The Beauties of Nature, and the Wonders of the World We Live In Part 22

Arcturus is, so far as we know at present, the swiftest, brightest, and largest of all. Its speed is over 300 miles a second, it is said to be 8000 times as bright as the Sun, and 80 times as large, while its distance is so great that its light takes 200 years in reaching us.

The distances of the heavenly bodies are ascertained by what is known as "parallax." Suppose the ellipse (Fig. 54), marked Jan., Apr., July, Oct., represents the course of the Earth round the Sun, and that A B are two stars. If in January we look at the star A, we see it projected against the front of the sky marked 1. Three months later it would appear to be at 2, and thus as we move round our orbit the star itself appears to move in the ellipse 1, 2, 3, 4. The more distant star B also appears to move in a similar, but smaller, ellipse; the difference arising from the greater distance. The size of the ellipse is inversely proportional to the distance, and hence as we know the magnitude of the earth's...o...b..t we can calculate the distance of the star. The difficulty is that the apparent ellipses are so minute that it is in very few cases possible to measure them.

[Ill.u.s.tration: Fig. 54.--The Parallactic Ellipse.]

The distances of the Fixed Stars thus tested are found to be enormous, and indeed generally incalculable; so great that in most cases, whether we look at them from one end of our orbit or the other--though the difference of our position, corresponding to the points marked January and July in Fig. 54, is 185,000,000 miles--no apparent change of position can be observed. In some, however, the parallax, though very minute, is yet approximately measurable. The first star to which this test was applied with success was that known as 61 Cygni, which is thus shown to be no less than 40 billions of miles away from us--many thousand times as far as we are from the Sun. The nearest of the Stars, so far as we yet know, is [Greek: alpha] Centauri, the distance of which is about 25 billions of miles.

The Pleiades are considered to be at a distance of nearly 1500 billions of miles.

As regards the chemical composition of the Stars, it is, moreover, obvious that the powerful engine of investigation afforded us by the spectroscope is by no means confined to the substances which form part of our system. The incandescent body can thus be examined, no matter how great its distance, so long only as the light is strong enough. That this method was theoretically applicable to the light of the Stars is indeed obvious, but the practical difficulties are very great. Sirius, the brightest of all, is, in round numbers, a hundred millions of millions of miles from us; and, though as bright as fifty of our suns, his light when it reaches us, after a journey of sixteen years, is at most one two-thousand-millionth part as bright. Nevertheless, as long ago as 1815 Fraunhofer recognised the fixed lines in the light of four of the Stars; in 1863 Miller and Huggins in our own country, and Rutherford in America, succeeded in determining the dark lines in the spectrum of some of the brighter Stars, thus showing that these beautiful and mysterious lights contain many of the material substances with which we are familiar. In Aldebaran, for instance, we may infer the presence of hydrogen, sodium, magnesium, iron, calcium, tellurium, antimony, bis.m.u.th, and mercury. As might have been expected, the composition of the Stars is not uniform, and it would appear that they may be arranged in a few well-marked cla.s.ses, indicating differences of temperature, or perhaps of age.

Thus we can make the Stars teach us their own composition with light, which started from its source years ago, in many cases long before we were born.

Spectrum a.n.a.lysis has also thrown an unexpected light on the movements of the Stars. Ordinary observation, of course, is powerless to inform us whether they are moving towards or away from us. Spectrum a.n.a.lysis, however, enables us to solve the problem, and we know that some are approaching, some receding.

[Ill.u.s.tration: Fig. 55.--Displacement of the hydrogen line in the spectrum of Rigel.]

If a star, say for instance Sirius, were motionless, or rather if it retained a constant distance from the earth, Fraunhofer's lines would occupy exactly the same position in the spectrum as they do in that of the Sun. On the contrary, if Sirius were approaching, the lines would be slightly s.h.i.+fted towards the blue, or if it were receding towards the red. Fig. 55 shows the displacement of the hydrogen line in the spectrum of Rigel, due to the fact that it is receding from us at the rate of 39 miles a second. The Sun affords us an excellent test of this theory. As it revolves on its axis one edge is always approaching and the other receding from us at a known rate, and observation shows that the lines given by the light of the two edges differ accordingly. So again as regards the Stars, we obtain a similar test derived from the Earth's movement. As we revolve in our orbit we approach or recede any given star, and our rate of motion being known we thus obtain a second test.

The results thus examined have stood their ground satisfactorily, and in Huggins' opinion may be relied on within about an English mile a second.

The effect of this movement is, moreover, independent of the distance. A lateral motion, say of 20 miles a second, which in a nearer object would appear to be a stupendous velocity, becomes in the Stars quite imperceptible. A motion of the same rapidity, on the other hand, towards or away from us, displaces the dark lines equally, whatever the distance of the object may be. We may then affirm that Sirius, for instance, is receding from us at the rate of about 20 miles a second. Betelgeux, Rigel, Castor, Regulus, and others are also moving away; while some--Vega, Arcturus, and Pollux, for example--are approaching us. By the same process it is shown that some groups of stars are only apparently in relation to one another. Thus in Charles' Wain some of the stars are approaching, others receding.

I have already mentioned that Sirius, though it seems, like other stars, so stationary that we speak of them as "fixed," is really sweeping along at the rate of 1000 miles a minute. Even this enormous velocity is exceeded in other cases. One, which is numbered as 1830 in Groombridge's _Catalogue of the Stars_, and is therefore known as "Groombridge's 1830," moves no less than 12,000 miles a minute, and Arcturus 22,000 miles a minute, or 32,000,000 of miles a day; and yet the distances of the Stars are so great that 1000 years would make hardly any difference in the appearance of the heavens.

Changes, however, there certainly would be. Even in the short time during which we have any observations, some are already on record. One of the most interesting is the fading of the 7th Pleiad, due, according to Ovid, to grief at the taking of Troy. Again, the "fiery Dogstar," as it used to be, is now, and has been for centuries, a clear white.

The star known as Nova Cygni--the "new star in the Constellation of the Swan"--was first observed on the 24th November 1876 by Dr. Schmidt of Athens, who had examined that part of the heavens only four days before, and is sure that no such star was visible then. At its brightest it was a brilliant star of the third magnitude, but this only lasted for a few days; in a week it had ceased to be a conspicuous object, and in a fortnight became invisible without a telescope. Its sudden splendour was probably due to a collision between two bodies, and was probably little, if at all, less than that of the Sun itself. It is still a mystery how so great a conflagration can have diminished so rapidly.

But though we speak of some stars as specially variable, they are no doubt all undergoing slow change. There was a time when they were not, and one will come when they will cease to s.h.i.+ne. Each, indeed, has a life-history of its own. Some, doubtless, represent now what others once were, and what many will some day become.

For, in addition to the luminous heavenly bodies, we cannot doubt that there are countless others invisible to us, some from their greater distance or smaller size, but others, doubtless, from their feebler light; indeed, we know that there are many dark bodies which now emit no light, or comparatively little. Thus in the case of Procyon the existence of an invisible body is proved by the movement of the visible star. Again, I may refer to the curious phenomena presented by Algol, a bright star in the head of Medusa. The star s.h.i.+nes without change for two days and thirteen hours; then in three hours and a half dwindles from a star of the second to one of the fourth magnitude; and then, in another three and a half hours, rea.s.sumes its original brilliancy. These changes led astronomers to infer the presence of an opaque body, which intercepts at regular intervals a part of the light emitted by Algol; and Vogel has now shown by the aid of the spectroscope that Algol does in fact revolve round a dark, and therefore invisible, companion. The spectroscope, in fact, makes known to us the presence of many stars which no telescope could reveal.

Thus the floor of heaven is not only "thick inlaid with patines of bright gold," but studded also with extinct stars, once probably as brilliant as our own Sun, but now dead and cold, as Helmholtz tells us that our Sun itself will be some seventeen millions of years hence.

Such dark bodies cannot of course be seen, and their existence, though we cannot doubt it, is a matter of calculation. In one case, however, the conclusion has received a most interesting confirmation. The movements of Sirius led mathematicians to conclude that it had also a mighty and ma.s.sive neighbour, the relative position of which they calculated, though no such body had ever been seen. In February 1862, however, the Messrs. Alvan Clark of Cambridgeport were completing their 18-inch gla.s.s for the Chicago Observatory. "'Why, father,'" exclaimed the younger Clark, "'the star has a companion.' The father looked, and there was a faint star due east from the bright one, and distant about ten seconds. This was exactly the predicted direction for that time, though the discoverers knew nothing of it. As the news went round the world many observers turned their attention to Sirius; and it was then found that, though it had never before been noticed, the companion was really shown under favourable circ.u.mstances by any powerful telescope.

It is, in fact, one-half of the size of Sirius, though only 1/10000th of the brightness."[72]

Stars are, we know, of different magnitudes and different degrees of glory. They are also of different colours. Most, indeed, are white, but some reddish, some ruddy, some intensely red; others, but fewer, green, blue, or violet. It is possible that the comparative rarity of these colours is due to the fact that our atmosphere especially absorbs green and blue, and it is remarkable that almost all of the green, blue, or violet stars are one of the pairs of a Double Star, and in every case the smaller one of the two, the larger being red, orange, or yellow. One of the most exquisite of these is [Greek: beta] Cygni, a Double Star, the larger one being golden yellow, the smaller light blue. With a telescope the effect is very beautiful, but it must be magnificent if one could only see it from a lesser distance.

Double Stars occur in considerable numbers. In some cases indeed the relation may only be apparent, one being really far in front of the other. In very many cases, however, the a.s.sociation is real, and they revolve round one another. In some cases the period may extend to thousands of years; for the distance which separates them is enormous, and, even when with a powerful telescope it is indicated only by a narrow dark line, amounts to hundreds of millions of miles. The Pole Star itself is double. Andromeda is triple, with perhaps a fourth dark and therefore invisible companion. These dark bodies have a special interest, since it is impossible not to ask ourselves whether some at any rate of them may not be inhabited. In [Greek: epsilon] Lyrae there are two, each again being itself double. [Greek: xi] Cancri, and probably also [Greek: theta] Orionis, consist of six stars, and from such a group we pa.s.s on to Star Cl.u.s.ters in which the number is very considerable. The cl.u.s.ter in Hercules consists of from 1000 to 4000. A stellar swarm in the Southern Cross contains several hundred stars of various colours, red, green, greenish blue, and blue closely thronged together, so that they have been compared to a "superb piece of fancy jewellery."[73]

The cl.u.s.ter in the Sword Handle of Perseus contains innumerable stars, many doubtless as brilliant as our Sun. We ourselves probably form a part of such a cl.u.s.ter. The Milky Way itself, as we know, entirely surrounds us; it is evident, therefore, that the Sun, and of course we ourselves, actually lie in it. It is, therefore, a Star Cl.u.s.ter, one of countless numbers, and containing our Sun as a single unit.

It has as yet been found impossible to determine even approximately the distance of these Star Cl.u.s.ters.

NEBULae

From Stars we pa.s.s insensibly to Nebulae, which are so far away that their distance is at present quite immeasurable. All that we can do is to fix a minimum, and this is so great that it is useless to express it in miles. Astronomers, therefore, take the velocity of light as a unit.

It travels at the rate of 180,000 miles a second, and even at this enormous velocity it must have taken hundreds of years to reach us, so that we see them not as they now are but as they were hundreds of years ago.

It is no wonder, therefore, that in many of these cl.u.s.ters it is impossible to distinguish the separate stars of which they are composed.

As, however, our telescopes are improved, more and more cl.u.s.ters are being resolved. Photography also comes to our aid, and, as already mentioned, by long exposure stars can be made visible which are quite imperceptible to the eye, even with aid of the most powerful telescope.

Spectrum a.n.a.lysis also seems to show that such a nebula as that in Andromeda, which with our most powerful instruments appears only as a mere cloud, is really a vast cl.u.s.ter of stellar points.

This, however, by no means applies to all the nebulae. The spectrum of a star is a bright band of colour crossed by dark lines; that of a gaseous nebula consists of bright lines. This test has been made use of, and indicates that some of the nebulae are really immense ma.s.ses of incandescent and very attenuated gas; very possibly, however, in a condition of which we have no experience, and arranged in discs, bands, rings, chains, wisps, knots, rays, curves, ovals, spirals, loops, wreaths, fans, brushes, sprays, lace, waves, and clouds. Huggins has shown that many of them are really stupendous ma.s.ses of glowing gas, especially of hydrogen, and perhaps of nitrogen, while the spectrum also shows other lines which perhaps may indicate some of the elements which, so far as our Earth is concerned, appear to be missing between hydrogen and lithium. Many of the nebulae are exquisitely beautiful, and their colour very varied.

In some cases, moreover, nebulae seem to be gradually condensing into groups of stars, and in many cases it is difficult to say whether we should consider a given group as a cl.u.s.ter of stars surrounded by nebulous matter or a gaseous nebula condensed here and there into stars.

"Besides the single Sun," says Proctor, "the universe contains groups and systems and streams of primary suns; there are galaxies of minor orbs; there are cl.u.s.tering stellar aggregations showing every variety of richness, of figure, and of distribution; there are all the various forms of star cloudlets, resolvable and irresolvable, circular, elliptical, and spiral; and lastly, there are irregular ma.s.ses of luminous gas clinging in fantastic convolutions around stars and star systems. Nor is it unsafe to a.s.sert that other forms and varieties of structure will yet be discovered, or that hundreds more exist which we may never hope to recognise."

Nor is it only as regards the magnitude and distances of the heavenly bodies that we are lost in amazement and admiration. The lapse of time is a grander element in Astronomy even than in Geology, and dates back long before Geology begins. We must figure to ourselves a time when the solid matter which now composes our Earth was part of a continuous and intensely heated gaseous body, which extended from the centre of the Sun to beyond the orbit of Neptune, and had, therefore, a diameter of more than 6,000,000,000 miles.

As this slowly contracted, Neptune was detached, first perhaps as a ring, and then as a spherical body. Ages after this Ura.n.u.s broke away.

Then after another incalculable period Saturn followed suit, and here the tendencies to coherence and disruption were so evenly balanced that to this day a portion circulates as rings round the main body instead of being broken up into satellites. Again after successive intervals Jupiter, Mars, the Asteroids, the Earth, Venus, and Mercury all pa.s.sed through the same marvellous phases. The time which these changes would have required must have been incalculable, and they all of course preceded, and preceded again by another incalculable period, the very commencement of that geological history which itself indicates a lapse of time greater than human imagination can realise.

Thus, then, however far we penetrate in time or in s.p.a.ce, we find ourselves surrounded by mystery. Just as in time we can form no idea of a commencement, no antic.i.p.ation of an end, so s.p.a.ce also extends around us, boundless in all directions. Our little Earth revolves round the mighty Sun; the Sun itself and the whole solar system are moving with inconceivable velocity towards a point in the constellation of Hercules; together with all the nearer stars it forms a cl.u.s.ter in the heavens, which appears to our eyes as the Milky Way; while outside our star cl.u.s.ter again are innumerable others, which far transcend, alike in magnitude, in grandeur, and in distance, the feeble powers of our finite imagination.

FOOTNOTES:

[66] Ball, _Story of the Heavens_.

[67] Ball, _Story of the Heavens_.

[68] Some authorities estimate it even higher.

[69] Ball.

[70] Hamerton, _Landscape_.

[71] Humboldt, _Travels_.

[72] Clarke, _System of the Stars_.

[73] Kosmos.