IN STARRY SKIES

Evening Post, Volume CXIX, Issue 98, 27 April 1935, Page 14

IN STARRY SKIES

THE GALAXY

(By "Omega Centauri.")

According to terrestrial standards the solar system is immense. If we make a model, taking a yard to represent a million miles, a pea will stand for the earth and the orbit of Pluto will require a field of 4J miles in diameter. But if we propopse to extend the model to the galaxy, on the same scale, we find that not a single member outside the solar system can find a resting place on earth. This just emphasises the fact that throughout vast regions, surrounding the solar system on every side, we can find no indication of the existence of any other body or system. But a glance at the sky shows at once that there are myriads of stars, and modern science has proved that stars are suns. Within his own system the supremacy of our. sun is absolute, but in the galaxy he is but a unit, and, indeed, a comparatively humble one. Stars are suns, but what can we say about their planetary systems? Strong [reasons may be given for the belief that such systems exist in great numbers, in the galaxy, as well as in other cosmic organisations, but we cannot, as yet, expect to obtain any direct observational evidence of the fact. Let us glance at future possibilities. Mars is at present shining very brightly in the

evening sky. -It was in opposition on April 6, and nearest to the earth on April 12. Its distance then was only 57,690,000 miles, which is less than 'one hundred-thousandth part of a lightyear. The distance of the nearest known star is 4.3 light years. If Mars were as far off as that its brightness would be reduced to a one hundred and eighty-five millionth part of its present splendour. It would-then shine as a star, of about the 27th magnitude, and would require a telescope with a mirror 4000 inches in diameter to secure it's photograph. But even that is an under-statement of the difficulty, for a planet, to show its full disc, must be on the far side of the star which gives it light. At greatest elongation from" it only half the illuminated disc would be turned towards us. Planetary systems, amongst the stars in the galaxyv for some time to come, will be seen only in'the'mind's eye; But with regard to the stars the case is very different. The ones we can .see .or photograph'are'independent sources of light. 'The,radiation pouring, continually from them bears a wonderful.story which'astronomers are learning to read more and more perfectly. : ' A glance at the sky.on a.clear night shows - the whole visible part of the heavens spangled with points of light. These appear at first sight to be scattered rather irregularly... The, yivid imagination of ancient peoples saw in many of the groups representations of men and maidens, lions arid bears, dogs and snakes, a bull, a crab, a ram and a scorpion, a great, ship, an eagle,, a swan, a crow, and a dove, a sea goat, a great river, a balance, and a lyre, a whale, and two groups of fishes. These constellations cover a great part of the sky, but looking through theni we see something that is in- reality far more important. A band of light, that we call the Milky Way, stretches right across the sky. Most of the stars, whose combined light makes it visible, are too faint to be seen as separate points. But every increase of telescopic power reveals the existence of multi-tudes-of, fainter suns. Most of these are not intrinsically, less brilliant than those known before, but appear feeble because of their immense distances. Nearly all the most brilliant stars have | individual names, the exceptions being j those top far south to be seen from the oldest centres of civilisation. Thus there are only .twenty stars brighter than magnitude 1.5, and of these the only ones without names, other than a Greek letter and1 the name of ths constellation, are Alpha and Beta Centauri and Alpha -CruciS;. In . the next twenty between magnitude 1.5 and magnitude 2, at least a dozen

have special names, but these, with the exception of Castor and Bellatrix, are seldom used. Now all these brilliant stars, the five thousand others that it is possible to see with the naked eye, and the hosts whose light blends into the glow of the Milky Way, as well as all the planetary, gaseous, and dark nebulae, that is practically all objects that can be seen without powerful optical, aid, belong to the galactic system. Special attention must therefore be devoted to the Milky Way. It is seen at once that this band is not uniform in breadth or brightness, but that it forms practically a complete ring. Its centre line cannot be defined with precision, but careful study and statistical investigations show that it is approximately a great • circle inclined at an angle of 62 degrees to the equator. The Poles of this great circle are in Coma Berenices and in Sculptor respectively. The width of the band of light varies from about five to thirty or more degrees. The brightest parts are in- Scorpio, Sagittarius, Scutum, and Cygnus, where the star clouds are conspicuous to the naked eye, though their full grandeur can be revealed only in giant telescopes. In many parts there are dark rifts or patches. The most noticeable of the former is the dark central band which divides the luminous belt into two nearly parallel streams extending from the Swan to the Centaur. One distinct dark patch is the Coal Sack near the Southern Cross, so familiar to all observers in the south. The observed central line of the bright band does not coincide exactly : with the galactic

equator, but is, on,an average, about one degree south of it. This. shows that our sun lies a little to the north of the central galactic plane. This gives us a slight advantage over northern observers in viewing the galaxy. The only portion which we can never see lies in the constellations Cassiopeia and Cephcus. But in this portion the stream is sufficiently wide for us to see part of it in the constellations Cygnus, Lacerta, and Andromeda. It then passes through Perseus and Auriga, over the horns of the Bull and the feet of the Twins, between Orion and the Little Dog, close beside the Great Dog, through a great part of the ship Argo, over the Cross and the Pointers and part of Ara. It widens out to cover nearly the whole of the Scorpion, and is divided by the dark belt in Ophiuchus, Sagittarius, the Eagle, and the Swan, thus completing the whole circle of the heavens. Owing to the high angle that the Milky Way makes with the equator, its position in the sky changes in a remarkable manner. Suppose we look for it-about nine o'clock in the evening near the beginning of .each month of the year. In January it passes from north to south, reaching half-way' to the zenith in the east. In February it comes nearer to the zenith and extends from N.N.W. to S.S.E. In March it passes from N.W. to S.E. through the zenith. In April it has slipped to the western side of the zenith. In May it extends from W. to E.S.E. with the Southern Cross nearly at its highest point. In June it passes from W.S.W. to east with the Pointers near its highest point. In July it goes from S.W. to^N.E. with the Scorpion in the zenith; in August from S.S.W. to N.N.E. with the sting of the Scorpion near the zenith. In September it stretches from S. by W. to N. by E., being, highest in the west. In October it is sinking towards the west and we can see more than half of it. In November it extends from S.E. round by the west to the N.E:, almost touching the horizon all the way. In December, the western • part has sunk below the horizon, and .that on the east has appeared. Again we can see far more than half, but it is a different part;that is hidden. During any particular night about the same change takes place in two hours that we have pictured as happening in a month owing to the change of date whilst observing always at the same hour. The galactic plane bears the same relation to the sidereal system that the ecliptic does to the solar system, and is the most fundamental plane of reference that we know.