CHAPTER XXXI.

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.