Assaj V2 N5 1930-Nov

acbt jJournal of tJJe ~5ttoltomital ~ori£t!1 of ~ olttb ~frira. Vol. II. NOVEMBER, 1930. No.5. THE EXPANSION OF THE UNIVERSE. By H. E. WOOD, M.Sc., F.R.A.S., Union Astronomer. (PRESIDENTIAL ADDRESS, SESSION 1929-30.) The first astronomers looked upon the earth as a: circular plane surrounded and bounded by the heaven which was a solid vault or hemisphere with its concavity turned downwards. The stars seemed to be fixed · on this vault: the moon and later the planets were seen to crawl over it. (G. Forbes, History of Astronomy.) A provisional estimate by Hubble of the distance of the Coma Berenices cluster of nebulae indicates a value of the order of 50,000,000 light years. (Annual Report of the Mount Wilson Observatory, 1928-29.) The diameter of the star Antares, as determined by the 20-foot interferometer attached to the 100-incn Mount Wilson telescope, is a,bout 400,000,000 miles. From this it may be deduced that the volume of this star is equal to that of 100,000,000 suns and that if it were.. brought to our solar system and substituted for our sun its boundaries would extend considerably beyond the orbit of the planet Mars. These two statements may be taken to epitomise the modern view of the universe as contrasted with the ideas of the ancients. It may not be unprofitable to consider how this wonderful modern conception of the universe has arisen. When Galileo with his first crude telescope saw that the planet Jupiter had four attendant 214 moons, his contemporaries refused to believe him and even would not look through his telescope to test his statement lest they should be convinced by their own eyes of the truth of that which they did not wish to believe. Science was then ruled by dogma, evolved from the imagination of the philosophers. Witness the attack made upon Galileo by one of his contemporary astrono mers. "The satellites are invisible to the naked eye and therefore can have no influence on the earth, and therefore are useless and therefore d6 not exist." If this style of philosophy had prevailed, then at the present day we should still be living in the mental and material darkness of the Middle Ages. But slowly and surely the scientific method of the direct appeal to nature has led to the building up of a conception of the universe such as far exceeds the limits of human imagination and philosophising. Let us consider a few of the main steps by which we have advanced from the crude and simple ideas of the first astronomers to our present knowledge: The greatest obstacle to the general acceptance' of the Copernican theory that the earth moved round the sun was the fact that such a movement would involve an annual parallax of the stars and none could be detected. Gradually it came to be recognised that this was due to the very great distance of the stars from the earth. Sir Isaac Newton (1642-1727) concluded that the parallax of the fixed stars must certainly be less than one minute of arc and that they therefore must be more than 360 times further away from the sun than the planet Saturn. And assuming that the fixed stars were equal to our sun in bulk and light, he concluded that their distance might be 100,000 times that of Saturn. In December, 1725, the English Astronomer Royal, Bradley, set out to make a series of observations of the star 'Y Draconis in an attempt to measure its parallax. He obtained a set of observations which would give him a good normal place for that epoch and then, since no sensible change in the parallactic displacement of the star could be expected for some time, intended to allow a considerable interval to lapse before he repeated the observations. Out of curiosity, as he says, he made a further observation only a week later and was surprised to find that the declination of the star had changed and in a direction contrary to that which would be expected if the star had an annual parallax. Naturally he sup posed that his observations were at fault and repeated 215 them, only to find that they were perfectly correct and that the star was moving in an unexpected direction. Three years later, having studied the phenomenon minutely, and with different instruments, he published his conclusions that it was due to the progressive motion of light and the earth's annual motion in its orbit. He also concluded that the parallax of the stars, especially of 'Y Draconis, could not have been as great as one single second, and that, therefore 'Y Draconis is more than 400,000 times as far away as the Sun. So, in attempting to measure the parallax of a faint star, Bradley had not only reached a correct idea as to the lower limit of star distances, but had made the outstand ing discovery of aberration due to the finite velocity of light. Almost a century later the first reliable measures of the parallax of a star were made. In the meantime other attempts to determine the distances of the stars had been made. Galileo had suggested that two stars lying close together, but in reality separated by a great gulf of space, must shift their mutual positions when observed from opposite points of the earth's orbit, and several had acted on this suggestion, but with no success. Sir William Herschel appreciated the possibilities of this method and resolved "to examine every star in the heavens with the utmost attention and a very high power to collect such materials for this research as would enable me to fix my observations upon those stars that would best answer my 'end, i.e., to assign definite distances to the nearest stars." In a few years Herschel had pub lished several catalogues of double stars, but it was still considered th~t they were purely optical phenomena or that their components lay by chance in the same line of sight from the earth. But after twenty years' observations, he was convinced that in at least 50 cases the proximity of the two stars was not fortuitous, but that the two stars were real binary combinations inti mately held together by the bond of mutual attraction. Here again, in trying to solve one problem, an unexpected discovery was made, and Herschel had extended our knowledge of gravitation from the solar system to the stellar universe. F. G. W. Struve in 1837 gave a list of stars for which, on account either of brightness combined with large Proper motion, or in the case of double stars of rapid orbital motion, there was a probability of a measurable 216 parallax. In this list was the double star 61 Cygni which was selected by Bessel for an investigation of it~ parallax. With the Konigsberg heliometer he measured the distances of two neighbouring faint stars from the point which bisects the distance between the two stars of 61 Cygni, making many sets of observations in a night and ultimately succeeded in obtaining a relative parallax of 0."3136 with a mean error of 0."0202. As a proof of the great accuracy of his observations, it may be quoted that the value now accepted for thk parallax of this star from modern observations is 0."300 with a probable error of 0."005. Before Bessel's observations had been made, Thomas Henderson, His Majesty's Astronomer at the Cape, had made many observations of the declination of a Centauri, and on his return to Scotland as Astronomer Royal had examined these observations for traces of parallactic displacement. He found that they indicated a parallax of about one second of arc, but withheld his res-ults from publication until he had confirmed them from observa tions of the star's right ascension. The great opject had. now been attained and the actual distances of some of the stars were measured. The method of measurement was perfectly simple, but the observations demanded the greatest refinement. Photographic methods of observation have now com pietely'dispiaced the early visual methods and have made it possible to measure with certainty a stellar parallax as small as 0."04, corresponding to a distance of about 80 light years or five million times the Sun's distance. Even for many of the bright stars, however, no measur able parallax had been found, so that more powerful methods available for the measurement of greater distances were wanted. Such new methods were very soon forthcominsr. Herschel, in his determination to measure the universe. finding that he was unable to measure the distances of the stars directly, had attempted to draw some conclusions froJ;ll the brightness of the stars. If all the stars were of about the same size and had the same conditions of temperature and density, then their apparent brightness would simply depend upon their distance from us. Herschel realised that he could hardly make this assump~ tion, but considered that if he dealt with very lar~e numbers of stars and used average values, then hIS 217 results would approximate to the truth. So from his method of star gauging, he found that our stellar system resembled a flattened disc extending to about 900 times the average distance of a first magnitude star in the direction of the Milky Way, and being only about one fifth as extended in a perpendicular direction. No information, however, could be given for any individual star unless there were some means of finding its real brightness, as distinct from its apparent brightness'.

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