THE SIZE OF THE UNIVERSE.

Auckland Star, Volume LXVIII, Issue 156, 3 July 1937, Page 35 (Supplement)

THE SIZE OF THE UNIVERSE.

Distances to Make the Brain Reel

WHAT SCIENCE HAS REVEALED IN 300 YEARS.

AS is well known, astronomy, like other physical science*, has made extremely rapid progress in the last half-century, and perhaps in no branch of astronomy have our ideas chanced more than in dealing with the size of the universe. All measurements of its size are comparatively modern. Even Kepler (1671-1630) had no idea of the distances of the stars that was based on measurement. Although he knew that the distances of tho stars must be great compared with the distance from the earth to the son, sines no perceptible movement of the stars was shown dne to the annual movement of the earth in its orbit round the sun, yet he considered that tho starry envelope surrounding the solar system was only a few miles in thickness. To Huygens (KB9-96) we are Indebted tor the first estimate of tho dwtance of ii star that was based on reasonable assumptions. He recognised that the sun was a star, and assuming that Siriua gave oat the same amount of light as the sun he concluded that the distance of Siriue was 23,000 times that of the sun. As H happens, Sirius gives out 26 times as much light as the sun, so that his distance should be multiplied by 6. Even then his distance Is only one quarter of recent measurements, but taking into account the difficulties of the measurement his result moat be considered quits a good one. To Huygens, then, the universe of stars was vastly greater than the solar system, and the telescope showed more and more faint stars, the number continually increasing as tho size of the telescope increased and the quality improved. Even the telescopes of Galileo (1004-1(142) showed that the Milky Way could be resolved into multitudes of stars. The Work of William HencM. It was William Hersehel (1738-1822) who first earried out systematic work on star counts and gave ns the first ideas on the structure of the universe that were founded on a basis of observation. Tho conclusion to which he was led from his star counts was that the galactic system formed a bun-shaped isolated system with an irregular boundary, "a very extensive, branding, compound congeries of many millions of stars." The diameter of this system was abont 6000 light-yean. (One light-year, the distance light travels in one year, is about six million million miles.) Beyond tho Milky Way were nebulae, of soma of which ho speaks as "Island universes." The nearest and largest of these wss the Andromeda nebula, the distance of whleh he estimated at 13,000 light-years, and the diameter 400 light-years. Xapteyn's Model Passing over various other workers, who did not make much alteration in Hersehel's value, which can therefore be taken as the general idea held of the size of the universe until towards the end of the nineteenth century, we come to {he greet name of Kapteyn of Groningen. Devoting over 30 yean of his life to th* main problem of the sise of our systeui, which involved many subsidiary problems, his final result waa published in 1022. He concluded that the galactic. system formed • bnn-shaped system, the greatest diameter lying in the galactic

in consequence these distances determined by Shapley have to be reduced. Hia more recent result, allowing for absorption, gives the diameter of the Byßtem as about 100,000 light-years. The number of stars in this system, based on the careful counte of Seares and van Rhijn, is thirty thousand million, grouped into various clusters, to one of which the sun belongs. . It is to Oort, of Leiden, that we owe the first knowledge of the rotation of the galactic system. In 1927 he found evidence that the galactic system was revolving about a centre, which he concluded wa« in the star clouds in Sagittarius. This result of Oort's has been confirmed by Lindblad, in Sweden, and by J. S. Plaskett, in British Columbia. "Island Universes." Bie time which the sun will take to complete its orbit about this centre is extremely long, being computed by

plane. At a distance of 28,000 lightyears from the centre in this plane the density of distribution had diminished to 1 per cent of tho density at the centre. The sun was 2200 light-yeara from this centre and 130 light-years north of the plane of symmetry. Greatly Enlarged Ideas. But before Kapteyn's final results were published, ideas of the size of the universe were undergoing profound change. In 1014 Shapley, then at Mount Wilson Observatory, commenced h studies of the globular clusters. He allowed that these are distributed symmetrically about the galactic system. Tn Shanlcv's model the stars, the m actic nebulae, dark nebulae, open dusters and galactic clouds lie mostly within 4000 or 5000 light-years of the JT oTV Milk, WW. strongly condensed near that I'W" 0 - The whole system is rotating with the centre of the of globular chis ers as centre. Tlic distance of_ the solar system from this centre is llcrht-vcars, while the distances of the globular clusters from the sun range from 20,000 light-years to 200,000 lignt 7< Tn"arrlving at these distances .Shapley made no allowance for the absorption of light in its passage through Thia it appears is not negligible, aw*

Plaskett *s about 200,000,000 years. So far, except for Herschei's estimate of the distance of the Andromeda nebula, no mention has been made of the spiral nebulae. Here also there have been wonderful developments in knowledge of recent years, chiefly due to Hubble, working with the 100-inch reflecting telescope at Mount Wilson Observatory. In 1925 he was able to resolve parts of two of the brightest nebulae into stars, and to determine the distance as 800,000 light-year 3. These, then, are far beyond the confines of our galactic system, and Herschei's term of "island universes" seems to apply aptly to them. The early measures showed one important difference between our galactic system and the Spiral Nebula in Andromeda, the former seemed many times as large 06 the latter. But more recent work, taking into account tlje absorption of light in space, has reduced our ideas of the diameter of the galactic system, and more sensitive instruments have increased the area over which the Andromeda nebula is found to extend, so that now it appears that the galactic system is not nearly so much larger than the Andromeda nebula as was thought. According to Eddington. the total number of the spiral nlbulae may be as large as one hundred thousand >»llion,

and the total number of stars that they contain one hu T "l , " ,r l million million—one followed by twenty zeros. The final model at which Hubble arrives it as follows:— First, the galactic system, including the sun as one of a multitude of »tars, and by no means a prominent one. This galactic system is surrounded by the globular clusters, and has associated with it two smaller systems, the Greater and the Smaller Magellanic Clouds. Beyond these are some half dozen other galaxies, including the Andromeda nebula, which, with our galactic system of the Milky Way, belong to a group of galaxies. These all lie within a million light-years from our galactic system. Beyond the Local Group. ( Then there is group we call Hubble's "general field." Among the nearer in this field is the Coma-Virgo cluster of galaxies, the distance of which Hubble gives as seven million light-years. This cluster consists of several hundred galaxies. Bevond these again are yet other galaxies and clusters of galaxies. The jmost distant yet investigated by Hubble, which appears so faint that its image just shows as a tiny spot on the photographs taken with the Mount Wilson 100-inch reflector, is at the enormous distance of 500 million light-years. This is the greatest distance yet measured by the astronomer, and marks the limit of the 100-inch telescope. But with the 200-inch telescope, with its

greater light-gathering power and greater freedom from city lights, it ie likely that the bounds of the known universe will be etfll further extended.