The conclusion that has been arrived at in regard to the cause of the variation of these stars is, that in each case the diminution of light is due to the existence of dark bodies, probably planets, which revolve round the central star.
This hypothesis was confirmed by Professor Vogel about 1889 by means of spectroscopic results.
Another interesting fact about stars is that they shine with various colours. The colours of stars are as various as the colours of the rainbow, and range through the whole spectrum, of red, orange, yellow, green, blue, indigo, violet, and white. What is more remarkable is the fact that the colours of the stars seem to change through great periods of time. If we turn to ancient records we learn that Sirius was red then, but is now green, while Capella was also red, but is now pale blue.
_Double and Multiple Stars._--Many stars when looked at through powerful telescopes are found to be double, triple, quadruple, and even multiple, although when looked at by the naked eye, they seem to be single in appearance.
An example of a double star is to be found in the constellation of Lyra.
A moderate telescope reveals this as a double star, while a still more powerful telescope reveals the strange fact that each apparently single star which forms the double is itself double, so that we have in this constellation a system of four stars, in which each pair revolves round a point situated between them.
Several thousand double stars are known altogether, while the motions of several hundreds of them have been detected with powerful telescopes.
Some of the double stars are as follows--Zeta Hercules, Eta Coronae Borealis, Gamma Coronae Borealis, Beta Cygni, Alpha Centauri.
The colours of some of the double stars are very beautiful. Some are yellow and blue; others, yellow and purple, while others are orange and green. Some of the double stars are only optical doubles, that is to say, they apparently seem close together, while as a matter of fact they are immense distances from each other, the apparent doubleness being due to the fact that they are more or less in the same line of vision. Real double stars, where the component stars are situated close together, are known as physical doubles, to distinguish them from the optical doubles.
_Binary Stars._--Another cla.s.s of double stars are known as Binary Stars. This cla.s.s of stars is composed of two stars which revolve around each other in regular orbits, and are among some of the most interesting objects in the heavens. About 1000 Binary stars are known altogether.
Their motions, however, are very slow, and only in a comparatively few cases have the dimensions of their orbits been ascertained. Some of the Binary stars are Zeta Hercules, which has a period of about 36 years; Eta Coronae Borealis, which has a period of 43 years; while the brightest star, Sirius, is also a Binary star, with a period of about 50 years.
_The Milky Way._--The Milky Way is the name given to that band of light which stretches across the sky at night-time, and forms a zone or belt that completely circles the celestial sphere.
This belt of light has maintained from the earliest ages the same relative position among the stars, and, when resolved by powerful telescopes, is found to consist entirely of stars scattered by millions across the expanse of the heavens.
The whole zone or belt is composed of nothing but stars, whose average magnitude, according to Herschel, is about the tenth.
Stars of all magnitudes are, however, found in this zone.
Of the brightest stars, about twelve are found in this region, while the majority of stars of the second, third, and fourth magnitudes are also found in or near it.
The great majority of star cl.u.s.ters are also found along the course of the Milky Way, while many of the irresolvable nebulae seem to congregate near the poles of this starry region.
The Milky Way is divided in one part of its course by a stream of stars, which seems to branch off as a separate stream, thus dividing it into two parts.
All these facts seem to point to the conclusion that the stars of the universe, instead of being scattered about haphazard in the s.p.a.ce, form a ring or layer, of which the thickness is very small compared with its length and breadth.
Our own solar system, according to Herschel, occupies a place somewhere about the middle of the thickness of the zone, and near the point where it divides into two parts.
Recent observations go to show that there is a tendency of the sun"s apex to drift along the edge of the Milky Way, and this drift seems to point to a plane of motion of the sun, nearly coinciding with the plane of the Milky Way.
ART. 118. _Stars and Kepler"s Laws._--We have learned in a previous chapter that the sun is the centre of a system which comprises a retinue of planets, with their attendant satellites, together with a number of asteroids or minor planets, with the addition of meteors and comets to complete the system.
Now if the sun is a star, then, according to our First and Second Rules of Philosophy, every star ought also to be the centre of a stellar system and the centre of two aetherial motions, that is, the Centrifugal and Centripetal forces, due to the pressures and tensions of the Aether medium. Further, every stellar system would be composed of exactly similar bodies to those which compose our solar system, as planets with their attendant satellites, together with meteors and comets; the whole of the stellar planets being bound to the central body by the combination of the two aetherial motions, and kept revolving round the central star by the rotating electro-magnetic Aether currents.
Such a hypothesis is entirely philosophical, as it is simple in conception, and fully agrees with our experience in relation to the only star of which we have any complete knowledge.
It is unthinkable to conceive of a star existing in so-called s.p.a.ce, and constantly radiating out its light and heat for no purpose at all. All Nature teaches us that there is not a single thing in existence but what has a definite purpose, and a definite place to fill in the universe.
Even the aetherial atoms, which form the foundation stones of the universe, have their own purpose to fulfil in the glorious scheme of the Universe conceived by the Eternal Infinite; and to suppose that a star has no purpose to fulfil, no task to perform, is to suppose something altogether opposed to the teaching of all Philosophy. Why even man, with his finite wisdom, would not be so foolish, so unwise, as to make a star, and set it in the firmament of heaven for no purpose at all! Are we therefore to suppose that the Divine Creator of all things possesses less wisdom than the creatures which He Himself hath made? Such an a.s.sumption would be a reflection not only on the wisdom of an All Wise Being, but would also be a reflection on our own ideas of philosophical reasoning.
Therefore the conclusion that we are compelled to come to, in relation to the millions of stars that exist in interstellar s.p.a.ce, is that every star is the centre of a stellar system, and the centre of two aetherial motions due to the pressures and tensions of the electro-magnetic Aether; while rotating round each star are the ever-circulating electro-magnetic Aether currents, which form the medium by which all the stellar planets with their attendant satellites are ever made to revolve around that central body which supplies them with their light and heat.
Some such conclusion as this Sir John Herschel arrived at, for in his _Treatise of Astronomy_, Art. 592, he writes: "Now for what purpose are we to suppose such magnificent bodies scattered through the abyss of s.p.a.ce? Surely not to illuminate our nights, which an additional moon of the 1/1000 part of our own moon would do much better. He must have studied astronomy to little purpose who can suppose man to be the only object of the Creator"s care, or who does not see in the vast and wonderful apparatus around us, provisions for other races of animated beings. The stars, doubtless, are themselves suns, and may perhaps each in its sphere be the presiding centre around which other planets or bodies may be circulating."
Further, with reference to the stability of each of these stellar systems, it is essential that the existence of a physical centrifugal force should be recognized, in order that the unity and harmony of the spheres should be maintained.
Professor Challis points this out very conclusively in the _Phil. Mag._ of 1859, where, writing on this point, he states: "It may also be remarked, that if the Law of Gravity be absolute, there is no security for the stability of a system of stars, whether the system be a Milky Way or a nebulous cl.u.s.ter. For, however small the mutual attraction between the const.i.tuent bodies may be, in the course of ages it must produce a general movement towards the centre or densest region. But the form of the Milky Way and of certain nebulae seems to present an utter contradiction to any such tendency." With the conception, however, of a physical centrifugal force or motion due to the pressure of a physical medium, the stability of even the Milky Way may be physically conceived and understood.
Again, when we consider the sun as a star, we find that it has two motions of its own, one of rotation on an axis, and the other of translation in an orbit, such rotation being due to the fact that it is a magnet and has ever circulating round it electro-magnetic Aether currents (Art. 91). By inference, therefore, we arrive at the fact that every star is a magnet, as suggested by Professor Schuster, and possesses rotation on an axis, such rotation being due to exactly the same cause as produces the rotation of any other planetary or solar body (Art. 92). Not only has each star a rotation on its axis, but it must also possess translational motion in an orbit, and that orbital motion must be due to exactly a similar cause as that which produces the orbital motion of the sun. Are there any indications given by astronomical observations which lead us to the conclusion that stars do possess such orbital motions? The answer is unanimously in the affirmative; for, although all the stars and the constellations retain apparently the same relative position to each other, yet they are all in motion. The actual translational motion of the stars is termed proper motion, and has been calculated with more or less success in relation to many of the stars nearest to us. There are other motions of the stars known as apparent motions, which are easily noted by any observer. These apparent motions are due to the rotation of the earth on its axis, and its...o...b..tal motion round the sun.
Nothing is more certain, however, than that careful astronomical observations have revealed the fact that stars have actual orbital motions of their own through s.p.a.ce. In many cases the orbital velocity has been approximately ascertained.
Halley discovered proper motions of certain stars as far back as 1715, when he found out, by comparing different observations, that Sirius, Arcturus, and Aldebaran had moved during the period which had elapsed since the respective observations were taken.
More recent observations tend to confirm the fact that stars have indeed proper motions, due to their actual translation through s.p.a.ce. It has been ascertained, for example, that Arcturus is travelling at least 54 miles per second.
The proper motion of the stars, however, only gives us an indication of their relative motion through so-called s.p.a.ce. It does not tell us whether the star is apparently receding from the earth, or approaching it.
Dr. Vogel has ascertained by a special system of photography in relation to the spectra of stars, that Rigel has a velocity away from the earth of nearly 39 miles per sec., Aldebaran of 30 miles per sec., and Capella of 15 miles per sec., while the Pole star is apparently approaching the earth at a rate of nearly 16 miles per sec.
Now if all the stars move through s.p.a.ce with varying velocities, as spectroscopic and telescopic observations seem to suggest, the question naturally confronts us as to what is the particular kind of orbit which each star completes? Is the orbit that of an ellipse, or a circle, or a parabola?
That it must have some kind of orbit is obvious from the proper motions exhibited by the several stars. We have already learned from Arts. 107 and 108 that the sun possesses an orbit, which orbit fulfils the first and second of Kepler"s Laws.
If therefore the sun, as representing all stars, is subject to Kepler"s Laws, then, according to our Second Rule of Philosophy by which we base our hypotheses on our experience, we are compelled to come to the conclusion that every star which possesses any motion at all through s.p.a.ce must also be subject to Kepler"s Laws, and therefore must each possess a controlling centre around which they severally revolve. Kepler himself was of the opinion that the stars were subject to the laws which go by his name, and this view of the subject was also accepted by Sir William Herschel.
Thus from philosophical considerations we affirm that each star, while it is itself the centre of a starry system, is also dependent upon and a.s.sociated with some other body, to which it is held bound by the electro-magnetic Aether, and around which it is made to revolve by the circulating electro-magnetic currents a.s.sociated with that central body.
So that by philosophical reasoning we are led to view the whole of the innumerable stars that flood interstellar s.p.a.ce, not as so many individual and isolated units, that have no relation to each other, but rather as parts of one great system, which in its entirety may form in its ultimate unity one harmonious whole, a universe.
As we come to consider star cl.u.s.ters and nebulae, we shall see how this idea of unity seems to be manifested throughout all celestial phenomena.
ART. 119. _Aether and Nebulae._--In addition to the host of stars that flood the infinite s.p.a.ce, there are other celestial bodies that meet the gaze of the astronomer as the telescope is turned upon the heavens.
These bodies, which are glowing ma.s.ses of gaseous matter, are termed Nebulae. The word Nebulae signifies a cloud, but they are not clouds in the same sense as we apply that term to ma.s.ses of vapour that exist in our own atmosphere. Sir Wm. Herschel did more towards the discovery of nebulae than perhaps any other astronomer, either before his time or since. His labours in the direction were completed and enlarged by his son, Sir John Herschel, who surveyed the Southern heavens in a way that had never been accomplished before.
The result of the combined labours of the two Herschels has placed information of the nebulae at our disposal which is invaluable. Several thousands of different nebulae are now known to us, and as the telescope is improved and its powers increased, fresh nebulae are being added to the number. Like stars, nebulae vary not only in size, but also in colour, shape, and even in the materials of which they are composed.
They also vary in brightness, the light from some being much fainter than the light from others.
It has been estimated by Huggins that the light received from a nebula will not exceed the light of a sperm candle looked at from a distance of a quarter of a mile. It is thought by some astronomers that the light received from a nebula is indicative of the stage of development to which it has arrived. Where the light is faint, the nebulae are in their first stages of formation, and where it is brighter it is indicative of a more advanced stage of development. Thus nebulae may consist of nebulous matter in various stages of condensation, but they are not yet in that condition which corresponds to the condition supposed to exist in our sun.
Nearly all the nebulae lie outside the Milky Way, so that it would seem as if in ages past all the nebulae that had ever existed in this starry zone had pa.s.sed out of their nebulous condition and been further condensed into suns or stars, as they are called. Astronomical observations teach us that there are very few nebulae indeed to be seen in this starry highway, the part of the heavens which are richest in them lying far beyond the confines of this zone. For many years certain aggregations of luminous points in the heavens were supposed to be nebulae, but by the aid of more powerful telescopes they have now been resolved into cl.u.s.ters of stars. One of these cl.u.s.ters is the cl.u.s.ter in Hercules, while another is the great nebula of Orion. In the case of the former, situated in the constellation of Hercules, we find a great number of very small points of light grouped together in a more or less globular form. When looked at through a small telescope, this object looks like a nebula, but looked at through Lord Rosse"s, or some other great telescope, it becomes at once resolved into an immense number of separate points of light, each one representing a star, there being between one and two thousand altogether in this constellation.
Cl.u.s.ters of stars are usually globular in form, though some are irregular in outline. The latter are generally rich in stars, with a less condensation of stars towards the centre. Sir Wm. Herschel considered the irregular cl.u.s.ters as being in a less advanced stage of condensation, as he was of the opinion that all groups ultimately tended to cl.u.s.ters which were globular in form. Before dealing with the different kinds of true nebulae we will now consider the question as to "What are Nebulae?"
ART. 120. _What are Nebulae?_--The question which presents itself to the mind of all astronomers when they have viewed the wondrous nebulae that exist in far-off s.p.a.ce is, "What are Nebulae?" This question is so closely identified with the question as to "What is Matter?" that the solution of the one will give us the key to the solution of the other.
It is now generally admitted, that nebulae are composed of a glowing ma.s.s of gaseous matter, that gaseous matter being partly composed of the gas Hydrogen. Dr. Huggins in 1864 first made the discovery of the existence of Hydrogen in certain nebulae by means of the spectroscope, which distinctly revealed certain lines that proved the existence of Hydrogen in the nebulae.
In the spectra of some of the nebulae, that of 31 Andromeda, for example, there are no dark lines shown, but only a continuous band of bright light, which would seem to indicate that there was no glowing gaseous matter in that nebula at all. But accepting the fact that the nebulae are composed of glowing gaseous matter, the problem confronts us as to where this gaseous matter comes from.