THE HEAVENLY BODIES

Taranaki Daily News, 24 July 1930, Page 7

THE HEAVENLY BODIES

discovery of nebulae WHAT THE TELESCOPE REVEALS. ADDRESS AT NEW PLYMOUTH. —, An address on nebulae was given by Dr. George Home, the president of the New Plymouth Astronomical Society, at the annual meeting of the society last evening. The doctor illustrated his lecture with lantern slides. ‘•lf you went out into the clear, moonless darkness when eager stars are diamond bright.” said Dr. Home, *“you could see, with a pair of ordinary human eyes, roughly about 3000 stars of varying degrees' of brightness. You would also see a luminous phosphorescent zone traversing the vault of the sky like a huge distant elongated glow-worm stripped from the roof of the Waitomo Cave, called by the Maoris To Ikaroa (long fish); or Mangoroa (long shark), and by us the milky way. You would also see towards the "South Pole region two rounded, detached fragments of the luminous belt which are called Magellan's clouds. “If you looked particularly at Orion’s . sword-handle you would see a haziness : around' and about the middle etar — rather overwhelmed by the -' ..brightness of the handle’s jewels. . - “Again, if ■ you regarded carefully a /somewhat, steariiy looking star in the . Centaur, forming one point of an almost equilateral /triangle with the brighter of the. pointers and the southern end- . star of tin? Cross,'you would notice that ;it was different from the diadems around it. “The luminous haze in Orion’s swordhandle, which a pair... of field glasses : brings out very clearly, is a true nebula. The steamy star in the Centaur is a globular cluster or great multitude of. individual .stars, thousands - and thousands of .them, and not bv. any means close together, but at their immense distance from us—2o,ooo light years—their combined light looks like " an unusual fourth magnitude star.

REVELATION. BY TELESCOPE.

'lf you look up the word nebula in ■ your Latin dictionary you will find that .. i it means a fog, or vapour, or cloud, and . this word nebula was at first applied * indiscriminately to all celcstral objects that appeared hazy .to the view, and could not be resolved into, separate, stars by ,‘the.".eye. Of ..course the telescope was necessary to demonstrate such sep■arat’ion and for a‘long time even after its'introduction, it was’thought” that a powerful enough instrument would show ' all. these" smjiil cloudlets to .be composed of individual stars. The greater telescopes, aided by- the camera, revealed ■more and more of.them, until now. mil-, lions are catalogued, but’the great revelation regarding J?-em came as long ago as 1864, when Sir William Huggins ap- ‘ plied his spectroscope to these more or less obscure small luminous phantoms of the starry firmament.” . . The first one he ; investigated,, said the lecturer, gave him a great surprise, for he found that the light from it, instead of being spread into a long, fluted ribbon like that of other stars, showed nothing more than a single line in the green. • The only possible ex- / planation of this was that the body investigated- was . nothing but a mass of radiating gas, though K ’had the discrete rounded shape' of the "class called "planetary' -nebulae”' by the Herschels, because .with: ' the large telescope they showed a rounded-disc like a planet instead of a brilliant, extremely fine point like a .star. j. Some nebulae: of -somewhat similar appearance, however.: such as the great nebula, in Andromeda/gave a continu- . ous spectrum like the stars. Thus Sir William Huggins-- demonstrated that there were two entirely different classes of nebula which could’ look A something like .one another even to the eye aided by the larger telescopes. ; Celestial photography was called in to assist hi the "cluster versus nebula” problem, and by long exposures the camera had given a> wealth of information as to the cgnstitution of star clusters. ' It had also revealed evolutionary aspects of the true nebulae, and in addition the presence of .darkj nonluminous masses of gaseous matter • pervading parts of the -.milky , way/ which would otherwise have been quite - beyond -the means'of detection and ana- ■ lysrn; ’ ‘ ' c '

SEPARATION OF NEBULAE

“Leaving the subject of star eluistere, . which are not clouds, but communities jof formed individual stars, and which . have been referred to only for the pur- . pose of drawing a distinction between them and the true nebulae, we find that these are classified by the spectroscope ■ into two great divisions called gaseous and spiral. “The gaseous nebulae,” said Dr. Home, “are again separated into two kinds—- ‘ the irregular and the planetary. The ir- . regular gaseous nebulae are found usually. in relation to the milky way and in association with stars of a higher temperature than our sun. “The larger planetfry nebulae, on the other hand, are fiund- frequenting . areas of space at some distance from . the milky way, and have the hottest type of stars as nuclei. “In these. nebulate—both the irrcgut lar and the planetary—we must be . looking at matter in its very simplest form—the raw material, as it were, out _.of which stars are made; and in the ■ case of the planetaries postulating tem- , peratures which must cause a partial dissociation of individual atoms, temperatures which cannot be reproduced . in our laboratories, running up to 20,000 ■ degrees or 30,000 degrees C. “The irregular nebulae, on the other hand, diffusely filling the inter- . jstellar spaces, real chacs, without form . and void must be in part associated with a temperature of « absolute zero, the extreme limit .of cold. So lines appear in the spectra of both irregular and planetary nebulae which cannot be identified with those of any atom as we know it. Two very prominent lines in the green part have no earthly analogy that we ' have arrived f as yet, and the gas, or state of matter producing these optical effects, has been provisionally labelled ‘nebulium.’ “The sun itself has furnished a parallel case. In the first spectroscopic observations of the sun lines of a gaseous element as yet unknown on the earth led observers to think that it was produced only in the sun, and it was called ‘helium,’ from ‘helios,’. the. Greek word .for the sun,. i l)ut. Jtjca nfe. to earth when the eiiiahatiOns fi-dm. radium were sort- . out ’by Sir Ernest Rut!>?r'.‘o -d. and

one of these gave identical spectroscopic lines. Now helium is' found in association with radio active elements in earthly springs and is even collected and used for inflating the balloons of airsjiips. Perhaps nebulium will also ‘come to earth’ with further unfolding of the mysteries of the universe, and a Danish physicist found something very like it in the' spectrum of ionised nitrogen at lo temperatures. “The spectrum of the great nebula in Orion gives colour to the idea that it consists largely of hydrogen and ionised helium, but its photographic brilliancy is not due so much to this element as to one producing a decided line of unknown origin, and the green lines are strong in the light derived from the centre of this nebula.” * PLANETARY NEBULAE. “The planetary nebulae,” ho said, “consiwt each of a luminous disc or ring surrounding a central star. They are shells ’of luminous gases which are brought into being and sustained by radiation pressure pushing away minute particles from atoms disintegrated by the high temperatures. When a certain distance is reached, an equilibrium between this outward'radiation pressure and the returning gravitational force established, resulting, in a more or less discrete and stationary shape with a well defined external contour. The temperature of the central star must be very high—more than 15,000 degrees C. —for the radiation pressure to be sufficient to keep the shell of gas in equilibrium and to illuminate it. ’ “The spiral nebulae were noticed by Sir William Herschel to concentrate in regions of space remote from the milky, way.. They are totally different in type from the irregular nebulae and also from the planetarice. They are compact, oval or round in general shape, and usually bright towards the centre. Some of them "appear as long, single, or paired parallel streaks of luminous material; this because they are edgeon towards us. They vary very great-ly-iii size, which is probably a broad, general indication of their distance from us. One quaint feature of .their movements is that they are practically unanimous in receding from us, usually at a' tremendous velocity—2oo to GOO miles per second. A close photographic study of their make-up reveafa sometimes luminous' spots or nuclei as -of condensation, sometimes long, drawnout streaks or arms with, bends curving awav from the ce'ntre and suggesting a wide and rapid' whirlpool or centrifugal motion. It is" as well not to be misled by that word 'rapid in this connection.'." Perhajps the rptationally swiftest of these is, a .nebula in the Great. Bear, which after eleven years of exact, minute photographic comparisons, is found to turn itself round, once in .58,000 years. That' is very fast. “Stars' and astronomers demand lots of time "going ' for a cosmic' body,’’ said Dr. Home.; “Observations on the erreat nebula'iii Andromeda for instance, lead to a 10,000,000 year estimate of the time that, it takes to take a complete turn.' As it revolves one edge is comiiinr towards us and the other is re-, cedin'* 0 from us, .and we are able to measure these rates of advance and recession by means of the displacements of certain lines in the spectra of the different portions. GLOWING WISPS OF GAS. “The so-called planetary nebulae and the spiral nebulae have more or less coherent physical shape, but in addition To these among the stars of the.milky way the telescope and camera reveal huge glowing, wisps of gas stretching from star to star, and as shapeless as clouds' trailed out by' the wind. These irregular nebulae apparently have many stars enmeshed in them. According to .Jeans the physical composition of these irregular nebulae must be an almost inconceivably tenuous gas occupying the space -between' the stars, here and there lit up by their radiation, here and there so opaque that they lie like pieces of dark curtain across.' the sky. So Jeans puts it: ‘The variations of density, opacity and luminosity' in combination produce all the fantastic shapes and varied' degrees' of light and shade we see in the Milky Way nebulae.’” This led on to a consideration of the dark nebulae which were specially studied photographically by Barnard, of .the Lick Observatory. Herschel’s tele-, scope' revealed them to liis eye and. suggested to him that they were holes in°the heavens. They were unfatfiom'able to him, and more than a century had to' pass before an adequate explan-' atiou of them was arrived at—that they were dark, -obscuring nlasses which cut off the light of ’the stars beyond them arid ' left visible' only those . stars on the near side of;. them. In some cases the actual edge and partial outline of these dark masses could be shown distinctly lit up by collateral stars, “Tlie dark space in . the Milky Way close to the Southern Cross, called by the Maoris the patiki, or.flounder, and by us 'tlie Coal Sack, .must be an extremely wide veil of. obscuring matter. The aggregate mass of it must be enormous, yet . this . apparition of matter-, luminous and non-luminoufl, between and among the stars, must, according to all calculations, consist of extremely small particles, particles small enough io. be repelled from the stars by radiation pressure, very few to the cubic foot, but occupying areas of unimaginable extent and depth.

“.To return to the spiral nebulae which Herschel described as island universes,” said Dr. Home, “the nearest of .these .is 850,000 light, years away. The great one in Andromeda is estimated to be further away still, and the most distant we know* of is about 140,000,000 light years distant. ' These remotenesses place them quite outside our own immediate universe, and by inference they are of the same physical structure and architecture as our universe is, an immensity of bright stars and variable stars and star clusters of -fe'tars born, and of nebulae which consist of stars beiim born, and of their birth material. Jeans states, that about 2,000,000 of these remote spiral nebulae are visible in the great 100-incli telescope. They appear to bo scattered with a suggestion of' uniformity through space and at an average distance from each other of the order of 2,000,000 light years. How far they may extend beyond and outside our present limit of estimate staggers the imagination, but new light has arisen in both tho realms of physics and philosophy,which will further elucidate the mysteries of both occupied and unoccupied space-time. - “Meanwhile,” he concluded, “in our attempts to explain them they seem to resolve themselves into mysteries profounder still.” At the end of the lecture Mr. W, H. Skinner moved a vote of thanks to Dr, Home for his address.