THE PULSATING SUN.

Otago Witness, Issue 3475, 19 October 1920, Page 51

THE PULSATING SUN.

By G. "6. Grew in the Graphic.

That bright disc in the sky, which is the source of all our heat and energy, and possibly of all life as we know it, is only a symbol of what the sun really is. Through a telescope it does not appear a bright disc at all, but one that is so covered by small clouds that its surface has been said to have a rice-grain structure; and at eclipses a great irregular area, the corona, is seen to stretch out far beyond its confines. That vast irregular extension of the sun suggests that the inner radiance of which it is the offshoot may be far less regular in form than it appears tjo be, and that the disc-like rotundity may be in part an optical illusion. For, after all, the sun cannot be imagined as a solid gaseous one, with an innner temperature incomparably higher than the paltry 6773 degrees Centigrade which it reveals to our measuring instruments. Once it was thought that when great sunspots swept across the sun its body was revealed in the depths of those monstrous whirlpools, but that, again, is a supposition now discredited. The darkened area of the spots, like the smooth roundness of the disc, is an optical illusion. On the whole the idea of a flaming globe Of gas holds the field ; but very little is known certainly about it, so little that science can hardly be said to have more than pricked its skin. The outer skin is certainly made up of the incandescent gases of hydrogen, helium, and calcium. Of that the methods of examination perfected some years ago by Professor Hale have made sure. But the little cirro-cumulus clouds, the "ricegrains" below them, are rather an awkward problem. They are metallic; and it was supposed at one time that they were the condensation clouds of metallic vapours below them, even as we might suppose water vapour in the earth's atmosphere to condense under certain .conditions of temperature into rain-water. But this explanation will not answer, because still higher in the sun's atmosphere are vapours of silicon and carbon which have nqt condensed. Why have-they ' remained gaseous when they are further removed from the central and hottest part of the sun? Is there a cool belt in the sun's atmosphere, above which it becomes hotter again? That is an awkward question, which, as a matter, of fact, presents itself in other hot stars beside the Sun—Sinus, for example. An explanation, not wholly satisfactory, is that the appearance of condensation in the rice-grains is another illusion, produced by irregularities of temperature. The darkness of ■ sun-spots might similarly be due to the increased opacity of the veil of gas due to the whirlpool motion. Both suppositions would leave us to imagine that the sun might be gas all through, though it -would be gas existing under almost inexplicable pressure at its core. That would leave several things still to be explained', one of which is the origin of the colossal bonfires which blaze up from the sun with the speed of "> explosions ; and which are very inadequately named prominences." Two of these eruptive giants showed themselves last year, one on May 29 and another on July 15. They were examined with unbroken attention by Mr Edison Pettit through the 40-inch telescope of the Yerkes Observatory, Williams Bay. Wisconsin; ahd an account of his observations appears in the Astrophysical Journal, published by the University of Chicago. '■■."■ The first of these eruptions spread itself over a patch of the sun's surface into -which the earth might have been dropped without causing a greater splash. It was 271,000 miles, in length.' The eruption itself shot upwards with enormous rapidity. At one time it travelled 287,500 miles in six hours, or at the rate of nearly 48,000 miles an hour, three and a-half miles a second. It continued its career for three days, when it reached its greatest height of 760,000 kilometres, nearly half a million miles. At the beginning it had a clawlike structure at its base, and there after went tip spirally, seeming like a great elastic band that was ever being stretched till it snapped. What, however, was its most remarkable feature, as observed by Mr Pettit, was that it did not shoot upwards as if impelled by a solitary explosion. It began to go up with great but uniform speed. But after a time its velocity, instead -of diminishing, as one might have expected, seemed to receive another impulse, which increased it. This mysterious phenomenon was repeated in the next great explosion, which took place six weeks later; and its occurrence has been corroborated by the examination of other smaller cataclysms. , These giant outbursts of gas rise with uniform motion,- but receive impulses which increase their speed after they have started. The kick does not last very long. It is prohablv a matter of a few seconds: but there it is. It seems, Mr Pettit observes, as if the force of gravitation, and other forces which tend to pull the explosive matter back into the sun, were always neutralised by forces acting in the reverse direction.

Is this the pulse of the sun? It might seem so unless some reason can be found for some form of electro-magnetic repulsion outside the skin of the sun's atmosphere. That the sun does pulsate there is evidence of another kind to show. From' observations, which have now lasted over ten years, Dr 0. G. Abbot and other observers in both hemispheres have made it certain that there are flunctuations in the sun's radiated heat, which occur at i irregular intervals of a few days. There are other variations of heat intermittent over intervals of months.

The amplitude of these changes Is" such that sometimes the sun sends to the earth one ienth less radiation than at others.

At present the times of these pulsations cannot be forecasted, but if the possibility of doing so ever becomes established, it would be possible to predict the outset of cold winters or cool summers on the earth. for a 6 per cent, ohange on the sun over a period of six months would probably alter the mean temperature at terrestrial inland stations by some 3£ degrees Fahrenheit. As yet, however, we can feel, but cannot count the pulse of the sun.