ASTRONOMICAL NOTES.

Press, Volume LXI, Issue 18372, 4 May 1925, Page 11

ASTRONOMICAL NOTES.

FOR MAY.

(SPZaALLT WKITTES FOB "THS PRESS.")

(By E. G. Horn;, M.A., F.R.A.S.)

The planet Mercury rises on May Ist at 5.14 a.m., ;ind on May 15th at 4.50 a.m. lis apparent magnitude will be 1.7 at the beginning of the month, but will increase to —0.2 by the end, when it should bo easily recognised. The planet will be found a little to the north of east. Venus is too close to the sun during the month to be visible. Mars sets well to the north of west on May Ist at 7.1S p.m., and on May 15th at 7.4 p.m. It is not a conspicuous object now, as its apparent magnitude has fallen to 1.8. Jupiter rises on May Ist at 9.33 p.m., and on May loth at 8.35 p.m., and Saturn rises on these dates at 5.15 p.m. and 4.13 p.m. respectively. There will be two oecultations of the planet Neptune by the moon during the month, viz., on May 2nd and May 29th. On the earlier of those dates the moon will not be above the horizon at Christchurch at the time of tho oecultation, but on the later tho phenomenon will be visible as a close graze, the centres of the earth, moon, and planet being in a line at 5.31 p.m. The many friends of Professor Bickerton in Chxistchurch will be gratified to learn that at its last annual meeting the Astronomical Society of New Zealand elected him an honorary life-mem-ber in recognition of the valuable work he did for so many years in stimulating an interest in astronomy in the community. It is hard to realise that nearly half a century has passed since Professor Bickcrton, in a series of papers read before the Philosophical Institute of Canterbury, put forward his theory of the origin of the novae or temporary stars which appear from time to_ time in the heavens; the truth of this theory—with its far-reaehiug consequences—he advocated with great persistency; by vivid lectures and by letters—always bright and often amusing —in "The? Press" and elsewhere, he compelled attention to his views, and for a time at least lifted our thoughts from the prosaic affairs of life into the higher realm of cosmological speculation, for which we should be duly grateful. By its recent action, the Society has shown its appreciation of the value of the work Professor Bickerton did in his day for astronomy. The Astronomical Society of New Zealand has now been in existence for four years, and those who were responsible for its foundation have good reason to be satisfied with the progress it has made. Its membership is steadily increasing, and it is gradually building up a library of astronomical literature, which is available for the service of its members. The valuable support it has received from the Press throughout the Dominion is shown by the increase in the number of astronomical "columns" now appearing in the daily and weekly papers. The copies of these columns, which are distributed monthly to the members of the Society, contain between them a large amount of varied information, which ig both educative and stimulating, apprising the recipients of recent astronomical discoveries, of interesting matters of observation in the near future, and of theories on fundamental matters which. aTe current in the older centres of research. Since its inception- the Society has issued two special bulletins, dealing with "The Transit of Mercury" and "Daylight Saving," and we may expect others of a similar nature from time to time when some astronomical question of more than usual interest to New Zealanders presents itself. We must not, however, stress too much this side of the Society's'work, valuable as it is, and overlook the special purpose for which the Society was formed, viz., to bring together into a single body the many persons in the Dominion interested in astronomy, so that by a collective effort a better direction and a great impetus could be given to any movement which aimed at promoting the progress of astronomy in our midst. , One matter in which all are deeply concerned is the establishment of a Dominion of whose staff and equipment all would be proud, for New Zealand is exceptionally well-placed geographically for the study of the southern skies. In urging the foundation of such an observatory or in helping forward any other astronomical question, the Society will exert a greater influence, as, by its growing numbers, it becomes more and more representative of local astronomical thought, and it is therefore very desirable that all who sympathise with the aims of the Society, and have not yet joined it, should speedily become members.

I The intensive study of stellar spectra daring the past twenty-five years has showed that the main body of tho stars can be divided into well-defined classes, which mark different stages in the normal evolution of these bodies. Starting as a gaseous mass of enormous dimensions and extreme tenuity—the giant M stage—the star passes with increasing brightness and temperature, but decreasing volume, through the giant classes K, G, F, and A, until it reaches its peak in the class B; the declining stages then set in with diminishing brightness and temperature, and the star passing through the dwarf classes A, F, G, and K, finally reaches .tie concluding phase of its lucent life as- a dwarf M. Each class is divided inio ten sub-classes, and hence it is possible for the expert to express the position of a 1 star in the evolutionary scale with very considerable exactness.

•A huge mass of information has now been accumulated concerning the spectra of the stars —the Henry Draper Catalogue, just issued by the Harvard Observatory, gives the spectra of over 225,000 stars, and from this material certain results may be mentioned on account of their connexion with what follows. In the first place the average velocity of each class of star increases with the age of the star; these velocities are found by means of the spectroscope, and the probable error in each determination is small The average radial velocity —the speed to-and-fro in the line of .sight—of the B stars is 6.5 kilometres per seeond, while for A stars it is 10.9, for F stars 14.4, for G stars 15.0, for K stars 16.8, and for dwarf M 17.1 kilometres per second. These figures show clearly that there is a direct connexion between the speed with which a star is moving through space and the position it has reached in the evolutionary sequence.

In the next 'place it is accepted at present as a working hypothesis that the energy which a star is perpetually radiating from its surface is supplied and maintained by the conversion of a part of the matter composing it into energy; hence the mass of a star is continually decreasing as it passes through the different stages of its existence. Combining these two conclusions, we infer that in the stellar universe the bodies of greatest mass have the least velocity, and those of least mass have the highest velocity. This has to be interpreted in a general sense, true for each of the main classes of stars, bat

not expected to hold good for each individual star.

Between the brightest stars and the faintest there is a tremendous difference of luminosity—one star may emit a, million times as much light as another, but it would seem that no such marked difference exists among the stars in gonviews of the formation of a star, we should expect to find stars whose masses are ten times that of the sun, and again others whose mass is only onetenth of the solar mass—that is to say, stellar masses should exist between limits which are close when compared with those between which the luminosities fall, and, so far as the normal members are concerned, we find that all observation and calculation goes to show that the mass of a star is a small multiple or.sub-multiple of that of our sun. .•■."•

All stars, however, cannot foe placed in the normal sequence—-from, giant-M, through B, down to dwarf M—and there is one class in. particular—known &s o—characterised by cortain distinctive features which: make its relation to the other classes of stars quite obscure. This typo has been specially studied: by Dr. J. S. Plaskett; T.S.S;, Director of ..the Dominion of Canada Astrophjreical Observatory, who has had,at his'disposal the magnificent 72-inch, reflector erected a few years ago at Victoria, 8.C., and to him is largely due the: knowledge we possess of this peculiar group of stars. ~ ...'? Prom a spectral point of view, the O-stars appear to link on fairly, well with the B stars; many stars originally classed as B were only found to belong to the O-class when their spectra had received closer examination; the spectra of one claBS appear to pass by gradations into those of the other. Hero the resemblance seems to end, and it is difficult to avoid the conclusion that very real differences separate-the two types. In the first placej Dr. Plaskett finds from a discussion ol all the data that the O-stars have masses which are on the average from 40 to 50 times that of the sun, with a probable range between 10 and 80 times the solar mass. The average mass of the O-stars thus far exceeds that of the ordinary star, and as the mass and luminosity of a star are intimately connected, we are not surprised to learn that the O-stars greatly excel the B-stars in brilliance and surface temperature. A further point very clearly elucidated by Dr. Plaskett is that, the average radial velocity of the O-stars is remarkably high, being no less than 25.5 kilometres a second. Here, then,..we. have the most massive, of the stellar bodies moving with an average speed far greater than that of t&e least massive of the ordinary run of stars, . the M dwarfs, which are only moving alt- tlio rate of 17.1 kilometres a second. Tlio association of large mass with small velocity, and of small mass with large velocity, which we have seen, holds good for the main body of stars, is distinctly violated in the case of the O-stars. This class of-star is evidently in an early stage of its development and we should have anticipated a low average speed for its members, whereas Dr. Plaskett finds the.very reverse of this. The massiveness and great velocity of the O-type stars mark them out as a separate class whose relation to the other members of our stellar system has yet to be determined,,but further discussion of tbiß.question must be postponed.