IN STARRY SKIES

Evening Post, Volume CV, Issue 5, 7 January 1928, Page 7

IN STARRY SKIES

THE CONSTELLATIONS THE SOUTHERN CROSS (By "Omega, Centauri.") It adds greatly to the pleasures of a starry night to be able to recognise with the naked eye the points of light which reveal their marvels only under high powers of the telescope. The stars are so numerous and are scattered so irregularly over tho extent of the sky that, for convenience, we instinctively arrange them in various groups. Most of these have been noted from time immemorial, and we still follow most of the arbitrary arrangements used by the ancient Chaldeans. Each grou r is called a constellation, and nearly all the groups which were visible in Chaldea five thousand years ago are known today by their ancient names," modified, of course, by the different races that have used them. One part of the sky, however, was invisible to these ancient observers. It surrounds the point which was then the South Pole of the heavens. The constellations within this region were named but little more than three hundred years ago. In learning the star groups, it is usual in New Zealand to start with the

Southern Cross, which is, here, the best ' known of the constellations. It is to be seen on every clear night, and it is quite easy to recognise. It therefore forms a splendid starting point for further study of the stars. It is partly sentiment that gives the Southern Cross its undoubted reputation. It is, to all , New Zealanders, a national emblem. To \ those returning after travel in the Northern Hemisphere it is the first token of home. To those coming South for the first time it brings the fascination of the. unknown. Opinions as to its intrinsic beauty as a constellation are very strongly divided. It was first named as a Cross by Amerigo Vespucci, the Spanish explorer, at the beginning of the sixteenth century, but was not definitely marked as a constellation until 1679, when it was mapped by Eoyer. Miss M. 6. Orr (Mrs. John Ever-shed) in her "Stars of the Southern Skies" tells us that it was described by the Florentine Andrea Corsali as so beautiful that up other constellation in the sky is worthy to be compared with it. However proud we may be of our Southern skies, we must feel *at this is an exaggeration. Surely Orion and the Scorpion, for example, are far finer. Miss Orr also mentions that Dante, in his Purgatory, speaks of four stars which glorify the Southern sky, and says that this has caused some to think that the constellation had been recognised as a Cross as early as the thirteenth century. She herself does not agree with this opinion. To most, who see it for the first time after knowing its reputation, the Southern Cross is disappointing. It is not really a cross at all, as there is no star in the middle. It is merely a quadrilateral, and not even the regular form known as a kite. Its diagonals are not at right angles to one another, and its symmetry is still further spoilt by a fifth star which appears quite out of place in the design. But we overlook all these defects and persist in loving it. Whatever its detractors may say, we feel that it is a beautiful group of bright stars, and, when examined in detail, it turns out to be unexpectedly full of interest. Its glory is immensely increased by the brilliant pointers which indicate its position even more clearly than the other celebrated pointer: in the Northern Hemisphere lead the eye to the Pole. Unfortunately in the Southern Hemisphere there is no brilliant Pole Star. Sigma Octantis is barely perceptible to the naked eye. But the Southern Cross, though 30 degrees away, gives us a good indication, of the position of the South Pole, for its axis points nearly towards it. If we continue the line from Gamma at the foot of the Cross to Alpha at its head four and a half times as far again, it will lead us to a point within two or three degrees of the Pole. A better result is obtained by making use of Achernar, which is about as far from the Pole as the middle of the Cross, but on the opposite side of it, and Beta Centauri the nearer pointer. The middle point of the line joining these two is within a couple of degrees of the Pole. If anyone wishes still greater exactness, he may take, instead of Beta Centauri, a point midway between Alpha Centauri and Beta Crucis. The line from this point to Achernar passes practically through the Pole, and its mid-point is only about one degree nearer to Achernar than the Pole itself is. We shall explain later how to pick out with field glasses Sigma Octantis, the Southern Pole Star; but any of the methods already described will enable us to fix approximately with the naked eye the point about which the heavens appear to turn. The line from this point to th© Southern Cross may then be regarded as the hour hand of a great clock in the sky. The direction of this hand at any moment really indicates the time. Unfortunately, however, it is sidereal time that is given, and it requires a little care to translate this into mean solar time. We hope to indicate an easy method, diagrammatically, later. But at present we must return to the Cross itself. Its stars are named in the order of their brightness, which happens also to be the order in which we come to them when we go clockwise round the constellation: Alpha, Beta, Gamma, Delta, and Epsilon. At 9 o'clock, Summer Time, on 6th January the Cross is near the horizon in the south-south-east, with the pointers nearly due south to the right of it. The head of the cross is then tilted a little towards the right. Twelve hours later the Cross will be the other way up and as far above the Pole as it is now below it, and Achernar will have come round to the position which the pointers now occupy. The Cross, although so small, is quite an interesting constellation. It illustrates beautifully the advantage gained by the use of a telescope. To the naked eye the stars in it all appear as single points of light. Fieldglasses show that this is not a true picture of their real character, and the telescope reveals a wealth of hidden wonders. A one-inch telescope shows that Alpha is at least a double. A thTee or four-inch discloses at least three stars in its system, and a six-inch telescope shows three more. The two principal ones of magnitudes 1J- and 2 are .only five seconds apart, a fifth magnitude companion is about ninety seconds from the close pair and three

other stars of the thirteenth aud fourteenth magnitude appear to belong also to the group. Tho principal stars are blue Orion stars, of class 81, and are nearly equal in mass. They are at a distance, of about 109 light years from us, and arc retreating more than 12 miles a second. They are in reality about 1000 and 650 times as luminous as our sun. Beta Crucis is a smaller star, 850 times as bright as our sun, which is approaching us at about eight miles a second, but is 408 light years away. In the same field of view through the telescope there is a star of magnitude 81 which is of a most intense blood-red colour, ifcta is surrounded at some distance by a ring of fainter stars, clearly visible to tho naked eye. Amongst these is a point of light which a field-glass resolves into a cluster. It is the celebrated Jewel Casket, Kappa-Crucis: It was described by Sir John Herschel as one of the most beautiful objects of its kind in the heavens. It is, he says, "an extremely brilliant and beautiful object when viewed through an instrument of sufficient aperture to show distinctly the very different colours of its constituent stars, whicli give it the effect of a superb piece of fancy jewellery." He made a map of 110 of its stars, from the 7th magnitude upwards, from observations made at the Cape of Good Hope between 1834 and 1838. In 1802 Abbott .suspected changes in their number, positions, and colour. In 1872 Eussell, at Sydney, examined it carefully and again mapped its constituent stars. He found 25 stars, not noted by Herschel, but could Jiot find five that were given by him. His conclusion was that many of the stars

| have drifted considerably since the Cape drawing was made. Twelve years ago a study of this cluster was made at the Wellington College Observatory with the 5-inch telescope, and a map was made showing over 60 stars. This cluster well deserves fuller observation. Gamma Crueis is of a clear orange yellow colour. With a low power it breaks up into three points of light forming a right-angled isosceles triangle. The main star, of magnitude 1.61, forms one corner of the base, the next brightest, of magnitude 6.55, is 101 seconds away at the right-angle. Gamma is a star of type M 6, and is one in whose spectrum bands due to titanium oxide can be seen through a small spectroscopie eyepiece attached to the five-inch telescope. Its distance is not known, but it is moving from us at a rate of more than thirteen miles a second. Delta Crucis is a B3 star which is retreating at a still more rapid rate. Its distance also is unknown. S. Crucis is a variable which changes from magnitude C.5 to 7.6. Its period is 41 days. The accompanying plan gives all the stars, which can be seen with the naked eye, within and close/ around the Cross. The numbers in/ the key indicate their magnitudes, but the decimal points have been omitted. Thus 308 stands for magnitude 3.08. The positions of two other clusters besides Kappa is indicated in the diagram. These are 292 Dunlop, which Herschel describes as very bright, very large, little condensed, and consisting of stars from the 12th to the 14th magnitude; and 311 Dunlop, just outside Beta, which Herschel notes as large, pretty, rich, irregularly round, composed of tenth magnitude stars. The Southern Cross is also associated with one of the most celebrated dark regions in the sky. This is the well-known Coal Sack. It is best seen on a clear night when there is no moon. It then seems to stand out with startling blackness. To the naked eye no stars appear within it. But long exposure photographs show that there are multitudes of stars, "but that their light is greatly obscured by the invisible cloud. These dark regions are no longer thought of as holes in the heavens. They are known to .be material clouds which cut off or diminish the light of the stars which lie beyond them. The Southern Cross is one of the constellations which lie upon the Milky Way, that wonderful stream of light which encircles the heavens. It is about at the point where this stream approaches most closely to the South Pole. This brilliant portion, stretching from Argo to Ara, is invisible from Europe, Canada, and nearly the whole of the United States. The portion which we miss between Auriga and Cygnus contains one brilliant constellation, Cassiopeia, but is not otherwise so full of interest. This is one only of the many ways in which the South, astronomically, is especially favoured. Unfortunately we have as ye* made comparatively little use of our great advantages.