ASTRONOMICAL NOTES.

Press, Volume LIV, Issue 16251, 29 June 1918, Page 10

ASTRONOMICAL NOTES.

FOR JULY. (Br E. G. Hogg, M.A., F.R.A.S.) On July Ist Mercury sets soon after the sun, but as the two are separating rapidly it should be seen in the western sky about the middle of the month; it sets on July 15th at 6.10 p.m. Venus is still an object of striking beauty *n the eastern sky; it rises on July Ist at 4.33 a.m. Mars is gradually fading in brilliance, but it is still slightly brighter than a first magnitude star. It will cross the meridian of Christ- 1 church on July Ist at 5.39 p.m., and will set that evening at 11.44 p.m. Jupiter is somewhat too close to the sun at present for observation; it will rise on July 15th at 6.8 a.m. Saturn will soon ceaso to be visible in the evening; it sets on July Ist at 7.17 p.m. At 7.30 p.m. on July sth the earth will be in aphelion or at it 3 greatest distance from the sun, which will be about 94,400,000 miles. Jupiter will approach very close to the moon on July- 7tlji, and on the 18th Mercury and Saturn will be separated by inly 26 minutes of aro from each other. Ihe following bright stars will cro»«i the meridian of Christchurch' on July Ist at the times and at the altitudes above the horizon given after each star. If the time 3 of meridian passages are required at other dates it will be necessary to subtrAct Smin 56sec for eaoJi succeeding day. Towards the north we have Alpha Virginis (Spica) 6.45 p.m., alt. 57deg 12min; Alpha Bootis (Areturus), 7.37 p.m., alt. 26deg 52 min; Beta BoStis 8.24 p.m., alt. 19deg 12min; Beta Librae 8.37 p.m., alt. 55deg 33min; Alpha Coronse Borealis 9.5 p.m., alt. 19deg 29min; Delta Scorpii 9.20 p.m., alt. 68deg 51min; Alpha Soorpii (Antaros) 9.49 p.m. Towards the south we have Alpha Eridani (Achernar) 7 p.m., alt. lldeg llmin; Alpha Centauri 7.59 p.m., alt. 73deg 2min; Gamma Triang. Aust. 8.36 p.m., alt. 65deg 9min; Beta Triang. Aust. 9.12 p.m., alt. 70deg 21min; Alpha BeticuU 9.3S p.m., alt. 16deg 13min; Alpha Triang. Aust. 10.5 p.m., alt. 64deg 39 min; Zeta Arse 10.17 p.m., alt. 77deg 40min. There are many objects of more than usual interest which will be well placed for observation this month. The striking globular cluster in Libra will cross the meridian towards the north at 8.38 p.m., altitude 44deg 4min; the globular cluster in Hercules culminates to tba north at"10.3 p.m., altitude lOdeg. The well-known coarse cluster in Scorpius will be almost overhead at 10.12 p.m.; many of the stars in it are seen to be arranged in lines approximately r.t right angles. Nn Scorpii, which culminates to the north at 9.31 p.m. at an altitude above the horizon of 6odeg 40min, is a double double star being made up of a close pair of magnitudes 4.3, 6.8, separated by 41 seconds of arc from a second, close pair of magnitudes 4.3, 6.5. Our information concerning the Nova which recently appeared in the .constellation of Aquila is still very

meagre. It may be summarised as follows: —It was first seen on June 9th, at 10.45 p.m., by Mr G. V. Hudson, of Wellington; its magnitude was then estimated to be .9, i.e., the Nova was slightly brighter than a first magnitude star; by June 11th it had attained a magnitude which made it the second brightest star in the northern hemisphere, i.e., it was inferior in brightness to Vega (mag. ..1) and superior to Areturus and Capella (mag. .2 each); since that date it has steadily declined in brightness. Unfortunately, the state of the sky at Christchurch during the past few days has been such as to prevent any serious attempt being made to estimate its brightness by comparison with stars of known magnitude. Tho Nova will cross the meridian of Christchurch on July Ist, at about 12.13 a.m., at an altitude of 46 degrees above tho horizon. ' In its brightness when first detected, and the brilliance it subsequently attained, Nova Aquilas much resembles Nova Persei, which was discovered by Dr. Anderson on February 22nd, 1901, at 2.40 a.m., when its magnitude was estimated at 2.7. On the evening of that day it was a first magnitude star, and 24 hours later it rivalled Vega in brilliance, but this stage lasted only for a short time, and by midnight of the 25th it had sunk to a first magnitude star. On March Ist its magnitude was 2, and by March Bth it had lost another magnitude. It fluctuated in brightness between magnitude 2 and 2.5 until March 18th, when in 24 hours it dropped to the fifth magnitude. Now began a series of most perplexing variations in the brightness of the Nova, the intensity of its light changing in an irregular manner at intervals of two or three days by two and a half magnitudes; it was, however, slightly on the down grade, so that towards the end of April its average magnitude was 5, and it then soon ceased to be visible to the naked eye. Further decrease of illumination continued until the Nova passed into the nebular stage, with a magnitude of 14. In 1907 Hartmann found that its spectrum had chaffed and wns then approximating to that of a Wolf-Eavet star, a result subsequently confirmed by Adams, at Mount "Wilson, as described in these notes for March last. A most casual observation of the Milky Way, or Galaxy, discloses that large irregular dark areas, black holes or "coal sacks," as they have been called, are strewn at intervals among the illuminated areas. Conspicuous dark patches are found near the Southern Cross, and in the constellation Sagittarius, while the camera shows their existence, in almost all parts of the Milky Way. Now, we know that the so-called "fixed" stars really move, the speed of the naked eye stars averaging about 16 miles a second, and that, though there is some tendency of stars to favour certain directions of motion, there can be no doubt that a large share of stellar motion is at random, or, in popular language, the stars are moving in all directions; hence when wo come to considor tho nature of those "coal sacks," or apparent "holes" in the Galaxy, we find it difficult to explain how, within a great volume of space which is 'rich in stars, there can be a smaller, but still enormous, volume of space, nearly free of stars. The random motions of the stars should have distributed them more uniformly than we obsorvo to be the case. The conception of a "hole" through the Milky Way surrounded by a plenitude of stars moving more or less at random so as to give a liigh density of star-distribu-tion right up to the sharply-defined edges of the "hole*' and yet leave the "hole" empty of stars is one very difficult to maintain; with the help of a.ll astronomical experience, we cannot ex-

plain the'phenomenon by the absence of stars. We seem to be driven to the conclusion that the stars are actually there, but their light does not reach us because it is cut off or absorbed by invisible material existing between us and the stars or, shortly, in our stellar universe and in the direction of the Milky Way are what are termed dark in addition to the irregular luminous nebulae which occur in such numbers in the Galaxy.

Returning now to Novae, we find that 33 such objects have been coserved during the past three cen turies, 23 of them since 1886, whan the photographic dry plate was appli"i systematically to the mapping of the heavens. This is an average (during the last three decades) of about three new stars every four years and. -.3 some of the fainter ones undoubtedly come and go unseen, it is evident that thev are by no means rare objects. The average distance of Jie lucent stars has been computed to be about eight light-years and, consequently, as the motions of the stars are for the greater part at random, the probability of a collision between two lucent stars is an extremely remote one. If a new star be the result of " the impact between a lucent star and a dark one or between t-.vo dark stars, a recent investigation far to show that the dark stars must be something like 8000 times as numerous as the lucent ones to render it probable that a collision may take place once in every two years on the avar age. Few astronomers are, however, prepared to admit that the- number of dark stars in our stellar system so overwhelmingly transcends that of the lucent stars. Of the 33 known Novae, four recently discovered ones are situated in spiral nebulae, and these do not for the present concern us, but with respect to the remaining 2!) a startling fact confronts us, viz., 25 of them have appeared in the Milky Way, and of the four others, two arc situated In weil-known nebulae, one was seen to have a nebulous halo round it, and t l e last was' but imperfectly observed, lha large irregular luminous nebulae are almost entirely confined to the_ Milkv Way, and this region is also rich in the* dark nebulae as already pointed out, and it is to this part of our stellar system that Novae are almost entirely "confined. It seems difficult to escane the conclusion that some definite" relation connects Novae and nebulae. In his address to the American Association of Science, held at _ New York in December, 1916, the president, Dr. W W. Campbell, Director of the Lick Observatory, and a leading authority on spectroscopic astrononiy, writes: "A temporary star is seemingly best explained on the theory uhat a dark or relatively dark star travelling rapidly through space has met with resistance, such as a great nebula or cloud of particles would afford Wliilo passing through the cloud the star is in effect bombarded at high velocity by the resisting materials. Ihe surface strata become heated and the luminosity of tho star increases rapidly." Campbell . finds interesting evidence in support of this view in Nova l'ersei. H© continues: —"Wolf at Heidelberg photographed in August an irregular nebulous object near the star. Ritchey's photograph of September showed extensive areas of nebulosity in all directions from the star. In October Perrine and Ritchey discovered that the nebular structure had apparently moved outward from the star. Going back to a March photograph, taken for a different purpose, Perrino found recorded upon it an irregular ring of nebulosity, closely surrounding the star, which was not visible upon later photographs. Tho region seemed to be full of nobulosity not visible to us under normal conditions. The rushing of the dark star into and through this resisting medium made the star the brightest one in tho northern sky for several days. The great wave of light going out from the star when at this maximum brightness travelled far enough in five weeks to fall upon nonluminous materials, and made a ring of nebulosity visible by reflection. Continuing its progress, with a speed of 186,000 miles a second, the wave of light illuminated the material which Wolf photographed far away from the star in August; the material which Ritchey photographed still farther away in September, and tho still more distant materials which Perrine and Ritchey photographed in October, November, and later months. _ We were able to see this material oniv as the very strong wave of light which left tho star at maximum brightness made she material luminous in passing." Here w© must now lea/> qnesli'.r. of the origin of Novae. As further information from the northern observatories reaches us it will be seen how far the observed facts of Nova Aquilre support or negative the theory briefly sketched in these notes; since the appearance of Nova Persei great advance has been made not only in spectroscopic equipment, but also in deciphering thr many riddles presented by spectra, so there is fair reason to hope that good progress will b© made shortly in solving the complex problem presented by Novae.