ASTRONOMICAL NOTES

Evening Star, Issue 22559, 29 January 1937, Page 16

ASTRONOMICAL NOTES

THE SKIES IN FEBRUARY [Written by A. G. C. Crust, M.Sc., for the ‘ Evening Star.’] POSITIONS OF THE STARS Local sidereal time, 6 hours; latitude, 46deg. S. The positions of the stars predicted for last month continue _to hold good locally four minutes earlier each evening—i.e., at 9.00 p.m. on February 15, and 8.00 p.m. on March 3. The Moon, will be at the last quarter phase at midnight, February 3-4. She will pass sdeg south of the planet Mars on the morning of . the 4th, and 2deg north of Jupiter early on the 9th, while a conjunction with Mercury will take place the same evening. New Moon falls on the evening of February 11; on the forenoon of the 13th the Moon will pass 28deg north of Fomalhaut; On the morning of the 14th Bdeg north of Saturn; and on the forenoon of the 15th 3deg north of Venus. At 9.00 p.m. on February 15 her position will be W.N.W. 2deg. She will attain first quarter phase on the afternoon of February 18, and will pass 6.6 deg north of Aldebaran on the forenoon of the 19th. At 8.40 p.m. on February 20 the Moon will be N., 21deg; on the afternoon of the 22nd she will pass lOdeg south of Pollux; and on the forenoon of the 25th sdeg south of Regulus. Full Moon will occur on the evening of the 25th, her position at 8.20 the same evening being 8.N.E., 12deg. The planet Mercury will be favourably placed for observation in the morning sky in February, being at greatest elongation west, 26deg from the Sun, on February 7.

The planet Venus continues to grow in prominence as the most brilliant object in the early evening sky. By six hours S.T. this month she is still visible, being W. by N., 3deg, at 9.00 p.m. on the 15th, and W.N.W., sdeg, at 8.00 p.m. on March 3. Venus will he at greatest elongation east, 47deg from the Sun, on February 5, and on the 16th she will pass very close to the star Delta Piscium, of mag. 4.6. The remainig planets are invisible at six hours S.T., and exhibit no phenomena of special interest during the mouth. N.Z. RESEARCH OH ECLIPSING BINARIES Hitherto the articles in the February Astronomical Notes have _ been concerned with a direct explanation of the notes themselves, from different 'aspects. This year an attempt is made to show how certain work carried on regularly in New Zealand is related to problems of general interest to the astronomical world. The advance of astrophysics in the near future must be determined in part by the activity of New Zealand observers, and in particular by the support generously accorded the star colour section pf the Astronomical Society by some young men from Otago and Southland. Owing to a very general failure on the part of the astronomical world to realise the advantages and the degree of accuracy attainable in ,the measurement of star colours by the New Zealand method, quite an avalanche of important discoveries has fallen to the credit of this country, and in spite of statements to the contrary that have recently appeared, it will have to he conceded sooner or later that New Zealand is making a very creditable advance in stellar research. While it is realised only too well in this country that important and most interesting work has been accomplished with the immense instruments and elaborate equipment available abroad, the section leaders of the New Zealand Astronomical Society are daily confronted with the fact that the mechanical advantages possessed by other lands have for years been made the facile excuse for inaction here, and not only for _ inaction, but for the non-recognition, in many quarters, of the activity which does exist. They have thus become conscious of a spirit of revolt, and no section has become so revolutionary in its aims as the star colour section. The splendid woife carried out by some of the world’s great observatories on the radial velocities of the stars has shown the existence of variations, which may arise in two different ways. There may be either an expansion and contraction in the atmosphere of a single star, as in many variables, or there may he the case of the true spectroscopic binary, in which the star is revolving around a common centre of gravity with an invisible companion. Such stars are termed “ single-lined spectroscopic binaries,” and together with the suspected cases, they have much "material of interest to the section.

Now let us consider the limitations of the methods employed, ingenious and indispensable for future research as they are. Often statistical results will yield information not obtainable from single cases. Thus it has been found that in single—lined binaries the mass of the companion, which is in every sense invisible, averages one-half of that of the primafy. Is the companion a iplanet, a dense dwarf, or similar to the usual secondary of a visual binary? The planet proposition is ruled out by the mass proportion, but the other possibilities are not indicated at all, unless we examine colour statistics. In the dwarf visual binaries, the companion star is the redder of the two, while in the giant ones, the companion is bluer than the primary. Exceptional cases are, of course, provided by the dense dwarfs, which are intensely blue, hut usually too faint to affect the colour of the system. If spectroscopic binaries resembled visual ones, the dwarfs would be redder than single stars of the same type, but our results show practically no difference, except in some of the hotter types, which tend to be bluer than the single stars. For the giants, the data are more complex and rather scanty. Further suggestions are afforded by our knowledge (apart from the recent discoveries) of eclipsing variables. To quote J. E. Gore, who wrote thus about 1900—“ The character of the variation is a peculiar one, and is quite different from other variables. All the light fluctuations take place in the course of a few hours, while for the remainder, and much the longer, portion, of the period the light remains constant at a maximum. The number of these variables known at present is comparatively small, but since the discovery of spectroscopic binaries it has become probable that they may be much more numerous than is generally supposed. Owing to the character of the variation, their discovery is a matter of no small difficulty. A minimum of a star of this class may be observed by chance, but, from ignorance of its period, we cannot tell when to expect another minimum, and consequently a long time may elapse' before the suspicion of variability can be confirmed.” (‘Stella Heavens, 1 p. 69.1 Since these remarks were written, a large number of faint eclipsing variables of largo range in magnitude have been detected,

and Wyse lias shown that in their spectral relationships they resemble the visual binaries. All these discoveries were based on changes of magnitude, butrfJur colour statistics suggest that u thelSnumerous dwarf systems they indicate a large percentage of relatively blue companions. Where the companion of a single-lined or justifiably suspected spectroscopic binary is the bluer of the two stars, its physical properties conspire to make the changes of magnitude small, thus placing a further difficulty in the way of °the usual means of detection, if the two stars do happen to eclipse. The relatively small mass implies a relatively small luminosity by Eddington’s mass-luminosity law, the intense blueness implies a small diameter for the companion, thus reducing the secondary minimum, and finally, if its density is very high, the luminosity of the companion may be far below that compatible with Eddington’s law, thus greatly reducing the change of magnitude at primary minimum. Now the measurement of colour takes advantage of the great difference in colour which exists in such an eclipsing system. The _ smallness of the comtoo, is apt to lend quite a dramatic quality to the colour change at primary minimum. So rapid have been soma of the changes observed that the possibility of checking a watch by them, to the nearest minute, is well within reach, but results of value to practical astronomy will require to be of a still more striking nature. Such a star should become bluer at seconder? minimum, though not much so if the difference in size between the two stars of its system is very threat The prolonged scrutiny of a given stellar region is a great advantage to the Star Colour Section, as, in all but quite overcast weather the same stars can be watched for three weeks at a time, and the independent evidence of two or more observers may be obtained at the same epochs. A similar procedure in the photographic measurement of colour indices would be both difficult and expensive. When a star has given suspicious colour results the writer applies careful magnitude compansons by the Argelander method, which can be carried on for very long periods and in most cases without a telescope. While site-testing in the inoD , ar shies of Central Otago in 1928 he obtained the opportunity of acquiring special skill in such work Also, photographs taken with a small camera are being studied, so that no Jack of confirmatory evidence will be available. Stellar photographs have to be given a time exposure with a wide aperture, and the driving clock of a telescope, to which the camera is firmly tied, is employed to keep the camera pointed to the same stars for periods of a quarter or half an hour. If a star is changing rapidly, the camera may be moved deliberately an equal distance every quarter of an hour, and the result will be a.series of images of varying intensity. Recently the photographic variation of Lambda Endani was checked in this way on a Verichrome film by comparison with two other blue stars. , The variation, from the visual colour and magnitude comparison, was 0.47 mag., and from the photograph, at least 0.41 mag. As the images were not sufficiently spaced, the difference is probably over-esti-mated, but a better film has already been secured, and is awaiting development. The recent discoveries concern some half-dozen stars, and though much remains to be done, they seem to prove the existence of a class of stars intermediate in density between the dense dwarfs such as the companion of Sirius, of whose material, it is said, a match box could contain one ton; and our densest metals. It is perhaps not generally realised how far we can go, in interpreting the results, with the resources available in New Zealand. A careful study of the colour curves will provide us with au independent check on the relative diameters of each totally eclipsing pair, and show how far Russell, Dugan, and Stewart’s colour index-diameter equations may be trusted, while the mass-luminosity law will always provide a minimum value for the masses and densities. Even tlie magnitudes may attain a high standard of accuracy by repeated observation, a striking example being offered by Lambda Eridani, whose period of 0.998 day secures for us a continuous study of its primary minimum for more than six months. There is, as yet, no pressing need for the services of the largest telescopes in the country—rather there is work for the smallest. The' co-operation of some overseas workers may perhaps be attracted by the rapid development of these New Zealand results, and it is likely that they will compel many people interested tin astronomy to form a much higher opinion of the accuracy of our method of observation. The criticism “that it is not very accurate ” can now be met with an inquiry into the fruits of “ more accurate ” methods, which may not be easy to answer. AVe hope to demonstrate the physical properties of these new bodies of high density sufficiently to show that the great gulf apparently separating the known dense dwarfs from the stars of ordinary density is being bridged. This gap in our knowledge has been encouraging indeed te theories of a catastrophic nature concerning the origin of very dense stars; theories disturbing to people whose scientific faith is founded on the constant and gradual operation of natural law. and not particularly encouraging to research. The known conditions of density under which matter exists are extended, and even the possibility of producing matter of imfamiliarly great density on this earth has been increased. Who can tell what new and useful chemical and physical properties such matter may possess?