IN STARRY SKIES

Evening Post, Volume CXXI, Issue 138, 12 June 1936, Page 18

IN STARRY SKIES

ftHE NEAREST NEIGHBOURS OF THE GALAXY 4* . " (By "Omega Centauri.") Our sun is one of hundreds of thoutsands ,of millions of stars in one particular stellar system that we call the Galaxy. The stars in it are not arranged with a >y approach to uniformity; but are gathered into,clusters and star clouds. It is, however, almost certain that the whole system if seen from without would have a very beautiful characteristic shape, that is hidden from us as we viewed it, of necessity, only from within. Herschel's researches showed that the system is very much flattened. This conclusion has been abundantly confirmed in recent years. Methods have been found for estimating distances which even light would take millions of years to cross.. Stars of every type, planetary anil diffuse nebulae and obscuring plouds of dark matter, are all found to be: contained in a space whose shape is often compared with that of a very flat watch. . Associated with this system are about' a hundred globular clusters. These are arranged in a system which has about the same centre as the Milky . Way, but which is less flattened. The .Stella system probably extends at least six or eight times as far in the galactic plane as at right angles to it. The system of the globular clusters extends about as widely as that of , the stars. Shapley mentions two clusters'N.G.C. 7006 and 2298 which are 260,000 light years ; apart. . But, at right angles to the galactic .plane clusters are found up t0'65,0W light years. Now what lies beyond? The Galaxy is not nearly as; isolated from similar cosmic systems as our solar system is, from their

stars. No other star has been found within 28 million diameters from the sun. But it would be possible to represent the Galaxy and lour or live other systems in a single diagram drawn to scale, if, we knew the distances and dimensions more exactly. We have seen that the tendency Just Bt present is to diminish the, ( estimates made a lew years ago of the size of the Galaxy'and, at the same time,. to. increase those of the nearest spirals.?Fof the sake of an illiiistratimtilet intermediate values. ;. Supp6se,the diameter of the Galaxy is taken as 20tf,00t> lightlyears and its thickness. 30,000 light years. Then a few globular clusters: will be found, three or four;ttynes; t as far a\yay from the galactic plane'-as. the star clouds./ Surrounding this .vast ' region is a comparatively-vacant space, before we come to some, other l ! interesting Systems. The plane containing the eiarth's orbit round tfie sun is tilted at ari Mgle of about 62 degrees to the galadtic plane. . Thus the bright of the passbs.thro.ugh Perseus .Cassiopeia and LacertO within about thirty degrees of the North Pole and through. Carina, Crux,. and Ara at about the same distance" fromi the south one. 'it happens that bours of the Galaxy are all to the south of it; Some of' these' aree'xtremely close, to us in comparison jvyith the distances of many objects that can now be observed 1 or photographed with great telescopes, but even they are a long way pil when measured Witfi terrestrial : standards. The two are' always; well above our horizon 1 , arid can be studied during any-clear night. We call Ihiem the Magellanic Clouds. We do not see'them as'they'are : iidw,' but as thtey v were nearly a hundred. thousand lyears ago. Their, distance'- used to be| &Ven' as 102 and 112 thousand light fyears respectively. But .from A study of the Cepheid variables in them Shapiey deduces a distance of only 86,000 light years for the' Large Cloud and 95,000 light years for the small one.. With these values it would take i light 108,000 years to cross from one ( side to the other of the Large Cloud, 6000 years to pass through the Small] Cloud/ and about 40,000 years to travel | frojn one cloud to the other. The next nearest known system is an irregular star cloud N.G.C. 6822 in Sagittarius,

at a distance of 700,000 light years. Then in the Northern Hemisphere of the sky, but still south of the Galaxy, arf the two nearest of the known Spiral Nebulae, viz.: the Great Nebula in Andromeda, and the beautiful one in > Triangulum. These are believed to be each about 900,000 or a million light years away, and to have diameters of about 65,000 and 30,000 light years respectively. These two spirals give a, good idea of what is believed to

be the constitution of the Galaxy. The Great Nebula of Andromeda we see obliquely, that in the Triangle broadside on. The former seems to have a brighter central cloud than anything in our Galaxy, but a small nebula looks as if it bears about the same relation to the great nebula that the Small Magellanic Cloud does to the Galaxy. M.33, the nebula in the Triangle, probably resembles our system still more closely except in size.

It is strange how opinions have varied with regard to the status of the spirals. In 1785 Sir William Herschel stated his belief that many of the nebula are external galaxies. In 178G he remarked that he had discovered 1500 "universes." But in 1791 he found a star in Taurus surrounded by nebulosity that was not at all of a starry nature, and spoke of a shining fluid of a nature totally unknown to us. He thus recognised that the one word nebula was used to describe very different objects. In 1845 Lord Rosse made many discoveries with his 6-foot reflector, which for a great many years was the largest telescope in existence. He w&S the first to detect the spiral form in what is now known as the Whirlpool. Nebula in the Hunting Dogs. He thought he was able to resolve into stars many nebulae, some of which are now known to be gaseous. A. certain proof of the'existence of gaseous nebulae was found by Huggins in 1864, when he turned his spectroscope on t9 a bright planetary nebula in Draco. During the next four years he examined the spectra of seventy nebulae. He found that the nebula in Andromeda, the Whirlpool nebula, and many others, as well as all the globular clusters, gave continuous spectra like those of stars. The extra-galactic nebulae are thus clearly differentiated from the planetary' and the diffuse galactic nebulat The, former are indeed cosmic systems similar in nature to the Galaxy itself. , ItJs .yery surprising to find the

contrary view upheld, in a book only I thirty' years old. "A Century's Progress in Astronomy," by Hector Macpherson, was published in 1906. In this we find the belief in the existence [of other galaxies beyond the Milky Way, referred to rather scornfully, as the, following quotations will show: earlier idea that the, nebulae were external galaxies was long, held majority, of astronomers, in preference to his later and more advanced -"It was clearly, shown; by William' Whewell and by , Herbert Spencer-that the. systematic distribution "of the nebulae : in regard to the stars precluded the' possibility of their being, external -galaxies.", "Proctor was not so fortunate in theorizing' as in , direct investigation. He thought that the Magellanic Clouds' were probably external galaxies; and, further, he put forward the idea that the Milky Way is a spiral. . . . But neither of these ideas has found favour with- astronomers."-. "But .the '.chief : work accomplished by. Proctor i was a revision of our knowledge of 'the' Universe which •he.thus describes: 'Within one and the same region, coexist stars of many orders of : real .'magnitude, the greatest being ; thousands of , times larger than the least. "All the'nebulae hitherto discovered, whether. gaseous or stellar, irregular planetary, ring formed or elliptic,' exist within the lirtiits of the Sidereal System." Later on Macpherson says: "The nebula' theory modified by. subsequent research, seems destined to "hold its own against all. attacks."

The- improvement of the' telescope, the application of photography to astronomy, and- the development of spectroscopic "ijfiethods of research have changed all-this. Portions' of many spiral pebulae have been resolved into clouds pf stars. Star clusters and variable stars, as well •• as gaseous nebulae, have been found within them. Distances are now estimated with gradually increasing accuracy by many indirect methods, amongst which the most powerful depends on the study of Cepheid variables. Fresh discoveries may'at any time change the details of the picture, but it seems certain that our Galaxy, gigantic as it is, forms but one unit in a much vaster assembly of cosmic systems. The hundred inch telescope is unable to sound the depths of this einormous gathering of "Worlds Without End," to use the happy title given to his last work by the Astronomer Royal. The Magellanic Clouds lead us towards Andromeda and The Whirlpool, and other forms entice us oh until we find innumerable specks on many:a photographic plate, which tejl of galaxies a hdndred million light years away.