The Astronomy of Milton's 'Paradise Lost', Thomas Nathaniel Orchard [best selling autobiographies .TXT] 📗
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The Sun is an example of this class of star, and constitutes the centre of the system to which the Earth belongs. Reasoning from analogy, it would be natural to conclude that there are other suns, numberless beyond conception, the centres of systems of revolving worlds, and although we are utterly unable to catch a glimpse of their planetary attendants, even with the aid of the most powerful telescopes, yet they have in a few instances been felt, and have afforded unmistakable indications of their existence.
Since the Sun must be regarded as one of the stellar multitude that people the regions of space, and whose surpassing splendour when contrasted with that of other luminaries can be accounted for by his proximity to us, it would be of interest to ascertain his relative importance when compared with other celestial orbs which may be his peers or his superiors in magnitude and brilliancy.
The Sun is one of a widely scattered group of stars situated in the plane of the Milky Way and surrounded by that zone, and, as a star among the stars, would be included in the constellation of the Centaur.
Although regarded as one of the leading orbs of the firmament, and of supreme importance to us, astronomers are undecided whether to classify the Sun with stars of greater magnitude and brightness, or assign him a position among minor orbs of smaller size. Much uncertainty exists with regard to star magnitudes. This arises from inability on the part of astronomers to ascertain the distances of the vast majority of stars visible to the naked eye, and also on account of inequality in their intrinsic brilliancy. Among the stars there exists an indefinite range of stellar magnitudes. There are many stars known whose dimensions have been ascertained to greatly exceed those of the Sun, and there are others of much smaller size. No approximation of the magnitude of telescopic stars can be arrived at; many of them may rival Sirius, Canopus, and Arcturus, in size and splendour, their apparent minuteness being a consequence of their extreme remoteness. If the Sun were removed a distance in space equal to that of many of the brightest stars, he would in appearance be reduced to a minute point of light or become altogether invisible; and there are other stars, situated at distances still more remote, of which sufficient is known to justify us in arriving at the conclusion that the Sun must be ranked among the minor orbs of the firmament, and that many of the stars surpass him in brilliancy and magnitude.
Double Stars.—To the unaided eye, these appear as single points of light; but, when observed with a telescope of sufficient magnifying power, their dual nature can be detected.
The first double star discovered was Mizar, the middle star of the three in Ursa Major which form the tail of the bear. The components are of the fourth and fifth magnitudes, of a brilliant white colour, and distant fourteen seconds of arc.
In 1678, Cassini perceived stars which appeared as single points of light when viewed with the naked eye, but when observed with the telescope presented the appearance of being double.
The astronomer Bode, in 1781, published a list of eighty double stars, and, in a few years after, Sir William Herschel discovered several hundreds more of those objects. They are now known to exist in thousands, Mr. Burnham, of the Lick Observatory, having, by his keen perception of vision, contributed more than any other observer to swell their number.
All double stars are not binaries; many of them are known as ‘optical doubles’—an impression created by two stars when almost in the same line of vision, and, though apparently near, are situated at a great distance apart and devoid of any physical relationship.
Binary stars consist of two suns which revolve round their common centre of gravity, and form real dual systems.
The close proximity of the components of double stars impressed the minds of some astronomers with the belief that a physical bond of union existed between them. In the interval between 1718 and 1759, Bradley detected a change of 30° in the position angle of the two stars forming Castor, and was very nearly discovering their physical connection.
In 1767, the Rev. John Michell wrote: ‘It is highly probable in particular, and next to a certainty in general, that such double stars as appear to consist of two or more stars placed very near together do really consist of stars placed near together and under the influence of some general law.’ Afterwards he says: ‘It is not improbable that a few years may inform us that some of the great number of double and triple stars which have been observed by Mr. Herschel are systems of bodies revolving about each other.’ Christian Mayer, a German astronomer, formed a list of stellar pairs, and announced, in 1776, the supposed discovery of ‘satellites’ to many of the principal stars. His observations were, however, not exact enough to lead to any useful results, and the existence of his ‘planet stars’ was at that time derided, and believed to find a place only in his imagination.
The conclusions arrived at by some astronomers with regard to double stars were afterwards confirmed by Herschel, when, by his observation of a change in the relative positions of many of their components, he was able to announce that they form independent systems in mutual revolution, and are controlled by the law of gravitation.
The number of binary stars in active revolution is known to exceed 500; but, besides these, there are doubtless numerous other compound stars which, on account of their extreme remoteness and the close proximity of their components, are irresolvable into pairs by any optical appliances which we possess.
The revolution of two suns in one sphere presents to our observation a scheme of creative design entirely different to the single-star system with which we are familiar—one of a higher and more complex order in the ascending scale of celestial architecture. For, if we assume that around each revolving sun there circles a retinue of planetary worlds, it is obvious that a much more complicated arrangement must exist among the orbs which enter into the formation of such a system than is found among those which gravitate round our Sun.
The common centre of gravity of a binary system is situated on a line between both stars, and distant from each in inverse proportion to their respective masses. When the stars are of equal mass their orbits are of equal dimensions, but when the mass of one star exceeds that of the other, the orbit of the larger star is proportionately diminished as compared with the circumference traversed by the smaller star. When their orbits are circular—a rare occurrence—both stars pursue each other in the same path, and invariably occupy it at diametrically opposite points; nor is it possible for one star to approach the other by the minutest interval of space in any duration of time, so long as the synchronous harmony of their revolution remains undisturbed.
(Drawn by Mr. J. E. Gore.)
When a pair of suns move in an ellipse, their orbits intersect and are of equal dimensions when the stars are of equal mass, their common centre of gravity being then at a point equidistant from each. Consequently, neither star can approach or recede from this point without the other affecting a similar motion, they must be at periastron and apastron together, and any acceleration or retardation of speed must occur simultaneously with each. Stars of unequal magnitude always maintain a proportionate distance from their common focus, and both simultaneously occupy corresponding parts of their orbits.
The nature of the motions of those distant suns, and the form of the orbits which they traverse, have been investigated by several eminent astronomers, and although the subject is one of much difficulty, on account of their extreme remoteness and the minute angles which have to be dealt with, necessitating the carrying out of very refined observations, yet a considerable amount of information has been obtained with regard to the paths which they pursue in the accomplishment of their revolutions round each other.
The orbits of about sixty stellar pairs have been computed, but only with partial success. Some stars have shown themselves to be totally regardless of theory and computation, and have shot ahead far beyond the limits ascribed to them, whilst others, by the slowness of their motions, have upset the calculations of astronomers as much in the opposite direction. So that out of this number the orbits of not more than half a dozen are satisfactorily known.
The dimensions of stellar orbits are of very varied extent. Some pairs are apparently so close that the best optical means which we possess are incapable of dividing them, whilst others revolve in wide and spacious orbits.
The most marked peculiarity of the orbits of binary stars is their high eccentricity; they are usually much more eccentric than are those of the planets, and in some instances approach in form that of a comet.
The finest binary star in the northern heavens is Castor, the brighter of the two leading stars in the constellation Gemini. The components are of the second and third magnitudes, and over five seconds apart. They are of a brilliant white colour, and form a beautiful object in the telescope.
In 1719 Bradley determined the relative positions of those stars, and on comparing the results obtained by him with recent measurements it was found that they had altered to the extent of 125°. Travelling at the same rate of speed, they will require a period of about 420 years to complete an entire circuit of their orbits. This pace, however, has not been maintained, for, their periastron having occurred in 1750, they travelled more rapidly in the last century than they are doing at present, and, as their orbits are so eccentric that when at apastron the stars are twice as remote from each other as at periastron, they will for the next three and a half centuries continue to slacken their pace, until they shall have reached the most remote points of their orbits, when they will again begin to approach with an increasing velocity; so that the time in which an entire revolution can be accomplished will not be much less than 1,000 years.[8]
As the distance of Castor is unknown, it is impossible to compute the combined mass of its components. They are very remote, their light period being estimated at forty-four years. Castor is doubtless a more massive orb than our Sun, and possesses a higher degree of luminosity.
α Centauri, in the Southern Hemisphere, is the brightest binary, and also the nearest known star in the heavens; its estimated distance being twenty-five billions of miles. Both components equal stars of the first magnitude, and are of a brilliant white colour. Since they were first observed, in 1709, they have completed two revolutions, and are now accomplishing a third. The eccentricity of their orbit approaches in form that of Faye’s comet, which travels round the Sun; consequently the stars, when at apastron, are twice their periastron distance. Their period of revolution is about eighty-eight years. The mean radius of their orbit corresponds to a span of 1,000 millions of miles, so that those orbs are sometimes as close to each other as Jupiter is to the Sun, and never so far distant as Uranus.[9] Their combined
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