solar system, and the force that makes an
apple fall to the ground. As Grant well says in his _History_:
“This may be justly asserted to be one of the most sublime truths
which astronomical science has hitherto disclosed to the researches of
the human mind.”

Latterly the best work on double stars has been done by
S. W. Burnham,[14] at the Lick Observatory. The shortest period he
found was eleven years (kappa Pegasi). In the case of some of
these binaries the parallax has been measured, from which it appears
that in four of the surest cases the orbits are about the size of the
orbit of Uranus, these being probably among the smallest stellar
orbits.

The law of gravitation having been proved to extend to the stars, a
discovery (like that of Neptune in its origin, though unlike it in the
labour and originality involved in the calculation) that entrances the
imagination became possible, and was realised by Bessel–the discovery
of an unknown body by its gravitational disturbance on one that was
visible. In 1834 and 1840 he began to suspect a want of uniformity in
the proper motion of Sirius and Procyon respectively. In 1844, in a
letter to Sir John Herschel,[15] he attributed these irregularities in
each case to the attraction of an invisible companion, the period of
revolution of Sirius being about half a century. Later he said: “I
adhere to the conviction that Procyon and Sirius form real binary
systems, consisting of a visible and an invisible star. There is no
reason to suppose luminosity an essential quality of cosmical
bodies. The visibility of countless stars is no argument against the
invisibility of countless others.” This grand conception led Peters to
compute more accurately the orbit, and to assign the place of the
invisible companion of Sirius. In 1862 Alvan G. Clark was testing a
new 18-inch object-glass (now at Chicago) upon Sirius, and, knowing
nothing of these predictions, actually found the companion in the very
place assigned to it. In 1896 the companion of Procyon was discovered
by Professor Schaeberle at the Lick Observatory.

Now, by the refined parallax determinations of Gill at the Cape, we
know that of Sirius to be 0″.38. From this it has been calculated that
the mass of Sirius equals two of our suns, and its intrinsic
brightness equals twenty suns; but the companion, having a mass equal
to our sun, has only a five-hundredth part of the sun’s brightness.

_Spectroscopic Binaries_.–On measuring the velocity of a star in the
line of sight at frequent intervals, periodic variations have been
found, leading to a belief in motion round an invisible
companion. Vogel, in 1889, discovered this in the case of Spica (alpha
Virginis), whose period is 4d. 0h. 19m., and the diameter of whose
orbit is six million miles. Great numbers of binaries of this type
have since then been discovered, all of short period.

Also, in 1889, Pickering found that at regular intervals of fifty-two
days the lines in the spectrum of zeta of the Great Bear are
duplicated, indicating a relative velocity, equal to one hundred miles
a second, of two components revolving round each other, of which that
apparently single star must be composed.

It would be interesting, no doubt, to follow in detail the
accumulating knowledge about the distances, proper motions, and orbits
of the stars; but this must be done elsewhere. Enough has been said to
show how results are accumulating which must in time unfold to us the
various stellar systems and their mutual relationships.

_Variable Stars._–It has often happened in the history of different
branches of physical science that observation and experiment were so
far ahead of theory that hopeless confusion appeared to reign; and
then one chance result has given a clue, and from that time all
differences and difficulties in the previous researches have stood
forth as natural consequences, explaining one another in a rational
sequence. So we find parallax, proper motion, double stars, binary
systems, variable stars, and new stars all bound together.

The logical and necessary explanation given of the cause of ordinary
spectroscopic binaries, and of irregular proper motions of Sirius and
Procyon, leads to the inference that if ever the plane of such a
binary orbit were edge-on to us there ought to be an eclipse of the
luminous partner whenever the non-luminous one is interposed between
us. This should give rise either to intermittence in the star’s light
or else to variability. It was by supposing the existence of a dark
companion to Algol that its discoverer, Goodricke of York,[16] in
1783, explained variable stars of this type. Algol (beta Persei)
completes the period of variable brightness in 68.8 hours. It loses
three-fifths of its light, and regains it in twelve hours. In 1889
Vogel,[17] with the Potsdam spectrograph, actually found that the
luminous star is receding before each eclipse, and approaching us
after each eclipse; thus entirely supporting Goodricke’s opinion.
There are many variables of the Algol type, and information is
steadily accumulating. But all variable stars do not suffer the sudden
variations of Algol. There are many types, and the explanations of
others have not proved so easy.

The Harvard College photographs have disclosed the very great
prevalence of variability, and this is certainly one of the lines in
which modern discovery must progress.

Roberts, in South Africa, has done splendid work on the periods of
variables of the Algol type.

_New Stars_.–Extreme instances of variable stars are the new stars
such as those detected by Hipparchus, Tycho Brahe, and Kepler, of
which many have been found in the last half-century. One of the latest
great “Novae” was discovered in Auriga by a Scotsman, Dr. Anderson, on
February 1st, 1892, and, with the modesty of his race, he communicated
the fact to His Majesty’s Astronomer for Scotland on an unsigned
post-card.[18] Its spectrum was observed and photographed by Huggins
and many others. It was full of bright lines of hydrogen, calcium,
helium, and others not identified. The astounding fact was that lines
were shown in pairs, bright and dark, on a faint continuous spectrum,
indicating apparently that a dark body approaching us at the rate of
550 miles a second[19] was traversing a cold nebulous atmosphere, and
was heated to incandescence by friction, like a meteor in our
atmosphere, leaving a luminous train behind it. It almost disappeared,
and on April 26th it was of the sixteenth magnitude; but on August
17th it brightened to the tenth, showing the principal nebular band in
its spectrum, and no sign of approach or recession. It was as if it
emerged from one part of the nebula, cooled down, and rushed through
another part of the nebula, rendering the nebular gas more luminous
than itself.[20]

Since 1892 one Nova after another has shown a spectrum as described
above, like a meteor rushing towards us and leaving a train behind,
for this seems to be the obvious meaning of the spectra.

The same may be said of the brilliant Nova Persei, brighter at its
best than Capella, and discovered also by Dr. Anderson on February
22nd, 1901. It increased in brightness as it reached the densest part
of the nebula, then it varied for some weeks by a couple of
magnitudes, up and down, as if passing through separate nebular
condensations. In February, 1902, it could still be seen with an
opera-glass. As with the other Novae, when it first dashed into the
nebula it was vaporised and gave a continuous spectrum with dark lines
of hydrogen and helium. It showed no bright lines paired with the dark
ones to indicate a train left behind; but in the end its own
luminosity died out, and the nebular spectrum predominated.

The nebular illumination as seen in photographs, taken from August to
November, seemed to spread out slowly in a gradually increasing circle
at the rate of 90″ in forty-eight days. Kapteyn put this down to the
velocity of light, the original outburst sending its illumination to
the nebulous gas and illuminating a spherical shell whose radius
increased at the velocity of light. This supposition seems correct, in
which case it can easily be shown from the above figures that the
distance of this Nova was 300 light years.

_Star Catalogues._–Since the days of very accurate observations
numerous star-catalogues have been produced by individuals or by
observatories. Bradley’s monumental work may be said to head the list.
Lacaille’s, in the Southern hemisphere, was complementary. Then
Piazzi, Lalande, Groombridge, and Bessel were followed by Argelander
with his 324,000 stars, Rumker’s Paramatta catalogue of the southern
hemisphere, and the frequent catalogues of national observatories.
Later the Astronomische Gesellschaft started their great catalogue,
the combined work of many observatories. Other southern ones were
Gould’s at Cordova and Stone’s at the Cape.

After this we have a new departure. Gill at the Cape, having the comet
1882.ii. all to himself in those latitudes, wished his friends in
Europe to see it, and employed a local photographer to strap his
camera to the observatory equatoreal, driven by clockwork, and
adjusted on the comet by the eye. The result with half-an-hour’s
exposure was good, so he tried three hours. The result was such a
display of sharp star images that he resolved on the Cape Photographic
Durchmusterung, which after fourteen years, with Kapteyn’s aid in
reducing, was completed. Meanwhile the brothers Henry, of Paris, were
engaged in going over Chacornac’s zodiacal stars, and were about to
catalogue the Milky Way portion, a serious labour, when they saw
Gill’s Comet photograph and conceived the idea of doing the rest of
their work by photography. Gill had previously written to Admiral
Mouchez, of the Paris Observatory, and explained to him his project
for charting the heavens photographically, by combining the work of
many observatories. This led Admiral Mouchez to support the brothers
Henry in their scheme.[21] Gill, having got his own photographic work
underway, suggested an international astrographic chart, the materials
for different zones to be supplied by observatories of all nations,
each equipped with similar photographic telescopes. At a conference in
Paris, 1887, this was decided on, the stars on the charts going down
to the fourteenth magnitude, and the catalogues to the eleventh.

[Illustration: GREAT COMET, Nov. 14TH, 1882. (Exposure 2hrs. 20m.) By
kind permission of Sir David Gill. From this photograph originated all
stellar chart-photography.]

This monumental work is nearing completion. The labour involved was
immense, and the highest skill was required for devising instruments
and methods to read off the star positions from the plates.

Then we have the Harvard College collection of photographic plates,
always being automatically added to; and their annex at Arequipa in
Peru.

Such catalogues vary in their degree of accuracy; and fundamental
catalogues of standard stars have been compiled. These require
extension, because the differential methods of the heliometer and the
camera cannot otherwise be made absolute.

The number of stars down to the fourteenth magnitude may be taken at
about 30,000,000; and that of all the stars visible in the greatest
modern telescopes is probably about 100,000,000.

_Nebulae and Star-clusters._–Our knowledge of nebulae really dates from
the time of W. Herschel. In his great sweeps of the heavens with his
giant telescopes he opened in this direction a new branch of
astronomy. At one time he held that all nebulae might be clusters of
innumerable minute stars at a great distance. Then he recognised the
different classes of nebulae, and became convinced that there is a
widely-diffused “shining fluid” in space, though many so-called nebulae
could be resolved by large telescopes into stars. He considered that
the Milky Way is a great star cluster, whose form may be conjectured
from numerous star-gaugings. He supposed that the compact “planetary
nebulae” might show a stage of evolution from the diffuse nebulae, and
that his classifications actually indicate various stages of
development. Such speculations, like those of the ancients about the
solar system, are apt to be harmful to true progress of knowledge
unless in the hands of the ablest mathematical physicists; and
Herschel violated their principles in other directions. But here his
speculations have attracted a great deal of attention, and, with
modifications, are accepted, at least as a working hypothesis, by a
fair number of people.

When Sir John Herschel had extended his father’s researches into the
Southern Hemisphere he was also led to the belief that some nebulae
were a phosphorescent material spread through space like fog or mist.

Then his views were changed by the revelations due to the great
discoveries of Lord Rosse with his gigantic refractor,[22] when one
nebula after another was resolved into a cluster of minute stars. At
that time the opinion gained ground that with increase of telescopic
power this would prove to be the case with all nebulae.

In 1864 all doubt was dispelled by Huggins[23] in his first examination
of the spectrum of a nebula, and the subsequent extension of this
observation to other nebulae; thus providing a certain test which
increase in the size of telescopes could never have given. In 1864
Huggins found that all true nebulae give a spectrum of bright
lines. Three are due to hydrogen; two (discovered by Copeland) are
helium lines; others are unknown. Fifty-five lines have been
photographed in the spectrum of the Orion nebula. It seems to be
pretty certain that all true nebulae are gaseous, and show almost
exactly the same spectrum.

Other nebulae, and especially the white ones like that in Andromeda,
which have not yet been resolved into stars, show a continuous
spectrum; others are greenish and give no lines.

A great deal has to be done by the chemist before the astronomer can
be on sure ground in drawing conclusions from certain portions of his
spectroscopic evidence.

The light of the nebulas is remarkably actinic, so that photography
has a specially fine field in revealing details imperceptible in the
telescope. In 1885 the brothers Henry photographed, round the star
Maia in the Pleiades, a spiral nebula 3′ long, as bright on the plate
as that star itself, but quite invisible in the telescope; and an
exposure of four hours revealed other new nebula in the same
district. That painstaking and most careful observer, Barnard, with
10-1/4 hours’ exposure, extended this nebulosity for several degrees,
and discovered to the north of the Pleiades a huge diffuse nebulosity,
in a region almost destitute of stars. By establishing a 10-inch
instrument at an altitude of 6,000 feet, Barnard has revealed the wide
distribution of nebular matter in the constellation Scorpio over a
space of 4 degrees or 5 degrees square. Barnard asserts that the “nebular
hypothesis” would have been killed at its birth by a knowledge of
these photographs. Later he has used still more powerful instruments,
and extended his discoveries.

The association of stars with planetary nebulae, and the distribution
of nebulae in the heavens, especially in relation to the Milky Way, are
striking facts, which will certainly bear fruit when the time arrives
for discarding vague speculations, and learning to read the true
physical structure and history of the starry universe.

_Stellar Spectra._–When the spectroscope was first available for
stellar research, the leaders in this branch of astronomy were Huggins
and Father Secchi,[24] of Rome. The former began by devoting years of
work principally to the most accurate study of a few stars. The
latter devoted the years from 1863 to 1867 to a general survey of the
whole heavens, including 4,000 stars. He divided these into four
principal classes, which have been of the greatest service. Half of
his stars belonged to the first class, including Sirius, Vega,
Regulus, Altair. The characteristic feature of their spectra is the
strength and breadth of the hydrogen lines and the extreme faintness
of the metallic lines. This class of star is white to the eye, and
rich in ultra violet light.

The second class includes about three-eighths of his stars, including
Capella, Pollux, and Arcturus. These stars give a spectrum like that
of our sun, and appear yellowish to the eye.

The third class includes alpha Herculis, alpha Orionis (Betelgeux), Mira
Ceti, and about 500 red and variable stars. The spectrum has fluted
bands shaded from blue to red, and sharply defined at the more
refrangible edge.

The fourth class is a small one, containing no stars over fifth
magnitude, of which 152 Schjellerup, in Canes Venatici, is a good
example. This spectrum also has bands, but these are shaded on the
violet side and sharp on the red side. They are due to carbon in some
form. These stars are ruby red in the telescope.

It would appear, then, that all stars are suns with continuous
spectra, and the classes are differentiated by the character of the
absorbent vapours of their atmospheres.

It is very likely that, after the chemists have taught us how to
interpret all the varieties of spectrum, it will be possible to
ascribe the different spectrum-classes to different stages in the
life-history of every star. Already there are plenty of people ready
to lay down arbitrary assumptions about the lessons to be drawn from
stellar spectra. Some say that they know with certainty that each star
begins by being a nebula, and is condensed and heated by condensation
until it begins to shine as a star; that it attains a climax of
temperature, then cools down, and eventually becomes extinct. They go
so far as to declare that they know what class of spectrum belongs to
each stage of a star’s life, and how to distinguish between one that
is increasing and another that is decreasing in temperature.

The more cautious astronomers believe that chemistry is not
sufficiently advanced to justify all of these deductions; that, until
chemists have settled the lately raised question of the transmutation
of elements, no theory can be sure. It is also held that until they
have explained, without room for doubt, the reasons for the presence
of some lines, and the absence of others, of any element in a stellar
spectrum; why the arc-spectrum of each element differs from its spark
spectrum; what are all the various changes produced in the spectrum of
a gas by all possible concomitant variations of pressure and
temperature; also the meanings of all the flutings in the spectra of
metalloids and compounds; and other equally pertinent matters–until
that time arrives the part to be played by the astronomer is one of
observation. By all means, they say, make use of “working hypotheses”
to add an interest to years of laborious research, and to serve as a
guide to the direction of further labours; but be sure not to fall
into the error of calling any mere hypothesis a theory.

_Nebular Hypothesis._–The Nebular Hypothesis, which was first, as it
were, tentatively put forward by Laplace as a note in his _Systeme du
Monde_, supposes the solar system to have been a flat, disk-shaped
nebula at a high temperature in rapid rotation. In cooling it
condensed, leaving revolving rings at different distances from the
centre. These themselves were supposed to condense into the nucleus
for a rotating planet, which might, in contracting, again throw off
rings to form satellites. The speculation can be put in a really
attractive form, but is in direct opposition to many of the actual
facts; and so long as it is not favoured by those who wish to maintain
the position of astronomy as the most exact of the sciences–exact in
its facts, exact in its logic–this speculation must be recorded by
the historian, only as he records the guesses of the ancient Greeks–as
an interesting phase in the history of human thought.

Other hypotheses, having the same end in view, are the meteoritic
hypothesis of Lockyer and the planetesimal hypothesis that has been
largely developed in the United States. These can best be read in the
original papers to various journals, references to which may be found
in the footnotes of Miss Clerke’s _History of Astronomy during the
Nineteenth Century_. The same can be said of Bredichin’s hypothesis of
comets’ tails, Arrhenius’s book on the applications of the theory of
light repulsion, the speculations on radium, the origin of the sun’s
heat and the age of the earth, the electron hypothesis of terrestrial
magnetism, and a host of similar speculations, all combining to throw
an interesting light on the evolution of a modern train of thought
that seems to delight in conjecture, while rebelling against that
strict mathematical logic which has crowned astronomy as the queen of
the sciences.

FOOTNOTES:

[1] _R. S. Phil Trans_., 1810 and 1817-24.

[2] One of the most valuable contributions to our knowledge of stellar
parallaxes is the result of Gill’s work (_Cape Results_, vol. iii.,
part ii., 1900).

[3] Taking the velocity of light at 186,000 miles a second, and the
earth’s mean distance at 93,000,000 miles, 1 light year=5,865,696,000,000
miles or 63,072 astronomical units; 1 astronomical unit a year=2.94
miles a second; and the earth’s orbital velocity=18.5 miles a second.

[4] Ast. Nacht., 1889.

[5] R. S. Phil. Trans., 1718.

[6] Mem. Acad. des Sciences, 1738, p. 337.

[7] R. S Phil. Trans., 1868.

[8] _R.S. Phil Trans._, 1783.

[9] See Kapteyn’s address to the Royal Institution, 1908. Also Gill’s
presidential address to the British Association, 1907.

[10] _Brit. Assoc. Rep._, 1905.

[11] R. S. Phil. Trans., 1803, 1804.

[12] Ibid, 1824.

[13] Connaisance des Temps, 1830.

[14] _R. A. S. Mem._, vol. xlvii., p. 178; _Ast. Nach._, No. 3,142;
Catalogue published by Lick Observatory, 1901.

[15] _R. A. S., M. N._, vol. vi.

[16] _R. S. Phil. Trans._, vol. lxxiii., p. 484.

[17] _Astr. Nach._, No. 2,947.

[18] _R. S. E. Trans_., vol. xxvii. In 1901 Dr. Anderson discovered
Nova Persei.

[19] _Astr. Nach_., No. 3,079.

[20] For a different explanation see Sir W. Huggins’s lecture, Royal
Institution, May 13th, 1892.

[21] For the early history of the proposals for photographic
cataloguing of stars, see the _Cape Photographic Durchmusterung_, 3
vols. (_Ann. of the Cape Observatory_, vols. in., iv., and v.,
Introduction.)

[22] _R. S. Phil. Trans._, 1850, p. 499 _et seq._

[23] _Ibid_, vol. cliv., p. 437.

[24] _Brit. Assoc. Rep._, 1868, p. 165.