Astronomy of the Southern Skies.

New Zealand Illustrated Magazine, 1 November 1900, Page 85

Astronomy of the Southern Skies.

By A. D. Austin, C.8., F.R.A.S.

WN the skies of the Southern Hemisphere W, there are numei-ous constellations, nl clusters of stars, nebulas, and other «gi celestial objects that are never seen in Europe, as they are always below the horizon in latitudes far north of the Equator. Many of these objects are of surpassing splendour, and the brighter skies of southern regions enable them to be seen with much greater distinctness, and with far greater frequency, than similar objects ax - e seen in the mist-laden atmosphere of Great Britain, So far as is at present known, the nearest fixed star in the whole heavens is Alpha Centauri, a brilliant double star in the constellation of Oentaurus, not far from the wellknown Southern Cross. The most magnificent cluster of stars in the visible universe is that known as Omega Centauri, also in the constellation of Centaurus. The large and small Magellan clouds are wholly dissimilar to anything visible in Europe, whilst the nebulse round Eta Arglis, in the constel- ' lation of Argo, the glorious Southern Cross, and the splendid portions of the Milky Way that include and surround these constellations, render the whole region very striking and deeply interesting.

Let us first deal with the star Alpha Oentauri, situated from our system by the enormous distance of twenty-five billions of Vol. lI.— No. 14.— 1.

miles. Cau wo put iv a small compass an easily-understood description as to how this distance is arrived at, and then give facts and data that will enable roadors to got some distinct ideas of what that distance really means ? Let us make the oudeavour. The first step in the trigonometrical survey of the heavens is to measure our own earth. In order to do this, if wo can measure a degree of latitude and then multiply that distance by three hundred and sixty — the number of degress in a circle — we at once get the circumforonce of the earth, from which all its other dimensions are readily obtainable. As degrees of

latitude, however, are not marked on the earth's surface, we must have recourse to the heavens. Suppose the meridian altitude of a star, looking north to an observer in Auckland, is eighty degrees, now suppose the observer went due south from Auckland till the same star measured seventy-nine degrees when it culminated, or reached its meridian altitude. If the .distance between the two places where the observations were made is measured, it would be the length of a degree of latitude, and if this distance is multiplied by three hundred and sixty, the girth of the earth is found. In practice, the measurement of an arc of the meridian, usually embracing several degrees of latitude, is one of infinite precision and exactness, but the principle is the same. The next step in our survey is to use the distaiice through the earth between two stations or observatories as a baseline to get the distance of the sun from us. The length of this .base is so small, compared with the vatst. distance of the sun, that the triangle is what is termed in geodesy an " illconditioned" one, but astronomers have to

make the best of it. It would be beyond the scope of this article to enter into the methods adopted to find the sun's distance. The transits of Venus across the sun's disc, viewed from stations on different sides of the earth, is the method most commonly known, but other and more accurate methods are now used. The mean distance of our luminary is given as 92,780,000 miles. Twice this distance, or 185,560,000 miles, is the length of the diameter of the earth's orbit, and this great distance is the base line for finding the distance of the stars. Stellar distances are, however, so enormous that a triangle, with the length of its known side, 185,560,000 miles, is still a very " ill-conditioned " one indeed, but is the best that the circumstances admit. There are certain minute stars in the direction of Alpha Centauri, but supposed to lie so immeasurably beyond it, that they are regarded as absolutely fixed, and their places in the heavens as entirely unaffected by our change of position, as the earth circles in its orbit round the sun. The position of Alpha Centauri is accurately measured with reference to some of these minute stars, let us say, to-day. In six months' time we shall have changed our position in space by 185,560,000 miles ; again, the position of Alpha Oentauri is measured with reference to the same small stars, and it is found that our change of position during that interval has resulted in a change of position of the bright star with reference, to the small ones, equal to about three-quarters of a second of arc. With this very small angle and the known length of our base line, Alpha Centauri is found to be at least 25,000,000,000,000 miles from us. It may here be mentioned that photography is now largely used in measuring the relative positions of stars, and is found to be a convenient and accurate method. The fact is, our sun and his attendant planets are surrounded by practically a vast void, at the least fifty billions of miles across.

the only occupants of this immense region being a few comets and unconsidered astronomical trifles in the way of meteorites. It is a very remarkable fact, and one that is not sufficiently considered, that out of the many millions of stars seen in the world's great telescopes, there is not one that pi'esents a distinct disc, as great and small, they all appear as points of light only ; also, as the milky way divides both southern and northern skies, roughly speaking, into two great halves, we are not far from the plane of the great ring or zone of stars forming the galaxy. Our position in the visible universe seems to be therefore a tolerably central one. Now, let us try to form some idea as to what twenty-five billions of miles means. Light travels at the rate of 186,000 miles in a second, and the time it would take light to reach us from a heavenly body is called the light journey, and is generally used to give an idea of stellar distances. It would take four and a quarter years for light emitted from Alpha Centauri to reach our earth. We cannot, however, form any clear idea of such a vast velocity as light travels at. Let us try some other standard. A cannon ball or rifle bullet would require more than two millions of years to go from the earth to the star, and an express train, travelling at sixty miles an hour without intermission, would require upwards of fortyseven millions of years to reach this, the nearest of all the stars. Compared with these vast intervals of time> the whole historic period is, as it were, but a question of yesterday, or as a single swing of the pendulum of the great clock of Eternity. When standing at a wayside station on one of the great lines of railway in England, and watching an express train tear through at the rate of sixty miles an hour, it seems almost past belief that it would take that train more than four hundred and twenty thousand centuries to cover the distance that intervenes between us and our nearest stella neighbours.

Another method to enable us to form some idea of the vast soale upon which the universe is built, is to toy to make a model of a part of it. Suppose wo adopt a scale for the model of a million miles to the inoh. Take a marblo rathor under an inch in diameter to represent the sun. A grain of coarse sand or a pin's hoad placed about seven feet eight inches from tho marblo would ropresont tho earth : then Alpha Centauri on the samo scalo would bo represented by another such marblo placed at a distanco of three hundred and ninetyfour miles ! If the marblo representing tho sun in the niodol is placed in Auckland tho marble representing the star would have to be removed to a point ono hundred miles tio the south of Wellington ! If this is tho scale for the nearest star, what are wo to say of the most distant ones that leave their images on sensitive photographic plates in star cameras after many hours of exposure ? Some of these stars are considered to bo many thousand times more distant. The magnificent cluster of stars known as

Omega Centauri is faintly seen in southern skies with the naked eye. When viewed through a powerful telescope, ifc is found to consist of about six thousand, four hundred stars of about the fourth magnitude, forming almost a blaze of light in the centre. It is beyond all doubt the richest and the largest object of its kind in the heavens. Some idea of the vast distance that separates this cluster of suns from us may be formed from the fact that the united light of the six thousand, four hundred stars shines in the sky with no greater light than a single star of the fourth magnitude would do. Large

numbers of the component stars of this cluster are variable, upwards of one hundred of these varying in brightness in short periods of under twenty-four hours. This globular cluster of suns is a universe in itself, and the conclusion seems to be that worlds revolving round these suns would have perpetual daylight. The blaze of light to inhabitants of these worlds would hide all external systems ; so that here we have the strange amomaly that an excess of light may hide more than it reveals. The Magellanic clouds are remarkable objects in the southern sky, forming two bright spots, roughly circular, of milky

light, looking like patches of the milky way, but are not in any way connected with the galaxy. The larger cloud, or Nebecula Major, occupies a space in the heavens about two hundred times the apparent size of the full moon. A good telescope shows that it consists of about two hundred clusters and nebulas, besides many thousands of stars. Mr. Russell, the Government Astronomer of Sydney, says : " The whole of this great cloud is a complex spiral nebula, with two centres, if I may so express it." This spiral character was first noticed by him on some photographs taken in 1890, and the discovery

is a very remarkable one, and corroborates Sir John Herschell's statement made long ago, " that the Nebecnlse are to be regarded as systems sui generis, and which have no analogues in the northern hemisphere." The best-known spiral nebula in the heavens is the very remarkable one in the constellation of Canes Nenatici, or the Greyhounds, and there are other spiral nebulae in other parts of the sky. In this instance, however, we have the whole of the great Magellan Cloud, with its swarms of nebulse and clusters, forming a gigantic spiral of itself. The smaller Magellanic Cloud, or Nubecula Minor, is fainter to the eye, and not so rich

in the telesoope. The photographs of it indicate a similarity in form to the wellknown Dumb-bell Nebula. It is broken into many knots, groups, and straggling branches, and is surrounded by a barren region remarkably devoid of stars. Close to it is the remarkable globular cluster known as Forty-seven Toucani. This cluster is similar to Omega Centauri, but is not so large, and contains, according to Professor Pickering, one thousand, four hundred and ninety-five bright stars and seven hundred and forty faint ones, giving a total of two thousand, two hundred and thirty-five components. These globular clusters are very remarkable objects, and from the seeming chaos of the general distribution of the stars in space, it is quite a relief to the mind when one of these glorious and compact universes, indicating design, enters into the field of the telescope. The nebula, surrounding the variable star Eta Argus, is sometimes called the Keyhole Nebula. Sir John Herschell says : "It is not easy for language to convey a full impression of the beauty and sublimity of the spectacle which the nebula offers as it enters the field of a large telescope, ushered in as it is by so glorious and innumerable a procession of stars, to which it forms a sort of climax, and is a part of the heavens otherwise full of interest." The nebula has undergone considerable changes in appearance since Herschell's time, and it is now found to consist of glowing gas. It occupies a space about five times the apparent size of the moon, and can therefore be seen with the naked eye. Mr. Russell, of the Sydney Observatory, says there is evidence that this nebula is also spiral in character. The well-known and fine group of the

Southern Cross is another glory of southern skies. The star at tho top of tho cross is known as Gamma Crucis, that at tho bottom as Alpha Crucis, and these two stars point almost exactly to tho south polo of tho heavens. As the cross swings round in tho sky, caused by tho rotation of tho earth on its axis, it always points to the samocouiro, and forms a sort of hour hand similar to that of a clock, by which Australasian bushmen can tell tho hour of tho night with considerable exactness. Of course, however, the position of the cross at any particnlnr hour of the night varies with tho season of tho year, and allowance has to be nmdo for this in using the cross as a celestial clock. There is a fine cluster surrounding tho star Oappa Crucis in this constellation of about one hundred stars of various brilliant colours, which, contrasting wonderfully with one another in a largo telescope, presents the appearance of a fine pieco of jewellery. This cluster is seen on tho photograph near the left-hand star forming the cross. The " Coal Sack "of Horscholl also appears in tho photograph below tho cluster just mentioned. This appears as si black spot or region amidst the general blazo of light nearly devoid of stars. Stars, however, appear on photographs oven in this bare region that cannot bo scon even with the most powerful telescopes. Who shall deal with tho profundities of infinite space, and tho countless hosts of heaven ! When viewing tho blazo of innumerable stars and systems in tho richer portions of either the northern or southern skies, we may well exclaim with the German poet, Richter, " End is there nono to tho universe of God. Lo ! also, there in no beginning."