IN STARRY SKIES

Evening Post, Volume CX, Issue 69, 18 September 1930, Page 20

IN STARRY SKIES

ASTRONOMY FOR GIRL

GUIDES

THE NEAREST STAR, THE SUN

(By "Omega Centauri.")

The stars are suns. To realise what they are like wo must know something about the sun. It is so large, so hot, and so bright that, when the facts are stated in figures, they convey but little meaning to our niinds. We owe to the sun not dnly our comforts and pleasures, but even tho possibility of life on this earth. If wo wero much nearer to it or much, further away life would be unbearable or.impossible. Fortunately for us tho speed with which the earth is flying along its orbit and the mighty pull of the sun's gravitation are so related that the earth's wanderings, are restricted and confined between definite limits. The actual distance of tho earth from the sun

varies, as w.e. have seen, by a Tittle over three million miles, but that is less than one thirteenth part of the total. We must try to get somo idea of tho sun's size, mass temperature, and brightness so that wo shall be able to use it as a. standard when studying the stars. Let us think first for a moment of the size of the sun.' Its diameter is a little over 109 times that of the earth. That means that if wo represent the earth by a ping-pong ball, wo shall have to -use a groat globe over twelve and a half' feet in diameter to represent the sun. Tho volume of tho sun exceeds that of the earth more than one million three hundred thousand times, that is as much as £5421 exceeds a single penny.

But the- mass or actual quantity of matter in a body is more1 important than its volume. Now" the earth is the densest of all. the planets, and this makes Up a little for its insignificant volume. Jupiter, for instance, whilst Kl]4 times as large as tlic\ earth, is only 318 times as. heavy. Tho density of the earth is almost exactly four times that -of the sun, so the mass of tho sun is not much over a third of a million times that of the earth. But even so it is 745 times as great ;j.s that of all the planets together. It is the mass of the sun which 'controls the movements of the earth and holds it in its elliptic orbit. But this would bo of no use to us if it were not for the sun's astonishing temperature. The earth is marvellously adapted for the preservation of life. We could not survive very much nearer to the sun or very much farther from it. Our very lives are dependent on our comparative proximity to this particular star, but fortunate indeed it is that no other star is at anything like the same distance. If tho .earth wore describing a figure of eight orbit- round a pair of suns; ' one-snruch hotter, than the other, the range .of, temperatures might well render.,, it .■.uninhabitable. But tho noxt; nearest, stai-..is about 270,000 times as far away as the sun, so the earth's allegiance Is■iindisturbed. The sun is-rather..exceptional in: this respect. Of'tho nearest stars about fifty per cent, arc- known to bo. components of double .or multiple systems. We may well thank'.ourI'-star-.for not btfiog one of these. :; - : .

. The. temperature Of the ;*sim is :::i-.;'i harder!tti picture iivour.niinds than '•■'.* size or Jts mass.:Wo experience such a small range. ■We think of ice as being extremely cold, boiling water as extremely; hot. On the centigrade scale this range, from ice to steam is

measured by-a hundred degrees. But ice is a long way from tho limit in coldness. Tho absolute zero of tempera turu is nearly two and throo-qmir-lur times us far below freezing point as boiling point is above it. Near tho iSouth Pole in winter temperatures are experienced . that are further below freezing point than those of; midsummer on the Equator arc above it. Now, using the centigrade' scale, freezing- point is called 0 degrees, the boiling point ol: water 100 degrees, and the absolute zero of temperature—273 degrees. On this scale iron molts at about 1500 degrees and boils at 2450. Now, the sun is so intensely heated that even in the colder parts of its atmosphere iron and other metals are in the state of vapour. We sometimes say that a kettle boils when wo moan only that the water in it does so. But in tho atmosphere of the sun an iron kottlc would flash - into -vapour, whilst oiio of copper or aluminium would disappear much faster. A metal kettle,indeed, could not'fall on to the sun, for long bsforp it reached even its fiery atmosphere it would cease to be a kettle and become a, cloud of vapour. But tho temperature of the interior of the sun, though expressed numerically, is utterly inconceivable. The outer atniosphore. is at some 6000 degrees centigrade. The interior, according to

Eddington, reaches about forty millions of degrees. At this temperature tho very atoms.-of which, tho sun is composed are broken to bits. The fragments, flying, electrons and atomic nuclei, arc' in a state of inconceivable agitation, colliding incessantly with ■ono another as they hurl themselves about with astounding speed. Although overy particle is endowed with such terrific energy, none can escape. The total mass is soonornious that gravitation ' can control even explosive velocities. The vast, globe is, however, pouring, out.vradiant energy incessantly into space. According to niodern ideas, it is gradually losing mass by radiation, but tho loss could not bo detected in a thousand, perhaps not in 'a; million, years. 1 The sun. thus presents to view a disc of dazzling brightness. The region from which we receive most of the. heat and' light is called the photosphere. ■ Surrounding this, .vapours ''of tho chromosphere-1 are at a lower temperature, and~so arc able-to ; absorb certain wavo lengths'.'of the radiation which passes-' through^ Violent eruptions continually take pla-co in the photosphere and vast flame-liko protuberances break through tho chromosphere and rise to enormous heights. Further out in air directions above the surface is the mysterious glow of the corona. The brilliant disc of- the photosphere is often scarred,by markings called sun spots. These, though much hotter and brighter thai' molten iron, appear, black, by'comparison with' the intense radiation' of the less disturbed regions of,'.: : the photosphere. These spots are .caused by violent cyclonic storms in the solar atmosphere. Radiation, as we have seen, is flying into" Space-in every direction from the entire v surface .of the sun. We are entirely dependent on the minute fraction of the.:total which the earth is able to. intercept.- Is~ this one-millionth part?- Not by, any nieans. . Nor is it one millionth part, of ■■'that millionth part..' It is less, - indqed, than ono lifty-thousandth part of: .'even the latter fraction. Yet all-life on earth depends on this minute, quota-;..'.

What wo havo ■ saitt;:ia this slight sketch can give .but:;a';faint and imperfect idea of the'wonderful character of the ruling :bb.dy. of tho solar system, but it may servo as a starting point-in the exploration, of the universe. . The sun' is a star. Wo shall have to consider, later whether it .is a typical one. But if we- remember, that every star is a sun ire shall realise how 'much gerater it must be • than even the largest .planeti,.,.■.'.•.■;•■•■;. ,:.