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

Much that is mysterious and uncertain is associated with those objects which appear to lie far beyond the limits of our sidereal system. It is now generally believed that they exhibit the earliest stage in the formation of stars and planets—inchoate worlds in process of slow evolution, which will eventually condense into systems of suns, and planetary worlds.

Nebulæ present every variety of form. Some are annular, elliptic, circular, and spiral; others are fan-shaped, cylindrical, and irregular, with tufted appendages, rays, and filaments. A fancied resemblance to different animated creatures has been observed in some. In Taurus there is a nebula called the ‘Crab’ on account of its likeness to the crustacean; another is called the ‘Owl Nebula’ from its resemblance to the face of that bird. The Orion Nebula suggests the opened jaws of a fish or sea monster, hence called the Fish-Mouth Nebula. There is a Horse-Shoe Nebula, a Dumb-Bell Nebula, and many others of various shapes and forms. They are classified as follows: (1) Annular Nebulæ, (2) Elliptic Nebulæ, (3) Spiral Nebulæ, (4) Planetary Nebulæ, (5) Nebulous Stars, (6) Large Irregular Nebulæ.

Annular Nebulæ.—These resemble in appearance an oval-shaped luminous ring; they are comparatively few in number, and not more than a dozen have been discovered in the whole heavens. The most remarkable object of this class is the Ring Nebula, which is situated between the stars β and γ Lyræ. It is visible in a moderate-sized telescope as a well-defined, flat, oval ring; its central part is not quite dark but is occupied by a filmy haze of luminous matter which is prolonged inwards from the margin of the ring. When examined with a high power the edges of the ring have a fringed appearance, and numerous glittering stellar points become visible both within and without its circumference. This nebulous ring, though a small object in the telescope, is of enormous magnitude, and if it were not more distant than 61 Cygni, one of the nearest of the fixed stars, its diameter would not be less than 20,000 millions of miles, but it has been estimated by Herschel that it is 900 times more remote than Sirius. How stupendous, then, must be its dimensions, and how bewildering to our conception is the profound immensity of space in which it is located! An annular nebula similar to that of Lyra, but on a smaller scale, is found in Cygnus, and within it there can be seen a conspicuous star. Another exists in Scorpio which contains two stars situated within the ring at diametrically opposite points to each other.

Elliptical Nebulæ.—The most interesting object of this class is the Great Nebula in Andromeda, called ‘the transcendentally beautiful queen of the nebulæ’—an appellation which it scarcely merits. This object, which is plainly visible to the naked eye, is of an oval shape, of a milky white colour, and is situated near the most northern star of the three which form the girdle of Andromeda. It was known to the ancients, and Ali Sufi, a Persian astronomer who flourished in the tenth century, alludes to it; but it did not attract much attention until the seventeenth century. Simon Marius was the first to observe this object with a telescope. This he did on December 15, 1612; he describes it as shining with a pale white light resembling in appearance the flame of a candle when seen through a semi-transparent piece of horn. When examined with a high magnifying power it is seen to occupy a largely extended area measuring 4° in length and 2½° in breadth. Its luminosity increases from the circumference to the centre, where there can be seen a small nucleus with an ill-defined boundary, which has the appearance of being granular, but its composition is not stellar. Two dark channels running almost parallel to each other and to the axis of the nebula have been observed by Bond; these, when prolonged, form into curves which terminate in two great rings. They are wide rifts which separate streams of nebulous matter, and are indicative that some formative processes may be going on within the nebula.

Astronomers have been baffled in their attempts to discover the nature of the Andromeda Nebula. Though great telescopes have been able to render visible thousands of stars over and around it, yet the nebula itself is irresolvable and bears no trace of stellar formation; neither, according to Dr. Huggins, is its spectrum gaseous, a circumstance which deepens the mystery associated with this object. Its distance is unknown, and its dimensions cannot be ascertained.

Other elliptical nebulæ are found in different regions of the heavens. In Ursa Major there is an oval nebula resembling that of Andromeda, but on a much smaller scale. It possesses a nucleus, and on the photographic plate there can be detected the presence of spiral structure, indicating the existence of streams of nebulous matter. Adjacent to this nebula is another of the same class with a double nucleus, and associated with it is a nebulous star.

Spiral Nebulæ.—The great reflector of Earl Rosse at Parsonstown was the successful means by which nebulæ of this form were discovered. This powerful telescope was capable of defining with greater accuracy the structural formation of those objects than any other instrument in use. It was ascertained that spiral coils and convoluted whorls enter into the structure of most nebulæ, indicating a similarity in the process of change which may be going on in these vast accumulations of cosmical matter. The most interesting specimen of a spiral nebula is situated in Canes Venatici. It consists of spiral coils emanating from a centre with a nucleus and surrounded by a narrow luminous ring. In appearance it resembles the coiled mainspring of a watch.

Planetary Nebulæ.—These have been so named on account of the resemblance which they bear to the discs of planets. They are of uniform brightness, circular in shape, with sharply-defined edges, and are frequently of a bluish colour. They are more numerous than annular nebulæ; three-fourths of their number are in the Southern Hemisphere, and they are situated in or very near the Milky Way. Those objects were first described by Sir William Herschel, who was rather perplexed as to what was their real nature and how he should classify them. He remarked that they could not be planets belonging to far-off suns, nor distant comets, nor distended stars. Consequently, he concluded rightly that they were nebulæ. When observed with large telescopes, they lose their planetary aspect, and their sharpness of outline is less apparent; their discs become broken up into bright and dark portions, and in some, numerous minute stars have been observed, whilst others have well-defined nuclei.

The most prominent nebula of this class is situated in the constellation Ursa Major, and is called the Owl Nebula, from its fancied resemblance to the face of that bird. Sir John Herschel describes it as ‘a most extraordinary object, a large, uniform nebulous disc, quite round, very bright, not sharply defined, but yet very suddenly fading away to darkness.’ When examined in 1848 with Earl Rosse’s reflector, two bright stars were discovered in its interior; each was in the centre of a circular dark space surrounded by whorls of nebulous matter—hence the origin of its name. This nebula gives a bright line spectrum indicative of gaseous composition. It is believed to consist chiefly of hydrogen and other gases which form a globe of such stupendous magnitude that, if we surmise its distance from the earth to be sixty-five light years—an estimate much too low—‘its diameter would exceed that of the orbit of Neptune upwards of 100 times.’[10] Within its compass the orbs of hundreds of solar systems as large as that of ours would be able to perform their revolutions, having spacious intervals existing between each system. Another interesting planetary nebula is in the constellation of the Dragon, near to the pole of the ecliptic; it is slightly oval, of a pale blue colour, and contains a star of the eleventh magnitude in its centre. It gives a gaseous spectrum. Attempts have been made to determine its parallax, but without success, and during the eighty years it has been under observation it has remained apparently motionless. Its light period, if estimated at 140 years, would indicate the existence of a globe with a diameter equal to forty-four diameters of the orbit of the planet Neptune.[11] A nebula of this class was discovered by Sir John Herschel in the Centaur. He described it as resembling Uranus, but larger; its colour was of a beautiful rich blue, and its light equalled that of a star of the seventh magnitude.

Nebulous Stars.—These stars are each surrounded by a luminous haze several minutes of arc in diameter and of a circular form. Sir William Herschel, by his observation of those objects, arrived at the conclusion ‘that there exists in space a shining fluid of a nature totally unknown to us, and that the nebulosity about those stars was not of a starry nature.’ Thirteen stars of this type have been enumerated by him and many others have since been discovered. The ‘glow’ which surrounds them has been observed in a few instances to have vanished without leaving any trace of nebulosity behind, but the causes which have brought about such a result are entirely unknown. The nature of those stars is involved in considerable obscurity, and one class of nebula would seem to merge into the other; nebulous stars with faint aureolæ do not differ much from small nebulæ interspersed with stellar points.

Large Irregular Nebulæ.—These are found in both hemispheres, and are remarkable on account of the varied appearances which they present, and the large extent of space which many of them occupy. In some, the nebulous matter of which they are composed can be seen like masses of tufted flocculi, sometimes piled up, and at other times promiscuously scattered, resembling in appearance the foam on the crested billows of a surging ocean rendered suddenly motionless, or cirro-cumuli floating in a tranquil sky. Islands of light with intervening dark channels, promontories projecting into gulfs of deep shade, sprays of luminous matter, convoluted filaments, whorls, wreaths, and spiral streams all enter into the structural formation of a great nebula.

The Great Nebula in Argo, in the Southern Hemisphere, is one of the most remarkable objects of this class. It consists of bright irregular masses of luminous matter, streaks and branches, and occupies an area about equal to one square degree. At its eastern border is situated the variable star η Argus, which fluctuates between the first and seventh magnitudes in a period of about seventy years.

A rich portion of the Galaxy lies in front of the nebula, which creates an effect as if it were studded over with stars. Sir John Herschel, in describing this nebula, writes as follows:—‘The whole is situated in a very rich and brilliant part of the Milky Way, so thickly strewed with stars that, in the area occupied by the nebula, not less than 1,200 have been actually counted. Yet it is obvious that these have no connection whatever with the nebula, being, in fact, only a simple continuation over it of the general ground of the Galaxy. The conclusion can hardly be avoided that, in looking at it, we see through and beyond the Milky Way, far out into space, through a starless region, disconnecting it altogether from our system.