THE NEW ASTRONOMY
During the latter half of the nineteenth century discoveries were made
which extended the powers of astronomical research into entirely new and
unexpected regions, comparable to those which were opened up by the
discovery of the telescope more than two centuries before. The older
astronomy for more than two thousand years was purely mechanical and
mathematical, being limited to observation and measurement of the apparent
motions of the heavenly bodies, and the attempts to deduce, from these
apparent motions, their real motions, and thus determine the actual
structure of the solar system. This was first done when Kepler established
his three celebrated laws: and later, when Newton showed that these laws
were necessary consequences of the one law of gravitation, and when
succeeding observers and mathematicians proved that each fresh irregularity
in the motions of the planets was explicable by a more thorough and minute
application of the same laws, this branch of astronomy reached its highest
point of efficiency and left very little more to be desired.
Then, as the telescope became successively improved, the centre of interest
was shifted to the surfaces of the planets and their satellites, which
were watched and scrutinised with the greatest assiduity in order if
possible to attain some amount of knowledge of their physical constitution
and past history. A similar minute scrutiny was given to the stars and
nebulæ, their distribution and grouping, and the whole heavens were mapped
out, and elaborate catalogues constructed by enthusiastic astronomers in
every part of the world. Others devoted themselves to the immensely
difficult problem of determining the distances of the stars, and by the
middle of the century a few such distances had been satisfactorily
measured.
Thus, up to the middle of the nineteenth century it appeared likely that
the future of astronomy would rest almost entirely on the improvement of
the telescope, and of the various instruments of measurement by means of
which more accurate determinations of distances might be obtained. Indeed,
the author of the Positive Philosophy, Auguste Comte, felt so sure of this
that he deprecated all further attention to the stars as pure waste of time
that could never lead to any useful or interesting result. In his
_Philosophical Treatise on Popular Astronomy_ published in 1844, he wrote
very strongly on this point. He there tells us that, as the stars are only
accessible to us by sight they must always remain very imperfectly known.
We can know little more than their mere existence. Even as regards so
simple a phenomenon as their temperature this must always be inappreciable
to a purely visual examination. Our knowledge of the stars is for the most
part purely negative, that is, we can determine only that they do _not_
belong to our system. Outside that system there exists, in astronomy, only
obscurity and confusion, for want of indispensable facts; and he concludes
thus:--'It is, then, in vain that for half a century it has been
endeavoured to distinguish two astronomies, the one solar the other
sidereal. In the eyes of those for whom science consists of real laws and
not of incoherent facts, the second exists only in name, and the first
alone constitutes a true astronomy; and I am not afraid to assert that it
will always be so.' And he adds that--'all efforts directed to this subject
for half a century have only produced an accumulation of incoherent
empirical facts which can only interest an irrational curiosity.'
Seldom has a confident assertion of finality in science received so
crushing a reply as was given to the above statements of Comte by the
discovery in 1860 (only three years after his death) of the method of
spectrum-analysis which, in its application to the stars, has
revolutionised astronomy, and has enabled us to obtain that very kind of
knowledge which he declared must be for ever beyond our reach. Through it
we have acquired accurate information as to the physics and chemistry of
the stars and nebulæ, so that we now know really more of the nature,
constitution, and temperature of the enormously distant suns which we
distinguish by the general term stars, than we do of most of the planets of
our own system. It has also enabled us to ascertain the existence of
numerous invisible stars, and to determine their orbits, their rate of
motion, and even, approximately, their mass. The despised stellar astronomy
of the early part of the century has now taken rank as the most profoundly
interesting department of that grand science, and the branch which offers
the greatest promise of future discoveries. As the results obtained by
means of this powerful instrument will often be referred to, a short
account of its nature and of the principles on which it depends must here
be given.
The solar spectrum is the band of coloured light seen in the rainbow and,
partially, in the dew-drop, but more completely when a ray of sunlight
passes through a prism--a piece of glass having a triangular section. The
result is, that instead of a spot of white light we have a narrow band of
brilliant colours which succeed each other in regular order, from violet at
one end through blue, green, and yellow to red at the other. We thus see
that light is not a simple and uniform radiation from the sun, but is made
up of a large number of separate rays, each of which produces in our eyes
the sensation of a distinct colour. Light is now explained as being due to
vibrations of ether, that mysterious substance which not only permeates all
matter, but which fills space at least as far as the remotest of the
visible stars and nebulæ. The exceedingly minute waves or vibrations of the
ether produce all the phenomena of heat, light, and colour, as well as
those chemical actions to which photography owes its wonderful powers. By
ingenious experiments the size and rate of vibration of these waves have
been measured, and it is found that they vary considerably, those forming
the red light, which is least refracted, having a wave-length of about
1/326000 of an inch, while the violet rays at the other end of the spectrum
are only about half that length or 1/630000 of an inch. The rate at which
the vibrations succeed each other is from 302 millions of millions per
second for the extreme red rays, to 737 millions of millions for those at
the violet end of the spectrum. These figures are given to show the
wonderful minuteness and rapidity of these heat and light waves on which
the whole life of the world, and all our knowledge of other worlds and
other suns, directly depends.
But the mere colours of the spectrum are not the most important part of it.
Very early in the nineteenth century a close examination showed that it was
everywhere crossed by black lines of various thicknesses, sometimes single,
sometimes grouped together. Many observers studied them and made accurate
drawings or maps showing their positions and thicknesses, and by combining
several prisms, so that the beam of sunlight had to pass through them
successively, a spectrum could be produced several feet long, and more than
3000 of these dark lines were counted in it. But what they were and how
they were caused remained a mystery, till, in the year 1860, the German
physicist Kirchhoff discovered the secret and gave to chemists and
astronomers a new and quite unexpected engine of research.
It had already been observed that the chemical elements and various
compounds, when heated to incandescence, produced spectra consisting of
coloured lines or bands which were constant for each element, so that the
elements could at once be recognised by their characteristic spectra; and
it had also been noticed that some of these bands, especially the yellow
band produced by sodium, corresponded in position with certain black lines
in the solar spectrum. Kirchhoff's discovery consisted in showing that,
when the light from an incandescent body passes through the same substance
in a state of vapour or gas, so much of the light is absorbed that the
coloured lines or bands become black. The mystery of more than half a
century was thus solved; and the thousands of black lines in the solar
spectrum were shown to be caused by the light from the incandescent matter
of the sun's surface passing through the heated gases or vapours
immediately above it, and thereby having the bright coloured lines of their
spectra changed, by absorption, to comparative blackness.
Chemists and physicists immediately set to work examining the spectra of
the elements, fixing the position of the several coloured lines or bands by
accurate measurement, and comparing them with the dark lines of the solar
spectrum. The results were in the highest degree satisfactory. In a large
proportion of the elements the coloured bands corresponded exactly with a
group of dark lines in the spectrum of the sun, in which, therefore, the
same terrestrial elements were proved to exist. Among the elements first
detected in this manner were hydrogen, sodium, iron, copper, magnesium,
zinc, calcium, and many others. Nearly forty of the elements have now been
found in the sun, and it seems highly probable that all our elements really
exist there, but as some are very rare and are present in very minute
quantities they cannot be detected. Some of the dark lines in the sun were
found not to correspond to any known element, and as this was thought to
indicate an element peculiar to the sun it was named Helium; but quite
recently it has been discovered in a rare mineral. Many of the elements
are represented by a great number of lines, others by very few. Thus iron
has more than 2000, while lead and potassium have only one each.
The value of the spectroscope both to the chemist in discovering new
elements and to the astronomer in determining the constitution of the
heavenly bodies, is so great, that it became of the highest importance to
have the position of all the dark lines in the solar spectrum, as well as
the bright lines of all the elements, determined with extreme accuracy, so
as to be able to make exact comparisons between different spectra. At first
this was done by means of very large-scale drawings showing the exact
position of every dark or bright line. But this was found to be both
inconvenient and not sufficiently exact; and it was therefore agreed to
adopt the natural scale of the wave-lengths of the different parts of the
spectrum, which by means of what are termed diffraction-gratings can now be
measured with great accuracy. Diffraction-gratings are formed of a polished
surface of hard metal ruled with excessively fine lines, sometimes as many
as 20,000 to an inch. When sunlight falls upon one of these gratings it is
reflected, and by interference of the rays from the spaces between the fine
grooves, it is spread out into a beautiful and well-defined spectrum,
which, when the lines are very close, is several yards in length. In these
diffraction spectra many dark lines are seen which can be shown in no other
way, and they also give a spectrum which is far more uniform than that
produced by glass prisms in which minute differences in the composition of
the glass cause some rays to be refracted more and others less than the
normal amount.
The spectra produced by diffraction-gratings are double; that is, they are
spread out on both sides of the central line of the ray which remains
white, and the several coloured or dark lines are so clearly defined that
they can be thrown on a screen at a considerable distance, giving a great
length to the spectrum. The data for obtaining the wave-lengths are the
distance apart of the lines, the distance of the screen, and the distance
apart of the first pair of dark lines on each side of the central bright
line. All these can be measured with extreme accuracy by means of
telescopes with micrometers and other contrivances, and the result is an
accuracy of determination of wave-lengths which can probably not be
equalled in any other kind of measurement.
As the wave-lengths are so excessively minute, it has been found convenient
to fix upon a still smaller unit of measurement, and as the millimetre is
the smallest unit of the metric system, the ten-millionth of a millimetre
(technically termed 'tenth meter') is the unit adopted for the measurement
of wave-lengths, which is equal to about the 250 millionth of an inch. Thus
the wave-lengths of the red and blue lines characteristic of hydrogen are
6563.07 and 4861.51 respectively. This excessively minute scale of
wave-lengths, once determined by the most refined measurement, is of very
great importance. Having the wave-lengths of any two lines of a spectrum so
determined, the space between them can be laid down on a diagram of any
length, and all the lines that occur in any other spectrum between these
two lines can be marked in their exact relative positions. Now, as the
visible spectrum consists of about 300,000 rays of light, each of different
wave-lengths and therefore of different refrangibilities, if it is laid
down on such a scale as to be of a length of 3000 inches (250 feet), each
wave-length will be 1/100 of an inch long, a space easily visible by the
naked eye.
The possession of an instrument of such wonderful delicacy, and with powers
which enable it to penetrate into the inner constitution of the remotest
orbs of space, rendered it possible, within the next quarter of a century,
to establish what is practically a new science--Astrophysics--often
popularly termed the New Astronomy. A brief outline of the main
achievements of this science must now be given.
The first great discovery made by Spectrum analysis, after the
interpretation of the sun's spectrum had been obtained, was, the real
nature of the fixed stars. It is true they had long been held by
astronomers to be suns, but this was only an opinion of the accuracy of
which it did not seem possible to obtain any proof. The opinion was founded
on two facts--their enormous distance from us, so great that the whole
diameter of the earth's orbit did not lead to any apparent change of their
relative positions, and their intense brilliancy which at such distances
could only be due to an actual size and splendour comparable with our sun.
The spectroscope at once proved the correctness of this opinion. As one
after another was examined, they were found to exhibit spectra of the same
general type as that of the sun--a band of colours crossed by dark lines.
The very first stars examined by Sir William Huggins showed the existence
of nine or ten of our elements. Very soon all the chief stars of the
heavens were spectroscopically examined, and it was found that they could
be classed in three or four groups. The first and largest group contains
more than half the visible stars, and a still larger proportion of the most
brilliant, such as Sirius, Vega, Regulus, and Alpha Crucis in the Southern
Hemisphere. They are characterised by a white or bluish light, rich in the
ultra-violet rays, and their spectra are distinguished by the breadth and
intensity of the four dark bands due to the absorption of hydrogen, while
the various black lines which indicate metallic vapours are comparatively
few, though hundreds of them can be discovered by careful examination.
The next group, to which Capella and Arcturus belong, is also very
numerous, and forms the solar type of stars. Their light is of a yellowish
colour, and their spectra are crossed throughout by innumerable fine dark
lines more or less closely corresponding with those in the solar spectrum.
The third group consists of red and variable stars, which are characterised
by fluted spectra. Such spectra show like a range of Doric columns seen in
perspective, the red side being that most illuminated.
The last group, consisting of few and comparatively small stars, has also
fluted spectra, but the light appears to come from the opposite direction.
These groups were established by Father Secchi, the Roman astronomer, in
1867, and have been adopted with some modifications by Vogel of the
Astrophysical Observatory at Potsdam. The exact interpretation of these
different spectra is somewhat uncertain, but there can be little doubt that
they coincide primarily with differences of temperature and with
corresponding differences in the composition and extent of the absorptive
atmospheres. Stars with fluted spectra indicate the presence of vapours of
the metalloids or of compound substances, while the reversed flutings
indicate the presence of carbon. These conclusions have been reached by
careful laboratory experiments which are now carried on at the same time as
the spectral examination of the stars and other heavenly bodies, so that
each peculiarity of their spectra, however puzzling and apparently
unmeaning, has been usually explained, by being shown to indicate certain
conditions of chemical constitution or of temperature.
But whatever difficulty there may be in explaining details, there remains
no doubt whatever of the fundamental fact that all the stars are true suns,
differing no doubt in size, and their stage of development as indicated by
the colour or intensity of their light or heat, but all alike possessing a
photosphere or light-emitting surface, and absorptive atmospheres of
various qualities and density.
Innumerable other details, such as the often contrasted colours of double
stars, the occasional variability of their spectra, their relations to the
nebulæ, the various stages of their development and other problems of equal
interest, have occupied the continued attention of astronomers,
spectroscopists, and chemists; but further reference to these difficult
questions would be out of place here. The present sketch of the nature of
spectrum-analysis applied to the stars is for the purpose of making its
principle and method of observation intelligible to every educated reader,
and to illustrate the marvellous precision and accuracy of the results
attained by it. So confident are astronomers of this accuracy that nothing
less than _perfect correspondence_ of the various bright lines in the
spectrum of an element in the laboratory with the dark lines in the
spectrum of the sun or of a star is required before the presence of that
element is accepted as proved. As Miss Clerke tersely puts
it--'Spectroscopic coincidences admit of no compromise. Either they are
absolute or they are worthless.'
MEASUREMENT OF MOTION IN THE LINE OF SIGHT
We must now describe another and quite distinct application of the
spectroscope, which is even more marvellous than that already described. It
is the method of measuring the rate of motion of any of the visible
heavenly bodies in a direction either directly towards us, or directly away
from us, technically described as 'radial motion,' or by the
expression--'in the line of sight.' And the extraordinary thing is that
this power of measurement is altogether independent of distance, so that
the rate of motion in miles per second of the remotest of the fixed stars,
if sufficiently bright to show a distinct spectrum, can be measured with as
much certainty and accuracy as in the case of a much nearer star or a
planet.
In order to understand how this is possible we have again to refer to the
wave-theory of light; and the analogy of other wave-motions will enable us
better to grasp the principle on which these calculations depend. If on a
nearly calm day we count the waves that pass each minute by an anchored
steamboat, and then travel in the direction the waves come from, we shall
find that a larger number pass us in the same time. Again, if we are
standing near a railway, and an engine comes towards us whistling, we shall
notice that it changes its tone as it passes us; and as it recedes the
sound will be in a lower key, although the engine may be at exactly the
same distance from us as when it was approaching. Yet the sound does not
change to the ear of the engine driver, the cause of the change being that
the sound-waves reach us in quicker succession as the source of the waves
is approaching us than when it is retreating from us. Now, just as the
pitch of a note depends upon the rapidity with which the successive
air-vibrations reach our ear, so does the colour of a particular part of
the spectrum depend upon the rapidity with which the ethereal waves which
produce colour reach our eyes; and as this rapidity is greater when the
source of the light is approaching than when it is receding from us, a
slight shifting of the position of the coloured bands, and therefore of the
dark lines, will occur, as compared with their position in the spectrum of
the sun or of any stationary source of light, if there is any motion
sufficient in amount to produce a perceptible shift.
That such a change of colour would occur was pointed out by Professor
Doppler of Prague in 1842, and it is hence usually spoken of as the
'Doppler principle'; but as the changes of colour were so minute as to be
impossible of measurement it was not at that time of any practical
importance in astronomy. But when the dark lines in the spectrum were
carefully mapped, and their positions determined with minute accuracy, it
was seen that a means of measuring the changes produced by motion in the
line of sight existed, since the position of any of the dark or coloured
lines in the spectra of the heavenly bodies could be compared with those of
the corresponding lines produced artificially in the laboratory. This was
first done in 1868 by Sir William Huggins, who, by the use of a very
powerful spectroscope constructed for the purpose, found that such a change
did occur in the case of many stars, and that their rate of motion towards
us or away from us--the radial motion--could be calculated. As the actual
distance of some of these stars had been measured, and their change of
position annually (their proper motion) determined, the additional factor
of the amount of motion in the direction of our line of sight completed the
data required to fix their true line of motion among the other stars. The
accuracy of this method under favourable conditions and with the best
instruments is very great, as has been proved by those cases in which we
have independent means of calculating the real motion. The motion of Venus
towards or away from us can be calculated with great accuracy for any
period, being a resultant of the combined motions of the planet and of our
earth in their respective orbits. The radial motions of Venus were
determined at the Lick Observatory in August and September 1890, by
spectroscopic observations, and also by calculation, to be as follows:--
By Observation. By Calculation.
Aug. 16th. 7.3 miles per second. 8.1 miles per second.
" 22nd. 8.9 " " " 8.2 " " "
" 30th. 7.3 " " " 8.3 " " "
Sep. 3rd. 8.3 " " " 8.3 " " "
" 4th. 8.2 " " " 8.3 " " "
showing that the maximum error was only one mile per second, while the mean
error was about a quarter of a mile. In the case of the stars the accuracy
of the method has been tested by observations of the same star at times
when the earth's motion in its orbit is towards or away from the star,
whose apparent radial velocity is, therefore, increased or diminished by a
known amount. Observations of this kind were made by Dr. Vogel, Director of
the Astrophysical Observatory at Potsdam, showing, in the case of three
stars, of which ten observations were taken, a mean error of about two
miles per second; but as the stellar motions are more rapid than those of
the planets, the proportionate error is no greater than in the example
given above.
The great importance of this mode of determining the real motion of the
stars is, that it gives us a knowledge of the scale on which such motions
are progressing; and when in the course of time we discover whether any of
their paths are rectilinear or curved, we shall be in a position to learn
something of the nature of the changes that are going on and of the laws on
which they depend.
INVISIBLE STARS AND IMPERCEPTIBLE MOTIONS
But there is another result of this power of determining radial motion
which is even more unexpected and marvellous, and which has extended our
knowledge of the stars in quite a new direction. By its means it is
possible to determine the existence of invisible stars and to measure the
rate of otherwise imperceptible motions; that is of stars which are
invisible in the most powerful modern telescopes, and whose motions have
such a limited range that no telescope can detect them.
Double or binary stars forming systems which revolve around their common
centre of gravity were discovered by Sir William Herschel, and very great
numbers are known; but in most cases their periods of revolution are long,
the shortest being about twelve years, while many extend to several hundred
years. These are, of course, all visible binaries, but many are now known
of which one star only is visible while the other is either non-luminous or
is so close to its companion that they appear as a single star in the most
powerful telescopes. Many of the variable stars belong to the former class,
a good example of which is Algol in the constellation Perseus, which
changes from the second to the fourth magnitude in about four and a half
hours, and in about four and a half hours more regains its brilliancy till
its next period of obscuration which occurs regularly every two days and
twenty-one hours. The name Algol is from the Arabic _Al Ghoul_, the
familiar 'ghoul' of the Arabian Nights, so named--'The Demon'--from its
strange and weird behaviour.
It had long been conjectured that this obscuration was due to a dark
companion which partially eclipsed the bright star at every revolution,
showing that the plane of the orbit of the pair was almost exactly directed
towards us. The application of the spectroscope made this conjecture a
certainty. At an equal time before and after the obscuration, motion in the
line of sight was shown, towards and away from us, at a rate of twenty-six
miles per second. From these scanty data and the laws of gravitation which
fix the period of revolution of planets at various distances from their
centres of revolution, Professor Pickering of the Harvard Observatory was
able to arrive at the following figures as highly probable, and they may be
considered to be certainly not far from the truth.
Diameter of Algol, 1,061,000 miles.
Diameter of dark companion, 830,000 "
Distance between their centres, 3,230,000 "
Orbital speed of Algol, 26.3 miles per sec.
Orbital speed of companion, 55.4 " " "
Mass of Algol, 4/9 mass of our Sun.
Mass of companion, 2/9 " " "
When it is considered that these figures relate to a pair of stars only one
of which has ever been seen, that the orbital motion even of the visible
star cannot be detected in the most powerful telescopes, when, further, we
take into account the enormous distance of these objects from us, the great
results of spectroscopic observation will be better appreciated.
But besides the marvel of such a discovery by such simple means, the facts
discovered are themselves in the highest degree marvellous. All that we had
known of the stars through telescopic observation indicated that they were
at very great distances from each other however thickly they may appear
scattered over the sky. This is the case even with close telescopic double
stars, owing to their enormous remoteness from us. It is now estimated that
even stars of the first magnitude are, on a general average, about eighty
millions of millions of miles distant; while the closest double stars that
can be distinctly separated by large telescopes are about half a second
apart. These, if at the above distance, will be about 1500 millions of
miles from each other. But in the case of Algol and its companion, we have
two bodies both larger than our sun, yet with a distance of only 2-1/4
millions of miles between their surfaces, a distance not much exceeding
their combined diameters. We should not have anticipated that such huge
bodies could revolve so closely to each other, and as we now know that the
neighbourhood of our sun--and probably of all suns--is full of meteoric and
cometic matter, it would seem probable that in the case of two suns so near
together the quantity of such matter would be very great, and would lead
probably by continued collisions to increase of their bulk, and perhaps to
their final coalescence into a single giant orb. It is said that a Persian
astronomer in the tenth century calls Algol a red star, while it is now
white or somewhat yellowish. This would imply an increase of temperature
caused by collisions or friction, and increasing proximity of the pair of
stars.
A considerable number of double stars with dark companions have been
discovered by means of the spectroscope, although their motion is not
directly in the line of sight, and therefore there is no obscuration. In
order to discover such pairs the spectra of large numbers of stars are
taken on photographic plates every night and for considerable periods--for
a year or for several years. These plates are then carefully examined with
a high magnifying power to discover any periodical displacement of the
lines, and it is astonishing in how large a number of cases this has been
found to exist and the period of revolution of the pair determined.
But besides discovering double stars of which one is dark and one bright,
many pairs of bright stars have been discovered by the same means. The
method in this case is rather different. Each component star, being
luminous, will give a separate spectrum, and the best spectroscopes are so
powerful that they will separate these spectra when the stars are at their
maximum distance although no telescope in existence, or ever likely to be
made, can separate the component stars. The separation of the spectra is
usually shown by the most prominent lines becoming double and then after a
time single, indicating that the plane of revolution is more or less
obliquely towards us, so that the two stars if visible would appear to open
out and then get nearer together every revolution. Then, as each star
alternately approaches and recedes from us the radial velocity of each can
be determined, and this gives the relative mass. In this way not only
doubles, but triple and multiple systems, have been discovered. The stars
proved to be double by these two methods are so numerous that it has been
estimated by one of the best observers that about one star in every
thirteen shows inequality in its radial motion and is therefore really a
double star.
THE NEBULÆ
One other great result of spectrum-analysis, and in some respects perhaps
the greatest, is its demonstration of the fact that true nebulæ exist, and
that they are not all star-clusters so remote as to be irresolvable, as was
once supposed. They are shown to have gaseous spectra, or sometimes gaseous
and stellar spectra combined, and this, in connection with the fact that
nebulæ are frequently aggregated around nebulous stars or groups of stars,
renders it certain that the nebulæ are in no way separated in space from
the stars, but that they constitute essential parts of one vast stellar
universe. There is, indeed, good reason to believe that they are really the
material out of which stars are made, and that in their forms,
aggregations, and condensations, we can trace the very process of evolution
of stars and suns.
PHOTOGRAPHIC ASTRONOMY
But there is yet another powerful engine of research which the new
astronomy possesses, and which, either alone or in combination with the
spectroscope, had produced and will yet produce in the future an amount of
knowledge of the stellar universe which could never be attained by any
other means. It has already been stated how the discovery of new variable
and binary stars has been rendered possible by the preservation of the
photographic plates on which the spectra are self-recorded, night after
night, with every line, whether dark or coloured, in true position, so as
to bear magnification, and, by comparison with others of the series,
enabling the most minute changes to be detected and their amount accurately
measured. Without the preservation of such comparable records, which is in
no other way possible, by far the larger portion of spectroscopic
discoveries could never have been made.
But there are two other uses of photography of quite a different nature
which are equally and perhaps in their final outcome may be far more
important. The first is, that by the use of the photographic plate the
exact positions of scores, hundreds, or even thousands of stars can be
self-mapped simultaneously with extreme accuracy, while any number of
copies can be made of these star-maps. This entirely obviates the necessity
for the old method of fixing the position of each star by repeated
measurement by means of very elaborate instruments, and their registration
in laborious and expensive catalogues. So important is this now seen to be,
that specially constructed cameras are made for stellar photography, and by
means of the best kinds of equatorial mounting are made to revolve slowly
so that the image of each star remains stationary upon the plate for
several hours.
Arrangements have been now made among all the chief observatories of the
world to carry out a photographic survey of the heavens with identical
instruments, so as to produce maps of the whole star-system on the same
scale. These will serve as fixed data for future astronomers, who will thus
be able to determine the movements of stars of all magnitudes with a
certainty and accuracy hitherto unattainable.
The other important use of photography depends upon the fact that with a
longer exposure within certain limits we increase the light-collecting
power. It will surprise many persons to learn that an ordinary good
portrait-camera with a lens three or four inches in diameter, if properly
mounted so that an exposure of several hours can be made, will show stars
so minute that they are invisible even in the great Lick telescope. In this
way the camera will often reveal double-stars or small groups which can be
made visible in no other way.
Such photographs of the stars are now constantly reproduced in works on
Astronomy and in popular magazine articles, and although some of them are
very striking, many persons are disappointed with them, and cannot
understand their great value, because each star is represented by a white
circle often of considerable size and with a somewhat undefined outline,
not by a minute point of light as stars appear in a good telescope. But the
essential matter in all such photographs is not so much the smallness, as
the roundness, of the star-images, as this proves the extreme precision
with which the image of every star has been kept by the clockwork motion of
the instrument on the same point of the plate during the whole exposure.
For example, in the fine photograph of the Great Nebula in Andromeda, taken
29th December 1888, by Dr. Isaac Roberts, with an exposure of four hours,
there are probably over a thousand stars large and small to be seen, every
one represented by an almost exactly circular white dot of a size dependent
on the magnitude of the star. These round dots can be bisected by the cross
hairs of a micrometer with very great accuracy, and thus the distance
between the centres of any of the pairs, as well as the direction of the
line joining their centres, can be determined as accurately as if each was
represented by a point only. But as a minute white speck would be almost
invisible on the maps, and would convey no information as to the
approximate magnitude of the star, mistakes would be much more easily made,
and it would probably be found necessary to surround each star with a
circle to indicate its magnitude, and to enable it to be easily seen. It is
probable, therefore, that the supposed defect is really an important
advantage. The above-mentioned photograph is beautifully reproduced in
Proctor's _Old and New Astronomy_, published after his greatly lamented
death.
But besides the amount of altogether new knowledge obtained by the methods
of research here briefly explained, a great deal of light has been thrown
on the distribution of the stars as a whole, and hence on the nature and
extent of the stellar universe, by a careful study of the materials
obtained by the old methods, and by the application of the doctrine of
probabilities to the observed facts. In this way alone some very striking
results have been reached, and these have been supported and strengthened
by the newer methods, and also by the use of new instruments in the
measurement of stellar distances. Some of these results bear so closely and
directly upon the special subject of the present volume, that our next
chapter must be devoted to a consideration of them.