IN STARRY SKIES

Evening Post, Volume CIX, Issue 38, 14 February 1930, Page 17

IN STARRY SKIES

MESSIER. 13

A SWARM OF GIANT SUNS

(By "Omega Cehtauri.")

This beautiful photograph was taken |by Professor G. \V. Eitchey, with tho 00in rofluotor at Moiuit Wilsou Obsorvatory on Oth, 7th, and Sth June, 1910. It shows a very celebrated and remarkablo objopt, tko great globular cluster Messier IS, the finest in tho Northern Hemisphere. It can be seen from Wellington, and it ■if possible to detect it even with the naked eye, when its oxact position ig.known, but when seen without tho telescope it is a hazy and inconspicuous object. During tho evenings of winter tho constellation Hercules appears low down near tho northern horizon, and directly between Jjyra and the Northern Crown. The cluster lies between the stars Eta and Zeta Herculis, dividing,tho distance

between them in the ratio of one to two. It is also in the continuation of the Una through Theta, Bho, and Pi Herculis, which leads the eye from Vega, the brilliant gem of Lyra, westwards towards tho Northern Crown.

Nearly ninety globular clusters are known, and with the exception of the spiral nebulae they are the most stupendous of celestial objects. • They have an astonishing family likeness to one another, but differ enormously in distance from us, and in apparent size. They appear almost exclusively in onelialf of the celestial sphere. Forty-four are within 30 degrees of a . certain point in tho constellation Sag'ittararius, and only four aro more than 90 degrees from that point. Thus the solar system appears to be situated near the boundary of the vast sphere which contains tho globular clusters. It is also in rapid motion with,regard to them, its speed relative to the.centre of the system being about 200 miles per second. The clusters have also high velocities relativo to one another. Southern observers are fortunate in having the. two nearest and most brilliant of the globular clustera perpetually in view. Omega Centauri, not far from the pointers to the Southern Cross, is only 43 degrees, and 47 Toucani, near the small Magellanic Cloud,:is only 17 degrees from the South Pole of the heavens. They are therefore excellently situated for observation here, and each can be seen easily, with the naked eye. Field glasses give us a much more impressive picture of them, but their full glory is only revealed by photographs taken with long exposures through great telescopes. Even a fourinch telescope, however, resolves them into innumerable .twinkling points of light. We may well wonder how many stars, a" globular cluster contains. In an enlarged positive of this photograph of Messier 13 Bitehey counted 40,520 star images, and he omitted the central region, two,, and a half seconds- in diameter, where the star images .blend iuto a confused mass. But Eitehey believes and expects to be able to prove by photography, that the larger clusters contain each some millions of .stars' In

picturing tlio size of bucli a system we must realise first its'immense distance. The light by whioli thp photograph of. this cluster was' taken liad been about 30j000 years on its journey towards us, whilst others are known to bo half a dozen times as far away. Many clusters "are so largo that it takes light 100 years to cross from one side of ono of them to the other. A typical globular cluster gives out at least 300,000 timos as much light as our sun. Tho brightest stars in it are giants like Antares or Butolgouso. What the smallest are like wo do nqt yet know.. Eitchey photographed stars in this cluster down to photographic magnitude 21.3 on the Harvard scale. These are nearly a hundred times fainter than the faintest stars detected visually with the greatest refractors. The stars in a globular cluster look to us to be extremely crowded, as indeed they arc in comparison with those about this sun, but they have notwithstanding 'plenty of room to move about. It has'been-esti-mated that.tho distance of any star in a cluster from its nearest bright neighbours would be some 25 thousand times tho diameter of the earth's orbit. There may, however, be innumerable faint ones in this space, that cannot yet be detected. We must wait for the great, reflectors that.-Kitehey is constructing. Tho stars in a cluster ai'e, of course, in

very rapid motion amongst themselves, but these movements have not yet been detected by observation. A transverse speed of a hundred miles a second would not change the apparent position by a single second of arc in a century. , We do not yet know tlie origin or the life history of one of these remarkable systems. To remove somo of the mystery which surrounds them is one of the tasks of the near future, and at present the auguries are full of hope. New Zealand should determine to secure one of Eitchey's-Chrediea telescopos, and take a creditable place in the exploration of the universe.

Few realise the astonishing skill and perseverance required to secure a photograph such as this. The total exposure waa 11 hours with the telescopic aperture cut down to 50in. A comparatively slow plate was used, on account of its thin film and fine granulation.) In the design of the mounting of the 60in telescope B'itchey had seen that the tangent screws transmitting the motion from the driving clock to the instrument were ground and polished with a refinement approaching that of the optical mirrors themselves. The mirrors had just been resilvered and were in perfect condition. Yet, during the long exposure, an average of two corrections per second were made to keep the image of the guiding star on the cross-lines in the eye-piece. Bef oeusing, accurate to a thousandth of an inch, took place every half-hour, by means of a, special knife edged device, when correction was also made for a very slight rotation of field. But the atmosphere is never absolutely steady. The Ijcsfc. moments had to be utilised, and whenever the definition was inferior the light had to be instantaneously cut off from the plate. Such skill and care produced photographs such as the world had never seen. Years of intense study and constant experiment have followed, and now we stand at the dawn of a new era. The designs are completed, the new instruments are begun. With lively anticipation and sanguine hope, we await the coming revelation.