ASTRONOMICAL NOTES.

Press, Volume LIV, Issue 16147, 27 February 1918, Page 9

ASTRONOMICAL NOTES.

FOR MARCH.

(Br E. G. Hogg, M.A.. F.R.A.S.)

The sun enters the sign of Aries on March 21st, on which date day and night will be of equal length over all the earth. Mercury is now too close to the sun to be seen, but by tjic end of the month may become visible in the evening. Venus rises on Marcii Ist at 3.<10 a.m. This planet is now a brilliant object in the eastern sky. and its brightness will increase still further during the next few days, a slow decline in brilliancy will then set in which Mill continue until September. Mars rises on March Ist, at 7.13 p.m. with a. brightness slightly exceeding that of Canopus. Its brilliancy is still on tho inercaso aiul on March loth will be only half a magnitude less than that of Sinus; it will then diminish, returning by the end of the month to the value it had on March Ist. Jupiter sets on March Ist at 10.9 p.m. and Saturn rises on that date at 5.28 p.m.

The following bright stars 'will cross the meridian of Christchurch on March Ist at tho times and at the altitudes above tho horizon given after cach star. If tho times of meridian passages arc required for other dates it will bo necessary to subtract 3min oGser; for each succeeding day. Towards the north we have Zeta Canis Maj. 7.43 p.m., alt. 76deg 31min; Epsilon Canis Maj., 8.21 p.m., alt. 75dcg. 20min; Pi Argus, 8.40 p.m.. alt. 83deg. 25min; Eta Cauis Maj., 8.47 p.m., alt. 75deg. 37min: Beta Canis Min. 8.49 p.m., alt. 38deg lmin; Alpha Geminoruni (Castor), 8.55 p.m., I4deg. 24min; Alpha Canis Min. (ProcyonY, D.l p.m., alt. 41deg. 2min; Beta Geminorum (Pollux), 9.G p.m., alt. lSdeg. lomin; Zota Argus, 9.27 p.m., alt. 85deg. lomin; Beta Cancri, 9.39 p.m., alt. 37dcg. 2min; Lambda Argus, 10.31 p.m., alt. 89dcg. 31min, Towards tho south wo liavo Alpha Argus (Canopus), 7.4S p.m., alt. SOdeg. 53min; Gamma Argus. 9.33 p.m., alt. SGdCig. 2Gmin; Epsilon Argus, 9.47 p.m., alt. 80dcg. 27min; Delta Argus, 10.8 p.m., alt. 79deg. "min; lota Argus, 10.40 p.m., alt. 74deg. 3(imin; Kappa Argus, 10.44 p.m., alt. 78deg. 52min; Delta Pavonis, 9.27 p.m., alt. 19deg. 55min; Alpha Pavonis, 9.45 p.m., alt. lOdeg. 32min; Beta Pavonis, 10.5 p.m., alt. SOdeg. 2min; Gamma Pavonis, 10.45 p.m., alt. ]9deg. 16min.

The evening sky is now- full of interest lor tho observer. The planets Jupiter, Saturn, and Mars arc well placed for telescopic examination, and many, of tho most striking constellations in tho heavens may now be seen. Of tho two stars, Castor and Pollux — tho Heavenly Twins —given in tho above list, it will bo easily noticcd that Pollux, though named Beta, is decidcdly brighter than Castor; there is, in fact, a difference of nearly ono magnitude between them. Castor Is tho only star among those mentioned above which occurs in Pickering's list of the sixty finest objects in the skv. It is a white double star, whose two components aro of magnitudes 2 and 2.8, and aro separated by 5.7 seconds of arc. At 10 p.m. the well-known naked eye cluster named Praseue in the constellation Cancer culminates towards tho north at an altitude of 26deg. lOmin. Tho magnificent globular cluster Omega Centauri is now well placed for observation in the southeastern sky. To locate its position draw a line from Beta Centauri—the less bright of tho two pointers of tfio Southern Cross—to the nearest star in the Cross; to the north of this line lies another star nearer Beta than the Cross, tho three stars forming a triangle with two equal sides. Now prolong the lino joining Beta to this star, nearly its OM-n length, and the eyo will encounter a faint misty star, whoso nature is easily discerned in a good field-gloss, while a largo telescope will reveal more than. 10,000 stars forming a system placed at so great a distance from us that it appears as a single star. Sir John Herschell states that when viewed in a powerful telescope it appears as a globo fully twenty minutes of arc in diameter, very gradually increasing in brightness* to the centro and composed of innumerable stars of the« thirteenth and fifteen magnitudes. Ho suggests that a star of tho thirteenth magnitude appears as two or more of tho stars of tho higher magnitude closely juxtaposed, but modern opinion rather tends to regard this differenco of ( magnitudc as pointing to the existenco in the cluster of two distinct types of stars —giants and dwarfs—a phenomenon ■which converging lines of evidence lead us to surmise may be a characteristic feature of the stellar universe to -which wo belong. It may be added that Omega Centauri was discovered by Halley in 1677 at St. Helena, whither he had proceeded in his twentieth year for tho purpose of studying the stars which lie around the southern nolo.

EL.J. writes enquiring -what hypotheses exist to account for the origin of Nova; or temporary stars, besides Professor Bickerton's partial-impact theory. Many suggestions have hern made that these stars result from tho collision of two stellar bodies, hut so difficult is it to explain tho appearances presented by their 6peetra on any hypothesis of collision that tho suggestion has never, so far as the present writer is aware, been formulated into a definite theory, except bv Professor Bickerton. who opines that in an oblique collision the matter struck from the impinging bodies would coalesce into a third hody which he identifies with the Nova. This theory has been before the scientific I world for some years, but can hardly be | said to have received much support from those competent to express an opinion on the subject. Lockyer sought the origin of temporary stars in a collision between a star and a meteor system, but this thoory failed to gain any general acceptance." The only theory, indeed, which receives serious notice in current literature, is the one put forward many years ago by Seeligcr, who attributed the blazing tip of n new star to the plunge of a previously dark body into a nebula. It was subsequently modifiod l>v Halm, of the Cape Observatory, and somo observations of recent times appear to lend support to Seeliger's hypothesis. The nevr stars seen in the heavens which have been examined spectroscopicallv, have fill run through very similar stages in their development, and have finally passed into gaseous nebula?. When stars are classified according to their spectra, into one wellmarked division fall what are known as Wolf-Rayet stars, whose spectrum is characterised by bright bands on a faint continuous background. Of the 91 stars of this tvpe known to exist. 70 arc situated in the Milky Wav and the whole of tho remaining 21 are in the Magellanic clouds. Novae are likewise almost entirelv confined to the' Milkv Way. Now the characteristic 1 radiations "of Wolf-Raycfc stars appear in tho spectra of temporaries before these take on a definite nebula status; indeed, the Wolf-Rayet and uebulav lines are often observed together, the latter gaining strength, as compared with the former, during the fading '-i the star. Early in 1901 there suddenly appeared a most brilliant new

star—Xova Persei; it ran through its stages and <>Tadually passed into a gaseous nebula. Tn 1907 Hartmann found that the spectrum of Nova Persei was no longer of the nebular typo, but more closely resembled that of an average Woll'-liayet star. Recently Adams and Pease, using the 60-inc'h reflector of tlic Mt. Wilson Observatory, succeeded in photographing the spectra of Nova Persei and of Xova Auriga, which appeared iri 1892. The result' has been a complete confirmation of Hartniunn's statement; both bodies arc now •Wolf-l?ayct stars, with nothing of the nebular character about them. Adams and Pease point out that Seeliger's hypothesis fits in well with their observations. On this view the disappearance of the nebular linos from the spectrum of an old temporary star indicates that it has passed right through the nebula, and that the latter, no longer agitated by Lhe presence of ils visitor, has ceased to emit light. We. thus trace, the several steps in the development of the .spectrum of a Xova. In the earliest stage, when it is brightest, its own radiations completely mask those of the nebula; we next find the spectra of star and nebula superposed, and finally the nebular spectrum masks that of the star. Tins third stage will probably bo maintained until the star leaves the nebida, which will now revert to its former state of relative quiescence, whilo the star will then exhibit the radiations of which it is capable. There has always been a difficulty iij fitting the Wolf-Rnyet stars into a general scheme of stellar evolution, but tho difficulty disappears ir Ave regard them as the final stage in the history of such exceptional phenomena as temporary stars, and therefore out of the direct line of normal evolution. It is, however, necessary to remind ourselves that though these interesting discoveries of l&te years appear to afford confirmation of tho truth of Seeliger's theory, there are many outstanding objections of a technical nature yhich have not yet been mot, and the hypothesis cannot be regarded as an accepted theory of the origin of temporary stars.