ASTRONOMICAL NOTES.

Press, Volume LX, Issue 18242, 28 November 1924, Page 2

ASTRONOMICAL NOTES.

FOE DECEMBER. (steculllt tteittzs ros "ins peess.") (BT E. G. Hogg, M.A., F.R.A.S.) The sun will enter the zodiacal sign Capricornus on December 22nd at 2.10 p.m., when-the solstice ocenrs. The sun on this day reaches its greatest altitude in the sky, which for the latitude of Christehurch is 69deg. 56min.; the distance of the earth from the sun will thou be about 91,470.090 miles. The planet Mercury will set on December Ist at 9.1 p.m., and on December 15th at 8.59 p.m.; its apparent magnitude on these dates will be and plus 0.1 respectively. It is now approaching the sun anil will be in conjunction with it on December 27th at 10.30 a.m., when its distance from the earth will be about 63 million miles. Venus rises on December Ist at 2.4G a.m., and on December 15th at 2.40 a.m.: Mars sots on these dates at 1.1 a.m. and 12.21 a.m. respectively. Jupiter sets on December Ist at 5.39 p.m.; it will be in conjunction with the sun on December "3rd, when its distance from the earth will be about 579.000.000 miles. Saturn rises on December Ist at 2.0S a.m., and on December loth at 2.7 a.m. .There will be an .occultation of the planet Neptune by the moon on December 17th. the earth, moon and planet being in a line at 2.30 a.m. The uheaomccon will appear as a close graze in the latitude of Christehurch, but as the planet will have an apparent magnitude of 7.7, there should be no difficulty in identifying it with the help of a small telescope. Early in the morning of December 6th the planets Venus and Saturn may be seen separated from each other by an apparent distance less than the diameter of the moon.

liiirvard Observatory is to be congratulated on having brought to a conclusion the Henry Draper Catalogue of star-spectra based on some 13,000 photographs of the sky. The classification of the stars according to their spectral class was begun by Miss Cannon in October, 1911, and finished by her in September, 1915, and in these nine volumes of "Harvard Annals" are given the positions, apparent magnitudes, and spectra of no less than 225,300 objects distributed over the sky. The cost of the catalogue, which has been close on a quarter of a million dollars, has been largely borne by Mrs Henry Draper; it is a fitting memorial of her husband, who did valuable pioneer work in the application of photography to astronomical research.

Mr Harlow Shapley, Director of the Harvard Observatory, has contributed to the May issue of the "Scientific Monthly" an interesting paper dealing with some of the results and inferences which may be drawn from the completed catalogue, but before entering on these it may be well to premise that for the purposes of analysis the stars visible to the naked eye and in the telescope are divided into a series of classes nanaed M.K.G.FABAF.G.K.M. —in which may be traced the evolution of a star from the M (giant) stage, when it is a gaseous body of stupendous magnitude and extreme tenuity, gradually becoming hotter, denser, and less red in colour until it reaches in the B stage its culmination as a white star, only to pass slowly.' on the descending scale through the stages A, r, G, K, of increasing density until finally it reaches the class M again as a red dwarf star. It is, however, necessary to state that all the giant M stars have not sufficient mass to reach the B class, and these may attain their maximum brilliance and surface temperature in the "A class, and thence descend to the dwarf stage.

Mr Shapley tells us that about 99 per cent, of the 225,300 stars catalogued fall into the descending series from B to dwarf M, and that about 20,000 of these stars have spectra so nearly identical with that of ths sun that no difference could be seen in the instruments employed to examine them. The classes most common are A and K, but it would be very misleading to imagine that the stars of this type are the most numerous in the heavens: the basis of the catalogue is apparent brightness rather than absolute brightness, and hence, while a dim star can only be seen when it is comparatively close to us, bright star may have the same apparent magnitude when it is at a vastly greater distance a.way: in fact, the highly luminous B stars show with apparent magnitudes brighter than 8.25 when at distances up to 3000 light-years, but G stars, such as the sun. ■ will be brighter than 8.25 only when within a distance of 240 light-years. Hence, when we come to consider the number of stars occurring in a given volume of space, we obtain a result totally "different from that presented by their distribution in tlie sky. We leara that the number of B and dwarf G stars visible in the direction of the Milky-Way is nearly the same, though the volume of space in which the former are disposed is about 2000 | times as great as that in which the | latter occur, and Mr Shapley con- | eludes that for every highly massive and luminous B star appearing in the catalogue, there are actually 1800 stars in the same stage of development as our sun. He also finds that the youngest of all the luminous stars—giant M—appear but once in space for every 350 solar stars. When we learn that dwarf K arid the stars are much more numerous than dwarf G, we begin to realise something of the true nature of the distribution of the lucent stars in our neighbourhood. It is probable that the stars within the central parts of our galactic system arc on the whole disposed somewhat, as in the space near us, but we must be cautious in drawing our conclusions. Shapley writes: "As a striking il)ustration of the small portion of the galactic system to which our spectroscopic, .studies are limited, we may observe that probably more than 95 per cent, of the stars of all classes and magnitudes in the Henry Draper Catalogue are within 1000 parsecs (3500 light-years) of the sun. Only about one-millionth of the space known to be closely populated with stars is covered at all well by this extensive compilation of spectra." We may here refer to a recentlypublished paper on the distribution of stars in the neighbourhood of the sun by K. G. Malmquist for the purpose of supplementing the figures given by Shapley. Malmquist finds that if the stars generally were disposed in space much as they are near the sun there would be 105 K dwarf stars for each K giant star, and about 600 M dwarfs for each M giant. These figures are not quite in accord with Shapley's, but the apparent discrepancy is due to the difference of material used. Both, however, agree in bringing out the vast excess of dwarfs over giants in tho stellar universe.

We must here note another point in dealing with these statistics of starfrequency. It ig widely held that our sun is a member of a local star-cloud or cluster, outlined by the brighter B stars we can see, in which two great streams of stars are drifting past each other, and hence what may be called our local conditions may differ considerably from those prevailing in other parts pf the> great lens-shaped

volume, bounded by the clouds of the Milky Way, in which are distributed most of the different objects—stars, gaseous nebulae, planetary nebulae, novae—seen through our telescopes or caught on photographic. plates. Our sun lies at a vast distance to the north ■of the medial plane of the Milky Way, and certainly very far from the centre of the sideral system, bhapley, indeed, believes that we are on the edge of that part of the Milky AY ay known as the great Cgynus star cloud, and to this eccentric position he attributes certain characteristics of the stellar motions we observe. But whether what we can discover near us is typical of other regions of the star-world or not there can bo no doubt at all that the dwarf stars occur in overwhelming numbers compared with the giants, and that there is a vast preponderance of those whose luminous careers are drawing to a close over those which arc in the early stages of their evolution. Or the existence of stars which have actually ceased to shine, we have little knowledge, and at present we are not aware of any means by which an enumeration of them can he made. They probably exist in vast numbers, and the dead stars may he as frequent in our universe as the lucent, but of this we cannot speak with any exactitude. We may not, however, always be in this state of ignorance. : All the stars move, and their motions are ultimately dependent on the gravitational attraction of the matter composing our sideral universe, and it may be that the time will come when, from the observed motions of individual stars and groups of stars, astronomers will he able to deduce what this sum-total of gravitating matter is, and by apportioning it between the lucent and dead stars be able to form some approximate notion of the number of the latter. Already some attempts have been made to solve the problem along this line of enquiry, but it cannot be said that any definite result is yet in sight. t In the passage of a star from the giant to the dwarf stage, it is believed that there is a gradual decrease of the mass of the star: the matter composing it js transformed into the energy manifested in its light and heat, and by the time a star has reached the dwarf stage it contains only a fraction of the matter it possessed when it shone as a giant M. The transformation of matter into energy probably proceeds much more rapidly whilst the development from M to B is being accomplished than when the star is slowly passing through the declining stages of its lucent life, and it becomes a dead or non-luminous sun because the heat generated by the transformation of the matter within it no longer equals the heat radiated from its surface: it is losing more heat than it* is gaining, and consequently its temperature falls until, after glowing for ages as a faint red star, it finally becomes extinct. It would seem that the ultimate fate of the stars glittering now in the sky is to become dark bodies, moving unchanged for countless aeons through space, except in the rare cases when, through collision with another stellar body, a dead sun may again shine for a brief time with some of its former splendour. It is, however, improbable that with its replaced mass such a star can ever again run through the c y^ e of changes from giant M to dwarf M, which characterised its previous existence and we must look elsewhere for the 'origin of the primal star-form —giant M.