STELLAR PHOTOGRAPHY.
Photography is not only a useful handmaid to astronomy as a whole, but
also it is so, peculiarly, to that division of astronomy with which we
are now chiefly concerned--to spectroscopy. A few pages may therefore
with advantage be given to the subject of celestial photography.
Photography is, indeed, the greatest possible help to the astronomer.
It pictures for him those stars which his eyes can see but dimly, or
even can not see at all; it paints for him those light-rays of which he
would obtain but a passing glance, and which he could not accurately
remember in all their details; it maps out for him the wide heavens,
which he, unaided, could never do with anything like equal completeness
by eye and hand alone.
Only recently a vast photographic map of the whole sky was undertaken.
About eighteen different observatories, in divers parts of the world,
divided the task among them, stars down to the sixteenth magnitude
being most carefully registered in a complete series of something like
fifteen hundred separate photographs. The whole result, when finally
completed, will be a grand achievement of the present century. Each
individual star, in the entire heavens all around our earth, from the
first to the sixteenth magnitudes, will have its exact position in
the sky accurately known, and the smallest change in the position of
anyone of those stars may then be detected.
Even in the delicate and abstruse operation earlier described, the
measurement of star-distances through annual parallax, photography
again steps in. Dr. Pritchard, Savilian professor of Astronomy at
Oxford, pressed photography into the service of this task also.
Measurements for parallax were made under his direction upon
photographic plates--a work of no small interest. Here, as elsewhere,
peculiar advantages belong to the photographic method when it can be
followed. Its records are lasting, the limited number of hours which
are fully suitable for direct astronomical work may be employed in
obtaining those records, and in broad daylight the examination of them
can be carried on.
One very curious use is made of star-photographs, more especially of
the photographs of unseen stars--that is, of stars too distant or too
dim to be detected by the eye. These photographs, when taken, may
be afterwards looked into further with a microscope. So, first, the
far-off, invisible suns of the universe are photographed on a prepared
plate, with the help of a powerful telescope, this being needful to
secure sufficient starlight; and then the tiny picture of those suns is
examined more closely with the help of a powerful microscope.
Spectroscopy has much to say to us. It tells us about the positions
of the different stars. It tells us about the structure of the stars.
It tells us about the various classes to which the various stars
belong. And also it tells us about the motions of the stars--not mere
apparent motions, caused by movements of our own earth, but true onward
journeyings of the stars themselves through the depths of space.
For by means of photography we do not obtain simple pictures of
the stars themselves _only_, but pictures also of the _spectra_ of
the stars. An instrument is made uniting the spectroscope with the
photographic apparatus, and this is called a spectrograph. By its
means the disintegrated or broken-up star-ray is photographed in its
broken-up condition, so that an exact picture is obtained of the bands
and lines characteristic of any particular star.
No easy matter, as may be imagined, are these spectroscopic
observations and these spectroscopic photographings of the stars. To
bring the image of a star exactly opposite a slender opening or slit,
perhaps only about the _three-hundredth part of an inch in width_,
and to keep it there, is a task which might well be looked upon as
practically impossible.
No sooner is a star found in the field of a telescope than it vanishes
again. As the astronomer gazes, the ground beneath him is ever whirling
onward, leaving the star behind; and although clockwork apparatus in
all observatories of any importance is made to counteract this motion
of the earth, and to keep the heavenly object, whether star or moon, or
comet, within view by following its apparent motion, yet, as can easily
be imagined, to follow thus the seeming motion of a dim star through an
opening so minute, demands exceeding care and delicacy of adjustment.
It is not indeed, for purposes of analysis, _always_ needful to pass
the light of a star through a narrow slit. In the case of a nearer
body, such as the sun or moon or one of the planets, light flows
from all parts of the body, one ray crossing another; and for the
examination of such light, a slit is imperatively needed, all side-rays
having to be cut off. But the whole of the brightest star in the sky
is only one point of light to us, and a slitless telescope may be
used with no confused results. Where, however, direct comparison is
required, the star-lines being made to appear, side by side or above
and below, with the solar spectrum, or with the lines of earthly
metals, then the slit becomes unavoidable.
By such comparison of the two, side by side--the light from an
intensely-heated earthly substance and the light from a star, the rays
of both being broken up in the spectroscope--the oneness or difference
of lines in the rays can be easily made out.
One main difficulty in such observations arises from the diminution
of starlight, caused by its passage through a prism. If a rope of a
dozen strands is untwisted, each of those strands is far weaker than
the whole rope was; and each strand of color in the twisted rope of
light is of necessity much more feeble, seen by itself, than the whole
white ray seen in one. Another difficulty in our country generally
is the climate, which gives so few days or hours in the year for
effective work. Yet the full amount accomplished by seizing upon every
possible opportunity is, in the aggregate, astonishing. A third and
very pressing difficulty attendant upon examination of star-spectra is
caused by the incessant motion of our air, through which, as through
a veil, all observations have to be made. The astronomer can never
get away from the atmosphere; and unless the air be very still--that
is to say, as still as it ever can remain--the spectrum-lines are
so uncertainly seen as to make satisfactory results impossible. Dr.
Huggins has sometimes passed hours in the examination of a single line,
unable to determine whether or no it precisely coincided with the
comparison-line of some earthly substance. In _this_ matter, no leaping
at conclusions is admissible.
The photography of stars would be easy enough if one could just
expose a plate to the shining of the star, and there leave it to be
impressed--there leave the star-ray to sketch slowly its own image. But
this is hardly possible. The unceasing motion of the earth, causing
the star perpetually to pass away from the telescopic field, and the
exceeding narrowness of the slit opposite to which the star has to
be kept in a stationary position, make the most accurate adjustment
needful.
Clockwork alone can not be trusted. If it could, the star and the
photographic apparatus might be comfortably “fixed,” and left to do
their own work. Instead of which, while a photograph is proceeding,
it is desirable that an observer should sit gazing patiently at
the telescope-tube, where the image of the star is seen, ready at
any moment to correct by a touch the slightest irregularity in the
clockwork motion of the telescope, and so to prevent a blurred and
spoilt reproduction of the star, or of its spectrum.
One hour, two hours, three hours, at a stretch, this unceasing
watchfulness may have to be kept up, and no small amount of enthusiasm
in the cause of science is requisite for so monotonous and wearying a
vigil.
As noted earlier, it has been found possible, if the photograph of a
star or nebula is not completed in one night, to renew the work and
carry it on the next night, or even for many nights in succession. This
is especially practicable in spectroscopic photography.
From four to five hours, sometimes from eight to ten hours, may
be needful before the clear image of the star or of its spectrum
appears--a dim little star probably to us, yet perhaps in reality a
splendid sun, shedding warmth and light and life upon any number of
such worlds as ours.
Eight or ten hours of photography at one stretch with a star are
impossible; for the stars, ever seemingly on the move, do not remain
long enough in a good position. For three to six hours, a telescope may
be made, by means of its clockwork machinery, to keep a star steadily
in sight, and all that while the photograph is progressing. If further
exposure is needed, the process has to be resumed the next night.
The more one considers the matter, the more plainly one perceives how
enormously our powers of sky-observation are increased by photography.
It is not only that one photographer, with his apparatus, may
accomplish in a single night the work of many astronomers who have
to depend upon the power of the eye alone. It is not only that, with
the help of photography, as much can now be done in a lifetime as
formerly must have occupied many generations. It is not only that the
photograph, once taken, remains a permanent possession, instead of a
record, more or less imperfect, in which otherwise the astronomer would
have to trust.
It is not even only that in the photograph details come out which could
not be detected by the eye, and that stars are actually brought to our
knowledge which no man has ever seen, which perhaps no man ever will
see from this earth with the assistance of the most powerful telescope.
For the weak shining, which can by no possibility make itself felt by
the retina of a man’s eye, _can_ slowly impress its picture on the
photographic plate. Hundreds of stars, thousands of stars, utterly
invisible to man, have had their photographs taken as truly as you have
had your photograph taken, and by the same process, only it has been a
longer business.
But in addition to all this, we see reproduced upon the plate those
ultra-violet and infra-red portions of the spectrum of light which but
for the handmaid, photography, would still be to us as things which
have no existence. And by means of photography we can observe and study
in those same unseen portions of the spectra, when looking into a ray
from sun or star, the innumerable dark lines, every one of which has
its own tale to tell. To these tales we must have remained blind and
deaf, but for photographic aid to our limited powers.
Look at some dim star in the sky, and try how long you can gaze without
blinking. You will very soon find that you have done your best, and
that to gaze longer only means a sense of fatigue in your eyes, a
growing dimness in the star. How different with the photographic plate!
There, no exertion is wasted, no weariness is felt. Faint though the
light may be, which travels earthward and falls upon the plate, it is
all collected, all used.
It is easy to photograph the sun, and this has been done. When we come
to the moon the operation is more difficult on account of the feeble
intensity of its light, and the difference of tint of the various parts
of its surface. Skill and perseverance have, however, surmounted the
greatest difficulties, and now we have photographs of the moon enlarged
to more than a yard in diameter, which show the smallest details with a
truly admirable clearness.
In the first second of time, your eye receives as much light from the
star as does the photographic plate in the same time. But during the
course of one hour, the plate receives and stores up about thirty-six
hundred times as much light as it or you received in the first instant.
There is the secret of the matter. The photographic plate does not only
receive, it can also keep and treasure up the light, and that our eyes
are not able to do.
If you magnify the amount received in one hour by five or ten, and
remember that it is all retained, then you will begin to understand
how feeble stars, unseen by man, should become known to us through
photography.