ASTRONOMICAL NOTES

Press, Volume LXX, Issue 21179, 1 June 1934, Page 7

ASTRONOMICAL NOTES

; » ! JUNE, 1934 j j U'fECIALUT V7RPJT*;; *O3 THE PftXSS.) | [By E. G. HOGG, P.8.A.8.] The sun will enter the zodiacal sign Cancer on June 22, which will be the shortest day of the year in the southern hemisphere; the meridian altitude of the sun at Christchurch on that day will be 23deg. 2min. The planet Mercury will set on June 1 at 5.36 p.m. and on June 15 at 6.10 p.m.: it will be seen well to the north of west and should be a conspicuous object at the beginning of the month; when its apparent magnitude will be 0.1., Venus rises on June I a!, 3.50 a.m. and on June 15 at 4.15 а.m., and Mars rises on these dates at б.31 a.m. and 6.27 a.m. respectively. Jupiter sets on June 1 at 2.28 a.m. and o:i June 15 at 1.33 a.m.; Saturn rises on these dates at 10.35 p.m. and 9.40 p.m. respectively.

The Interstellar Calcium Cloud Some 30 years ago Dr. J. Hartmann, of the Potsdam Astrophysical Observatory, while studying the star Delta Orionis. made an epoch-marking discovery which has led to a great extension of our knowledge of the distribution of matter within our galaxy. The star in question is the faintest of the three forming the belt of Orion; it is a spectroscopic binary of apparent magnitude 2.48. Its spec- , trum shows absorption lines of hydrogen. helium, and other elements suggestive of a high temperature, and these lines shift regularly from their normal position to tho red and to the I violet in a period of about six days, indicating that the brighter star is moving with a speed of about 100 kilometres a second round a companion ] whose luminosity is so faint that its spectrum is not recorded on the photographic plate. Hartmann's discovery consisted in finding that two absorption lines due to calcium did not participate in the displacements suffered by the other lines but remained nerfectly stationary. This peculiar behaviour of the calcium lines was confirmed later by other observers, and it was established that practically all binaries whose surface temperatures reached 1 15.000 deg. Cent, possessed stationary | calcium lines in their spectra.

1 Radial Motions ! Jn 1924 Dr. Plaskett, director of the Astrophysical Observatory of the Dominion of Canada, was able to announce that these stationary calcium lines were not confined to binaries but are also present in the spectra of stars of the required temperature. When a star is moving relative to the earth its spectrum is displaced by an amount, proportional to the component in the line of sight of its velocity towards or from the earth; Plaskett found that in numerous cases the radial velocity of a star determined from the lines of hydrogen, helium, silicon, etc., disagreed entirely with that deduced from the calcium lines, and hence it seemed clear that the calcium vapour which was the cause of the strange phenomena presented by the lines due to it did not share in the ttar's motion and therefore could not exist in its atmosphere, but must be somewhere in the sr>ace lying between the star and the"observer. j Eddington's Explanation The solution of the problem raised by these stationary calcium lines in the spectra of hot stars was given by Sir Arthur Eddington in his Bakerian lecture in 1926. Our sun is a member of a local star-cloud lying well within the galaxy, and it has been known for many years that, relative to the nearest stars, it is moving in the direction of the constellation Hercules at a speed of about 20 km. a second. An examination of all the obr servations of the calcium lines has revealed that, no matter what the velocity of the star showing the stationary calcium lines may be, its velocity as calculated from the dis-

placement of the calcium lines is invariably of the order of the component of the motion of the solar system towards Hercules. Observation showed that the calcium lines in all the hotter stars in Hercules and thg nearby constellations gave velocities of approach corresponding to 15-20 km. a second, while in the opposite direction of the heavens—such as the constellations Orion. Cauis Major, etc. —the velocities derived from the calcium lines were of the order 15-20 km. a second and recessional. These results led to the hypothesis of separate clouds of calcium vapour which were in some way associated with the hotter stars and at rest with regard to the stellar system. Further research carried on by Dr. Otto Struve, director of the Yerkes Observatory, led him to announce in 1926 that, while the velocities relative to the sun derived from the calcium lines were in general merely the reflex of the solar motion, there 'were many cases where the agreement was not exact, and it was inferred that the calcium clouds were not entirely at rest but possessed small "residual" velocities through space.

■ Galactic Rotation t The full significance of these residual cloud-velocities became appar- • ont when in 1927 J. H. Ooi-t, of Lei- • den, published his first papers on the • problem of the rotation of our Galaxy. 1 It has long been the opinion of astronomers that the stellar system to which Ave belong is similar in structure to the spiral nebulae or island uni- ■ verses which occur in such vast num- ; bers in outer space. The form of these | bodies suggests strongly that they are ■ rotating and it is held by many that ; such rotation has been demonstrated ; in the case of certain of the nearer , spirals from the evidence yielded by : the comparison of recent photographs ' of them with others taken many years | previously. Be this as it may there is now good reason to believe that our universe does rotate about .1 centre lying some 30,000 light-years distant in the direction of the constellation I Sagittarius in a period of about 150 j million years. It would be best to rc-j : gard these figures as provisional and open to correction in the light of further knowledge. The local star-cloud to which our sun belongs is comparable to one of the knobs or condensations seen in most spirals and it must be regarded as rotating as a whole about the above centre while each of its component stars has its own individual motion. On this view it appears that the sun is moving at about the rate of 300 km. a second in virtue of its galactic rotation, while superimposed on this is a motion, relative to the other members of tho star-cloud, of 20 km. a second.

j Returning now to the "residual" j velocities of the calcium clouds found j | by Dr. Struve, there is goocl reason to' believe that these clouds also share in the rotational movement possessed by the stars and nebulae of the Galaxy, and results published by Plaskett and Pearce last year go far to show that the velocity .of the calcium cloud in any particular region when it has been measured is within 1 per cent, of that of the neighbouring bright stars. Our picture then of our stellar universe must therefore be supplemented by supposing that the myriads of bright and dark stars ahd bright and dark nebulae, which compose the main part of its mass, are accompanied in their rotation by fairly generally diffused clouds of calcium vapour which lag slightly behind f'neir more solid companions. "It. is perhaps early to say that the calcium vapour is uniformly distributed throughout the Galaxy as its detection is conditioned by the presence of very hot stars but there is accumulating evidence to show its wide-spread dispersion. When the stars suitable for 'showing the existence of the calcium clouds are near us, the spectral lines of calcium are very fine but, the more distant these stars are. the more marked and broader do they become. This is explained by the fact that the light reaching us from a near star has traversed only a comparatively short distance ; through the calcium vapour and has • consequently suffered but slight ab- i sorption, while that coming from a J very distant star has passed through ] a greater amount of Calcium vapour < and been subjected to greater absorp- j tion. So clear is this characteristic • of the calcium lines that it is capable 1

of being utilised as n measure of the relative distances of the very hot stars from us. j The 200-Inch Trlcsfofif More than live years h;ivc elapsed .since attention was first drawn in these notes to the intention of the authorities of the California Institute of Technology to construct a reflecting telescope with a diameter of 200 inches, or double that of the magnificent Hooker telescope at Mount Wilfon. which has during its brief existence of 13 years made such vast additions to our knowledge in all fields of astronomy, and opened up so many enthralling problems for solution. Receilt information from America shows that the plans of the makers of the new instrument, have been considerably modified since they were first placed before the scientific world. It was originally intended that the material for the disc on which the mirror was to be figured should be fused quartz, and that, the reflecting medium should be a thin film of silver electrically deposited. It would, however, appear that when during the World War the European supplies of optical glass were cut off and the American scientists were forced to undertake the production of such glass, they evolved during their experiments a superior pyrex jLilass whose volumechanges very slightly with variation of the temperature, i This pyrex lens will be made at. the Corning Glass Works; the glass will be taken from the furnace at a temperature of 1500deg. cent., trucked to fne mould, and poured at about 1000 degrees. It will be allowed to cool to about 500 degrees and kept in that condition for about four months, when it is expected that a uniform temperature will have been attained throughout the mass; a further period of about four months will be required to allow it to cool down to its final state, when it will be ready for figuring. A f ar more revolutionary change than the substitution of pyrex zlass for quartz in the materia] of the lens is | to be made in giving the figured surface its reflecting power as it is intended to coat the prepared surface with a layer of aluminium instead of silver, which has been the standard material for many decades—a change arising from some very successful experimentation by Dr. J. Strong of the California Institute of Technology. In the process employed by him it is of the utmost importance to get the surface which is to be plated really clean; what is good enough for silver is by no means satisfactory lor aluminium, and Dr. Strong, we read, blasts off the 'final contamination by a bombardment with ions and electrons.

The next puzzle is to find a way of getting the aluminium to go where it is required, and this is done by melting the aluminium on a tungsten ' " ie r iSht size and shape. It the wire is too fine the molten aluminium will dissolve it and burn out. The wire in the form of a helix is heated electrically until the aluminium evaporates, when it is deposited on cooler surfaces near by. We are told that this process must take place in a high vacuum and that, when coating the mirror of the 36inch reflector of the Lick Observatory, a vacuum chamber big enough for several people to sit in was required. The chamber for the 200-inch mirror • almost 17 feet across) will be gigantic in comparison, but the technical difficulties involved in its construction will doubtless be overcome. Another point in favour of the new medium is that aluminium can be 'laundered." The surface develops an invisible coat of aluminium oxide which protects the metal without tarnishing like silver. It can be washed over and over with soap and water, and it is generally more durable than silver. It lias the further advantage of reflecting ultra-violet light far better than silver docs. Full success has been achieved in"the construction of j I a plane mirror—l2o inches square—which will be required in testing the concave surface of the reflecting mir|ror during the process of figuring. Probably the disc of pyrex glass has been cast by now and is being allowed to cool, and we can only hope that no untoward circumstances will prevent the accomplishment in due course of time of the great effort now being made to provide an instrument superior by far to anything we now possess and capable of revealing secrets of nature which the Hooker telescope cannot disclose.