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

Sir William Herschel, to whom we are indebted for most of what we know of the Milky Way, commenced a series of observations in 1785 with the object of acquiring a knowledge of the structure of the sidereal heavens. In the accomplishment of this object, to which he devoted a considerable part of his life, he undertook a systematic survey of that portion of the Galaxy which is visible in the Northern Hemisphere. By a method called star-gauging, which consisted in the enumeration of the stars in each successive telescopic field as the instrument moved slowly over the region under observation, he found that the depth of the star strata could be approximately ascertained by counting the stars along the line of vision; those were most numerous where the visual line appeared of the greatest length and fewest in number where it was shortest. Herschel perceived the internal structure of the Galaxy to be exceedingly intricate and complex, and that it embraced within its confines an endless variety of systems, clusters, and groups, branches, sprays, arches, loops, and streaming filaments of stars, all of which combined to form this luminous zone. ‘It is indeed,’ says a well-known astronomer, ‘only to the most careless glance, or when viewed through an atmosphere of imperfect transparency, that the Milky Way seems a continuous zone. Let the naked eye rest thoughtfully on any part of it, and, if circumstances be favourable, it will stand out rather as an accumulation of patches and streams of light of every conceivable variety of form and brightness, now side by side, now heaped on each other; again spanning across dark spaces, intertwining and forming a most curious and complex network; and at other times darting off into the neighbouring skies in branches of capricious length and shape which gradually thin away and disappear.’ Sir John Herschel, who was occupied for four years at the Cape of Good Hope in exploring the celestial regions of the Southern Hemisphere, describes the coming on of the Milky Way as seen in his 20-foot reflector. He first remarks ‘that all the stars visible to us, whether by unassisted vision or through the best telescopes, belong to and form part of a vast stratum or considerably flattened and unsymmetrical congeries of stars in which our system is deeply and eccentrically plunged; and, moreover, situated near a point where the stratum bifurcates or spreads itself out into two sheets.’ ‘As the main body of the Milky Way comes on the frequency and variety of those masses (nebulous) increases; here the Milky Way is composed of separate or slight or strongly connected clouds of semi-nebulous light, and, as the telescope moves, the appearance is that of clouds passing in a scud, as sailors call it.’ The Milky Way is like sand, not strewed evenly as with a sieve, but as if flung down by handfuls (and both hands at once), leaving dark intervals, and all consisting of stars of the fourteenth, sixteenth, twentieth magnitudes down to nebulosity, in a most astonishing manner. After an interval of comparative poverty, the same phenomenon, and even more remarkable, I cannot say it is nebulous, it is all resolved, but the stars are inconceivably numerous and minute; there must be millions and all almost equally massed together. Yet they nowhere run to nuclei or clusters much brighter in the middle. Towards the end of the seventeenth hour (Right Ascension) the globular clusters begin to come in; they consist of stars of excessive minuteness, but yet not more so than the ground of the Milky Way, on which not only they appear projected, but of which it is very probable they form a part. ‘From the foregoing analysis of the telescopic aspect of the Milky Way in this interesting region, I think it can hardly be doubted that it consists of portions differing exceedingly in distance, but brought by the effect of projection into the same, or nearly the same, visual line; in particular, that at the anterior edge of what we have called the main stream, we see foreshortened a vast and illimitable area scattered over with discontinuous masses and aggregates of stars in the manner of the cumuli of a mackerel sky, rather than of a stratum of regular thickness and homogeneous formation.’

The profound distance at which the stars of the Galaxy are situated in space precludes the possibility of our obtaining any definite knowledge of their magnitude and of the extent of the intervals by which they are separated from each other, nor can we learn anything of the details associated with the systems and combinations into which they enter. It is believed that the majority of the stars in the Milky Way equal or surpass the Sun in brilliancy and splendour. They are tenth to fifteenth magnitude stars; now, the Sun at the distance indicated by these magnitudes would in the telescope appear a much fainter object; he would not reach the fifteenth magnitude. Consequently, the galactic stars are regarded as his peers or superiors in magnitude and brilliancy. Those myriads of suns are all in motion—in nature a stationary body is unknown—and they are sufficiently far apart so as not to be unduly influenced by their mutual gravitational attraction; a distance perhaps equal to that which separates our Sun from the nearest fixed star may intervene between each of those orbs. In the deepest recesses of the Milky Way, Sir William Herschel was able to count 500 stars receding in regular order behind each other; between each there existed an interval of space, probably not less extensive than the interstellar spaces among the stars by which we are surrounded.

The richest galactic regions in the Northern Hemisphere are found in Perseus, Cygnus, and Aquila. Night after night could be spent in sweeping the telescope over fields where the stars can be seen in amazing profusion. In the interval of a quarter of an hour, Sir William Herschel observed 116,000 stars pass before him in the telescope, and on another occasion he perceived 258,000 stars in the space of forty-one minutes. In the constellation of the Swan there is a region about 5° in breadth which contains 331,000 stars. Photography reveals in a remarkable manner the amazing richness of this stelliferous zone; the impress of the stars on the sensitive plate of the camera, in some instances, resembles a shower of descending snowflakes.

Though Sir William Herschel was able to fathom the Galaxy in