The Great Nebula Around Eta Argus.

New Zealand Illustrated Magazine, Volume IV, 1 April 1901, Page 533

The Great Nebula Around Eta Argus.

By A. D. Austin, C.E., P.R.A.S

fHIS magnificent nebula surrounds the variable star Eta Argus, situated in the constellation of Argo, not far from the constellation of Crux, - v" or the Southern Cross ; and like that constellation is in the Galaxy or Milky Way. When the Cross is upright in the heavens, and therefore at its greatest altitude, the nebula is to the right of the Cross, about the same height as the middle of it, and distant nearly twice the length of the Cross from its centre. As this nebnlais seen with the naked eye — or to put it another way, without optical assistance — the above directions enable its position to be located with ease. The nebulous haze occupies a space in the heavens about five times the apparent space taken up by the full moon, but as the moon is close to us, and the nebula at a vast distance from us, any comparison of their real relative dimension is, of course, out of the question, as it could only be made in terms of thousandsof millions or even billions. This nebula, like the Southern Cross, is never seen in Europe, being always below the horizon of any place far north of the equator. The general form of the nebula can be made out with an opera or field glass on any clear night when the moon is absent from the sky. It has sometimes been called the Keyhole Nebula, owing to a dark rift or opening in it, presenting in a powerful telescope the appearance of a keyhole. The nebula has undergone considerable changes in shape since it was first drawn by Sir JohnHerschell, at the Cape of Good Hope, in 1834 to .1838.

A conservative estimate of the Dumber of nebulae in the sky, according to the late Mr. Keeler, Director of the Lick Observatory, is 120,000, and most of these, he says, have a spiral character. This spiral character of Nebulae is considered by many to be

confirmatory of the Nobulu Hypothesis of Laplace. In tho Androraedn Nebula, for instance, we seem to see nebulous rings in actual process of formation, rings apparently in rotation round a strongly condensed central nucleus, and which, in course of ages, will probably condense into planets revolving round a central sun. The Andromeda Nebula, from its vast size and distance, will probably result in tho formation of a gigantic system. The genera 1 tendency of the study of tho problem of the origin of the solar system is to somewhat discredit many of the details of Laplace's Hypothesis, but at the same time to establish on a firmer basis the broad view that the solar system has been formed by a process of condensation out of a very diffused mass of attenuated matter, resembling some of the nebula) as seen through telescopes, and this leads to the conclusion that the stars generally have had a similar origin.

There are distinct indications that the nebula around Eta Argus is of a spiral character. The spectroscope shown that it consists of luminous or glowing gas. It takes a very long exposure to obtain good photographs of this nebula, owing to its yellow colour. A photograph of the Great Nebula in Orion, with one hour's exposure, shows more detail, and is more dense than v photograph of the Argus nebula with an exposure of four hours. The photograph from which the illustration accompanying this article is reproduced had an exposure of twenty-four hours. This, of course, means that the sensitive plate had to bo exposed several nights in succession to the same part of the sky, it being covered during the daylight intervals. Telescopes used for photographic purposes and star cameras are kept in motion by clockwork exactly corresponding to the earth's rotation on its

axis. By this means the earth's movement is neutralised, the telescope or camera appearing to follow the stars as they move from*east to west across (he heavens.

The distances of nebulas from ouv system cannot be ascertained, hs tliere are no actual points in a nebulous haze by wliicli its distance could be even approximately obtained as in the case of the stars, which present minute points of light, enabling the distances of some of them to be roughly arrived at. We have little more than

negative evidence to go upon in estimating' the distances of nebulas. It is, however, considered thsit the light-journey. could only be measured l>v ociiluvics. As liirlit. Inivels

at about 186,830 miles in a second, these remarkable objects must be removed by enormous distances from us. Professor Pickering, of Harvard College Observatory, thinks that it would take light a thousand years to reach us from the nebula in Orion, which would place it about two hundred and fifty times the distance of the nearest known of the stars, Alpha Centauvi, which is about twenty-five billions of miles.

It is almost impossible to get any clear conception of such numbers as billions or trillions. Some idea may be formed perhaps in this way. Let us take the star known as 61 Cygni, whjch. is one of the very closest of the stars to us. Its distance is assumed

NORTH.

to be about forty- five bill ions Jof miles from our earth. Take an average sample of wheat, and assume an average yield of twenty-five bushels to an acre; it would then require the crop from 3,000,000 acres to produce as many grains of wheat as there are miles in the distance to 61 Cygni ! Trillions do not enter into astronomical calculations except in dealing with the weights of the sun, planets, etc. Taking the present wheat crop of the world at 2,500,000,000 bushels it would take the whole wheat crop for 700 years to amount to a trillion gi'ains ! There are numerous stars scattered over the crreat nebula of Eta Argus ; some of these may possibly have a physical connection with it, while others belong to the Milky Way. In no part of the extent of this nebula does it show an appearance of resolvability into stars, being in this respect analogous to the nebula of Orion. It may, therefore, be in no way connected with the Milky Way, having nothing in commou with it, and although we see it projected on the ground of the galaxy, it is probably at an immeasurable distance behind it. The star Eta Argus, which the nebula appears to surround, is a very remarkable variable star. When first noted by Halley, in 1677, it was a star of the fourth magnitude. After undergoing fluctuations in brightness, it was estimated as a star of the first magnitude in 1827. In the following year it was found to be of the second magnitude. In 1837 Herschell was astonished to find it of the first magnitude. After again waning, it was in 1843 nearly equal to Sirius in brightness. It was still a first magnitude star in 1856. In 1878 it had fallen to a seventh magnitude star. It is now probable that it is slowly rising to another maximum period. Wolf suggested a period of fortysix years, and Loomes sixty-seven, but it would appear that it has no regular period. The star is reddish in colour, and the spectroscope shows the hydrogen lines to be well marked. Most of the stars in the neighbourhood of the nebula are of the first type, since it is in the spectra of the stars of

the Milky Way that the hych'ogon lines are seen. It is estimated that the world's greatest telescopes, such as the Yorkes, the Lick, and the one at the Paris Exhibition, show stars as low as the seveuth magnitude. Photogi'aphy, however, goes still further, and stars are now photographed that cannot be seen in any telescope yet constructed. The light from these, sunk in space as they are at appalling distances from us, leave their impressions on sensitive photographic plates after long exposures. The whole heavens are now being photographed, the leading observatories of the world each taking a separate portion of the- sky. Sydney and Melbourne are two out of the eighteen observatories engaged in this work. It is intended to includo stars down to the fourteenth magnitude, and it is estimated that the photograph will contain about 20,000,000 of stars, 2,000,000 of which will be measured on the photographs and catalogued. There will then be a permanent record of the state of the visible universe (as far as it can be ascertained) for future comparison and reference. The word fixed-star is an entire misnomer. There is nothing in the visible uuiver.se, so far as is known, that is at rest. All is ceaseless movement. The stars are all moving at varying speeds, some of which have been ascertained and found to be from twenty to 250 miles in every second of time. It would at first view appear that the motion would soon alter the appearance of the star groups and constellations, but such is not the case. The distances of the stars are so enormous that in the lifetime of a man no change is observable to unaided vision in their relative positions, although to the astro ■ omer, with his delicate instruments of precision, changes can be detected, and these movements are termed " proper motion." If it is asked in what direction do the stars move, and do they move in orbits ? the reply is that they appear to move in every conceivable direction, and whether in vast orbits or not is unknown. The spectroscope, besides enabling us to

THK TUIFID NEBULA IN SAGHTAKIUS Photographed with the Ci'ossley Eeflector of the Licit Observatory.

detect minerals and gases in the stars similar to those found in the earth, also enables the astronomer to ascertain whether ■ stars in the line of sight are approaching or receding from our system, and at what rate they are travelling. After some thousands of years the appearance of the constellation, so familiar to most people, will have entirely changed. Some stars will have disappeared altogether, and many new ones will adorn the sky.

We are apt to regard all the stars as luminous bodies, but it is almost certain that to every bright star there are great

numbers of dull and cold bodies in space — burnt-out suns, in short — that have " had their day and ceased to be." Some theories have of late been advanced as to whether these dark bodies moving rapidly through space do not occasionally collide, and by the enormous heat generated or caused by the impact become again self-luminous bodies, and begin anew their careers as suns. Whether such is the case or not, we may rest assured that Design directs the stupendous whole, and that no " wreck of matter and crash of worlds " will take place a single minute before the appointed time.

It is tolerably certain that our solar system has been in existence for many, millions of years. Daring all this time no disaster has befallen it from collision with any other heavenly body, although, as we have seen, it is surrounded by countless millions of these moving rapidly, and apparently without any settled plan, in all directions. Our sun, too, and his family of planets are also moving rapidly through space ; but as Thomson says : " Man marks not Thee : marks not the mighty hand, That, ever busy, wheels the silent spheres." The nebula theory of Laplace is not opposed to religion or to the belief in a First Great Cause. Creation by evolution is really more marvellous than creation by direct methods. A man can make a machine, but he cannot cause a machine to develop itself. The universe, if not an ordered cosmos, is a chaos. It must be one or the other; it cannot be a mixture of the two. There is far too much method, adaptation of means to ends, regularity and continuity in the visible universe for it to be regarded as chaos. It is, therefore, a cosmos, the outcome of Purpose and Design. The question arises, do the stars continue to exist without limits in space, or has the visible universe definite boundaries, outside of which is a vast void ? Science has not answered this question satisfactorily as yet. In rnauy parts of the heavens there is a thiumng-out of the stars, and distinct indication that in these directions at least a boundary to the universe to which we belong is reached. The probabilities seem to be that the visible universe is limited in extent, although so vast. But far removed from this universe, countless other universes may exist in the realms of infinite space more glorious than our own, that no mortal eye may ever see. Here we have reached the limits of the human mind, and we may well exclaim : — Hide me from this insufferable infinitude, Or narrow my thoughts to this earthly ball, With its fleeting cixcumstance of time and sense, Its trivial round and its petty cares.

Is not one universe enough for man to road ? Is nob man too mysterious for himself to fully know P Can he journey with the iniinito — with the spued of thought, And see what Almighty Power and inscrutable Design Perform in the realms of never-ending space ? Until perchance all creations coii.se, and naught remains But unutterable nothingness of eternal night , The endless void of blackness beyond reason's bounds. How soon are tho limits of the human mind reached in dealing with these questions! How little man really knows of Creation ! How small ih the past he haw ever or can ever see ! So far as the visible universe is concerned all seems growth and decay. Solar systems have their births and deaths, and nothing is permanent, although millions of years, or even millions of ages, may elapse between tho birth and d.>ath of suns and their attendant worlds. Remorseless time, however, engulfs the whole at last; to begin it may be renewed careers in other forms and other systems. Whatever theories of Biblical inspiration readers of this article may have, surely every mind cannot but be struck' with the insight of the Psalmist, and the majesty of tho language in which so long ago he expressed thoughts now known to science as facts, thoughts uttered in :>mnitive times, when astronomy could hardly be said to have existed. " Of old hast Thou laid the foundations of the earth : and the heavens are the work of Thy hands. "They shall perish, ■ ut Thou shall endure: Yea, all of them shall wax old as doth a garment ; as a vesture sh It Thou change them, and they shall be changed. " But Thou art the same, and Thy years shall have no end."* This article will not deal with the Trifid Nebula in the constellation of Saggitarius, of which a reprod action of the latest photograph is given.

* Psalm cii., 25, 26, 27.