The Astronomy of Milton's 'Paradise Lost' by Thomas Nathaniel Orchard (easy books to read in english TXT) 📕

Should those suns be differently coloured, each emitting its own peculiar shade of light as it appears above the horizon, the varied aspects of the perpetual day enjoyed by the inhabitants of those circling worlds present to the imagination harmonies of light and shade over which it is pleasant to linger.

Temporary, Periodical, and Variable Stars.—It may seem remarkable that among so many thousands of stars which spangle the firmament, there should occur no very perceptible change or variation in their aspect and brilliancy. From age to age they present the same appearance, shine with the same undiminished splendour, and rise and set with the same regularity. So that from time immemorial the stars have been regarded by mankind as the embodiment of all that is eternal and unchangeable. Yet, the serenity of the celestial regions does not always remain undisturbed—at occasional times a ‘Nova,’ or new star, blazes forth unexpectedly in the heavens, and perplexes astronomers; and, after shining with a varying degree of brilliancy for a few weeks or months, gradually diminishes in size and brightness and eventually becomes lost to sight.

A record has been kept of about twenty temporary stars that have been observed at various periods since the time that reliable data of those objects have been published. Pliny mentions the appearance of a new star in the time of Hipparchus (134 B.C.); it was seen in the constellation of the Scorpion, and it is said that it was the apparition of this star which induced the celebrated astronomer to construct what is known as the earliest star catalogue. A new star is said to have become visible when the Emperor Honorius ruled, and another during the reign of the Emperor Otho, about 945 A.D. In May 1012 a new star appeared in Aries, and in July 1203 another was observed in Scorpio, which resembled Saturn. The most remarkable star of this kind was one observed by Tycho Brahé, which appeared in the constellation Cassiopeia. He first perceived it on November 11, 1572. In lustre it equalled Jupiter, and when at its brightest rivalled Venus; it was visible at noonday, and at night its light could be perceived through strata of cloud which rendered all other stars invisible. The star maintained its brilliancy for three weeks, when it became of a yellowish colour and perceptibly decreased in size; it afterwards assumed a ruddy hue resembling Aldebaran, and, diminishing gradually in magnitude and brightness, ceased to be visible in March 1574. It twinkled more than the other stars, and during the time it could be perceived its position remained unchanged. In 1604 a conspicuous new star burst forth in Ophiuchus. It surpassed in brilliancy stars of the first magnitude, and outshone the planet Jupiter, which was in its proximity. Kepler observed this star, and described it as ‘sparkling like a diamond with prismatic tints.’ It soon began to decline after its appearance; in March 1605 it had shrunk to the dimensions of a third-magnitude star, and in a year later it became entirely lost to view. Other stars of the same class, though of a less conspicuous character, have been observed at occasional times. Anthelme, a Carthusian monk, discovered one near β Cygni in 1670; another appeared in Ophiuchus in 1848; one in Scorpio in 1860; one in Corona Borealis in 1866; in Cygnus in 1876; in Andromeda in 1885; and in Auriga in 1892.

Various theories have been advanced in order to account for the sudden outbursts of those stars, the light from which has probably occupied not much less than one hundred years in its passage hither. It has been suggested that the collision of two suns, or of two great masses of matter, would create such phenomena; but, apart from the improbability of such a catastrophe occurring among the celestial orbs, the rapid subsidence in the luminosity of the observed objects would indicate that the outburst was produced by causes of a more rapidly transitory nature than what would result from the collision of two condensed masses of matter. A collision occurring between two swarms of meteors has been suggested as one way of accounting for the sudden appearance of those stars; but another, and more plausible, explanation is that they are produced by a great eruption of glowing gas from the interior of a sun, causing an enormous increase in its luminosity, which subsides after a time, and is succeeded by a normal condition of things. It has been observed that all those temporary stars, with the exception of two, have appeared in the region of the Milky Way. In this luminous zone the condensation of small gaseous stars and nebulæ is more pronounced than in any other part of the heavens, and this would seem to indicate that there may be cosmical changes taking place among them which need not be associated with the occurrence of catastrophes resulting in the conflagration of worlds, and that Nature, in accomplishing her purposes, does not overstep the uniform working of her laws, upon which depend the stability and existence of the universe.

Periodical and Variable Stars are distinguished from other similar objects by the fluctuations which occur in the quantity of light emitted by them. The difference in the luminosity of some stars is at times so marked that, in a few weeks or months, they decline from the first or second magnitudes to invisibility, and, after the expiration of a certain period, they again gradually regain their pristine condition. When these changes take place with regular recurrence, they are called ‘periodical;’ when they occur in a variable and uncertain manner, they are called ‘irregular.’ About 300 stars are known as variable, but the majority of them are telescopic objects. Their periodical changes of brilliancy present every degree of variety; in some stars they are scarcely perceptible and occur at long intervals; in others, changes of brightness occur in a few hours or days, by which the light emitted is intensified many hundreds of times.

Some stars accomplish their cycle of change in a few days, many in a few weeks or months, and there are others which do not complete their periods until the expiration of a number of years.

One of the most remarkable of variable stars is called Mira ‘the wonderful,’ in the constellation Cetus. When at its maximum brilliancy it shines for two or three weeks as a star of the second magnitude. It then begins to gradually decline, and at the end of three months becomes invisible. It remains invisible for five months, and then reappears, and during the ensuing three months it regains by degrees its former brilliancy. Mira completes a cycle of its changes in 334 days, and, during that time, oscillates between a star of the second and tenth magnitude. The variability of Mira Ceti was first observed by David Fabricius in the sixteenth century.

Another remarkable star is η Argus, which is surrounded by the great nebula in the constellation Argo Navis. It is invisible to the naked eye, but in the telescope it has a reddish appearance, and is slightly brighter than the stars in its vicinity. It was first observed by Halley in 1677, and it was then of the fourth magnitude. In 1751 it had risen to the second magnitude, and maintained its position as a star of this class until 1837, when, on December 16 of that year, its brilliancy suddenly increased, and it equalled in a short time α Centauri. It reached its maximum in 1843, and then it was surpassed only by Sirius. It maintained its brilliancy for about ten years. In 1858, it declined to the second magnitude, in 1859 to the third, and, gradually diminishing, it became invisible to the naked eye in 1868. It is now of the seventh magnitude, and is again increasing, and may soon resume its position among the other stars. It is believed to have a period of seventy years, and in that time its light ebbs and flows between the seventh and first magnitudes.

The most interesting variable star in the heavens is Algol (the demon), in the constellation Perseus. Its light fluctuations can be observed without the aid of a telescope, and it completes a cycle of its changes in two or three days. For about two days and thirteen hours it is conspicuously visible as a star of the second magnitude; it then begins to decline, and in about four hours sinks to the dimensions of a fourth-magnitude star; it remains in this condition for twenty minutes, and then increases gradually until, at the expiration of four hours, it regains its former brilliancy, which it sustains for two days and thirteen hours, when it again goes through the same cycle of changes in a precisely similar manner to what has been described. Astrologers have ascribed many evil influences to the demon star, which adorned the head of Medusa; nor did it escape the observation of ancient astronomers that this malevolent orb is—as a modern writer amusingly remarks—slowly winking at us from out the depths of space.

Variable stars are found in greater numbers in some parts of the heavens than in others. Those of a white colour, and with shorter and more regular periods, are most numerous in the region of the Milky Way; those that are small, with long periods and of a reddish hue, are more widely removed from that zone. Stars of this class are all very remote, and no attempt has as yet been made to ascertain the parallax of Algol.

Several theories have been suggested in order to account for the periodical brilliancy of those stars. It has been suggested that the stars have opaque non-luminous patches on their surfaces, and that during axial rotation their light ebbs and flows according as the dark or bright portions are turned towards us. This theory is highly improbable. Another and more plausible reason, especially with regard to short period variables, is, that around those stars there revolve opaque bodies or satellites which at times intercept a portion of their light by producing a partial eclipse of their discs, similar to that caused by the dark body of the Moon when passing between the Sun and the Earth.

It is now known that in the case of variables of the Algol type, the periodical fluctuations of their light arises from this cause, and that round Algol there is a dark world or satellite travelling, which completes a revolution of its orbit in about sixty-nine hours, and that, during each circuit, it intercepts one half of the light of its primary by partially eclipsing the orb, and thereby creating a diminution in its apparent magnitude which becomes perceptible at recurring intervals.

Star Groups.—These are plentifully scattered over the heavens and, by their conspicuous brilliancy, add to the grandeur and magnificence of the midnight sky. The Hyades in Taurus, of which Aldebaran is the chief, forming the eye of the Bull, attract attention.

The stars in Coma Bernices form a rich group; the sickle in Leo, the seven stars in Ursa Major, and those in Cassiopeia and Aquila are familiarly known