IN STARRY SKIES

Evening Post, Volume CXVI, Issue 85, 7 October 1933, Page 21

IN STARRY SKIES

(By "Omega Centauri.")

NOVA AURIGAE, 1891

On December 10, 1891, a photograph of part of tho constellation Auriga was taken at tho Harvard Observatory. On the negative was a tiny black spot which would not have been there had tho photograph-been taken a night or two' earlier. In fact, a photograph of tho same region had. been taken by Dr Max Wolf at Heidelberg fortyeight hours earlier. In this tho place occupied in the" Harvard ncgativo by this particular black spot was a perfect blank. . But. these, fasts were not known till much later. During the next seven or eight weeks a dozen morephotographs were taken at Harvard, all recording the presence of the star. But they were stored away for future examination. Suddenly the astronomic world was raised to feverish activity. On February 1, 1892, when the stranger had shone for nearly two mouths with a brightness easily perceived by tho naked, eye, Dr. Thomas D. Anderson, of Edinburgh, sent an anbnymous postcard' to Dr. Copcland at the Royal Observatory, Edinburgh, who found tho stranger easily with tho help' of an opera glass. Notice was immediately telegraphed to other observatories. The star°by this time had sunk to the sixth magnitude, barely visible to the naked eye, but it aroused intense excitement amongst astronomers, for they were better equipped than ever before to read its story. It was most unfortunate that all tho early stages had been missed. Dr. Anderson explained later that ho was almost certain that he had seen the star on the morning of January 24, and quite certain ho saw it at least twico during the following week, but on each occasion until tho morning of January 31 ho mistook it for 20 Aurigae, though ho noticed that it was brighter than he expected. As soon as its character was recognised there was work both for daylight and dark. Photographic records had to bo oxamined and visual and spectroscopic observations made. In somo scores of

photographs taken, before December, IS9I, no trace of the Nova could be found. All taken after December 10 showed it clearly. It appears to have been rather an unusual Nova, botl^its lise and fall .being less sudden than that of a typical one. It is generally said to have had two or. three maxima. Tho photographic records seem to mdieato that it reached magnitude 4.4 on De-comber 20, that by January 20 it had fallen below the fifth photographic magnitude, that it rose again to_ 3.0 on February 3, and then sank rapidly. Probably tho real maximum occurred soon after December 10, .and escaped, observation- altogether. If this is so the fluctuations in brightness between December 2P and Marpli 9 arc just those of a typical Nova. By April it had fallen to the nftcenthth or sixteenth magnitude, but in August it rose again to tho ninth;. It then took about eighteen months to fall to.tho eleventh magnitude. But the most important fact in-'the study of-Novae connected with this: star is that its spectrum was recorded photographically, at a number of different observatories. ■ The first interpretations wcro very startling.- It was said that two bodies wcro involved, and that they had a velocity, relative to one' another of 820 miles per second. One was said, to' bo approaching us at ■over-400 miles a second, the other .flying from us. Then a still more surprising assertion, was made. The star-ap-., proaching us wnssaid to ; have a dark lino spectrum, the other one consisting of bright lines, whilst tho chemical com position of the two appeared indentical the bright lines of one spectrum being matched by dark ones of the other. Of course, when Professor Bickerton heard that the spectrum proved two stars to be' involved and that their relative speed . was. over 800 - miles per second, ho was delighted. But as soon as a reproduction of the photographic spectrum reached New Zealand he saw at a glance that tho spectrum was that of the Third Body alone, with a. constitution such as described in the last article. But-other, astronomers continued to ascribe the lines to different bodies. Vogel noticed that tho bright lilies showed a- double maximum, and suggested that there were two bright line and one dark line stars involved in tho encounter, .the- latter approaching at over 400 miles per second,1 whilst of the fonne^ one w;is approachingut 22 miles per second, the other receding at .100 miles per second. Astronomers hesitated to suggest that any of these stars had actually collided. Many spoke of a near approach, and Dr. Huggins, whilst refusing to consider even a partiiil collision, suggested that if the bodies wore sufficiently diffuse there might possibly have been some interpenetration1 and mingling of I'lo--rare gases of their outer atmospheres. Monck suggested that the phenomena might bo explained by a star br, aswarm of meteorites rushing through .a gaseous nebula. But more surprises were to come. If stars arc.drawn.near to one another and pass unscathed, they must quickly lose, as they retreat, the enormous velocities produced by gravitation during their approach. Now the velocities deduced from tho. spectrum in this case failed to behave as was expected. They remained practically, unchanged. Of course, this is normal in the case of tho third body, for the velocities 'of the light gases aro so much above the critical velocities of cscfipo that they aro not appreciably diminished by gravitation. But more difficulties had to be faced by the orthndox cosmogonists. Three bodies failed to account for the complexities oC the spectrum. .Several sels of lines, each with its separate amount of shift,

and so indicating SOlll9 special velocity, stood out clearly. Soon it was evidciit that if this intorprottttion.wtts.adoptwl, instead of three stars at least «x would be-rcquired. So Mlhs Agnes Clo kc.in "Problems in Astrophyßios," «1. "Tho collision theory collapsed umlci (ho weight of the facts it had to carry" A very different conclusion would have been reached If she, _had taken into account the pecuha• ehai«. teristics of the Third Body. Aftoi 11. niaiuiuß above the sixth magnitude to over three moiiths, with changes in brightness of less than two magnitudes a precipitate decline set in. early in March, 1892. The light foil to one tenthousandth part of-its former value p about five weeks. . For the next foul months the sun interfered with observation. When tho Nova was. re-exam-ined in August it was found that a re-1 markablc transformation, predicted by Bickcrton as characteristic of tho normal 'development of tho third body, had taken plate. Professor Barnard, with the 36-inch Lick telescope, saw the Nova as a small bright nebula witn a star-liko nucleus of the tenth magnitude. The nebula was pretty bright and about three seconds in diameter, and surrounded by a fainter glow. At first Nova Aurigac had appeared decidedly yellow, vow it assumed a greenish tint. The aspect1 and the positions of the lines in its. spectrum appeared to be identical with those of a planetary nebula.- Wo have seen that this.is the'normal fate of a third body which is too massive to be dissipated entirely into . space. . Tho heaviest elements tumble back to form the stellar nucleus, whilst those of intermediate weight form' a semi-permanent .hollow gaseous shell. ■ Nova Aurigae. remained near the tenth magnitude, with some fluctuations, for about four years. By 1897 it had sunk to below the thirteenth magnitude and still appeared as a minuto planetary nebula. It may be well to quoto here ono or two sentences from Professor Biekerton's paper, read before the Philosophical Institute of Canterbury on July 4, 1878:— ".Applying .the spectroscope to such a star, we get first a continuous spectrum; then black. lines, quickly followed by bright lines and spectrum; then-bright, lines alone. . . . Lastly, as heat and-pressure diminish by the 'dissipation, of tho body into space, wo get1

fewer and fewer.lines, until only those substances, in greatest quantity, or of greatest power of giving lines at lowest temperature and pressure, remain luminous, and we have a, nebula left; or in the case of the total dissipation of the the, gaseous mass, all evidence of its oxistenee will disappear."