NOTES. 



ASTRONOMY. 



By A. C. D. CROMMELIN, B.A., D.Sc, F.K.A.S. 



THE SIZES AND DISTANCES OF THE STARS — 



There was an interesting paper on this subject, by the 

 Astronomer Royal, at the March meeting of the Royal Astro- 

 nomical Society. He dealt with the faint stars within 10° of 

 the North Pole, as these were observed by Carrington, at 

 Redhill, sixty years ago, and have been recently re-observed 

 at Greenwich, so that their Proper Motions are well determined. 

 While the Proper Motion of an individual star does not give 

 us its distance, it is possible to obtain formulae giving the 

 relation between proper motion and distance for groups of 

 stars. The Astronomer Royal obtains a formula for star 

 distribution that fits the data from observation very well. 

 Extending his figures to the entire sky, he finds that there 

 would be three hundred and twenty stars with parallaxes 

 greater than a tenth of a second. The number with a 

 parallax greater than a fifth of a second would be one eighth 

 of this, or forty. It will be remembered that Mr. Eddington 

 recently gave a list of seventeen stars with a parallax greater 

 than one-fifth of a second ; a few more have since been 

 found, and it is likely that there are several stars with an 

 equal parallax that have not yet been studied for the purpose, 

 so that the two methods are in fair accordance. The 

 Astronomer Royal finds that a large proportion of the stars 

 discussed lie between parallax 0"-005 and 0"-0025, and that 

 ninety-five per cent, of them are intrinsically brighter than 

 the Sun. Mr. Eddington finds that in another region only 

 one per cent, of the stars in the catalogue he used are fainter 

 than the Sun. He points out that this does not mean that the 

 Sun really occupies so insignificant a place in the stellar host, 

 for if we consider the stars of large parallax, the Sun comes 

 quite high up on the list as regards intrinsic lustre. But 

 when we go out to great distances, the intrinsically faint stars 

 become too faint to come into our catalogues at all, and only 

 those of high lustre survive. At a distance whose parallax is 

 one-hundredth of a second, the Sun would be of magnitude 

 10-3, which is about the faintest included by the Astronomer- 

 Royal in his discussion ; at greater distances only stars of 

 great lustre are included, so that the catalogue is not a fair 

 sample of all the stars really existing in remote space. 

 Incidentally, the paper made a suggestion that a short name 

 should be given to the distance corresponding to a parallax of 

 one second ; the term " Astron " was tentatively put forward, 

 but Professor Turner expressed some apprehension that this 

 might be taken to mean the astronomical unit of length, viz., 

 the mean distance from the Earth to the Sun. It is certainly 

 desirable to have a name both for this unit and for the other, 

 but it will be well to have a little discussion before they are 

 adopted. The Astronomer Royal urged that the "Astron" 

 should be generally used for stellar distances instead of the 

 Light-Year. In view of the large use of the Light-Year that 

 is made in many treatises on Astronomy, it is fortunate that 

 there is a simple relation between the two ; one " Astron " is 

 almost exactly three and a quarter Light-Years. The beginner 

 may like to see how this number is found. Light takes 498-2 

 seconds to travel from Sun to Earth. The number of seconds 

 in a year is 365JX 24X3,600 or 31,557,600. Divide this by 

 498 '2 we get 63,346 astronomical units in a Light-Year. ;: But 

 there are 206,265 astronomical units in an " Astron," this 

 being the number of seconds of arc in the unit of Circular 

 Measure. The " Astron " comes out as 3-2561 Light-Years, 

 but three and a quarter is near enough for all purposes. I 

 think the question of suitable names for these two units might 

 be a good subject for discussion in our correspondence 

 columns. Apropos of the Sun's distance, I was recently 



examining Delambre's Solar Tables, which appeared in 1806, 

 and was astonished to find that they used the very same 

 value of the Sun's parallax (8"- 80) that is now used in the 

 Nautical Almanack. Encke's famous value, 8"- 5776, 

 announced some years later, was thus a change for the worse. 



THE MEROPE NEBULA IN THE PLEIADES.— 

 Lowell Observatory Bulletin No. 55 contains an account of 

 the photography of the spectrum of the nebula ; an exposure 

 of twenty-one hours was given with the twenty-four inch 

 refractor, with the surprising result that the spectrum is con- 

 tinuous, with five distinct hydrogen lines and three fainter 

 helium ones, the spectrum being quite like that of Merope 

 itself. This was a great surprise, as the nebula resembles 

 that of Orion, and little doubt was felt that its spectrum 

 would be gaseous. Tests were made to see if diffused light 

 from Merope could have caused the continuous spectrum ; the 

 results appear to negative this idea. It is concluded that the 

 nebula shines by reflected light from Merope. It is calculated 

 that Merope would appear nearly as bright as our Full Moon 

 from the region of nebula photographed, and as we can photo- 

 graph a landscape by moonlight, the assumption of reflected 

 light seems reasonable. Mr. Slipher, the author of the 

 Bulletin, also suggests that the Andromeda nebula (for which 

 a similar spectrum has been found) may also shine by reflected 

 light, but this would only be possible if there were a very 

 bright central star, which is veiled from us by a dense screen 

 of dust or other opaque material. This seems a somewhat 

 strained hypothesis. We have, however, evidence of the 

 existence of such opaque screens in the numerous dark lanes 

 that are shown in photographs of the Milky Way. It would, 

 however, require an artificial and improbable arrangement that 

 the opaque veil should hide the central star from us, while 

 leaving it free to illuminate the nebula. 



DISCOVERY OF A COMET IN SOUTH AUSTRALIA. 

 — Mr. Dodwell, the Director of the Adelaide Observatory, has 

 communicated the discovery of a faint comet by Mr. Lowe, at 

 'Laura, South Australia, at the end of December. Owing to 

 some delay in the announcement, and the vagueness of the 

 description, the comet was not seen elsewhere, and only a 

 very rough determination of the orbit is possible. I take the 

 opportunity of pointing out that should any readers of this 

 column pick up a comet, they can make observations of 

 sufficient accuracy to be of real value by carefully drawing 

 the small stars visible in the field of the telescope, and fixing 

 the position of the comet as accurately as possible by align- 

 ment among the stars, also noting the time when the position 

 of the comet was noted. A rough clue to the position must 

 also be given, which may be obtained by looking along the 

 outside of the telescope tube, and inserting the position on a 

 star map. Tracings of the sketches should be sent to some 

 Observatory, when the region can be identified with the aid of 

 a good star map or a photograph, and a very fair position of 

 the comet deduced. Had Mr. Lowe done this, we should 

 have known the orbit of his comet, and it would have been 

 possible to recover it at other observatories. I have myself 

 tested the possibilities of the method, and find that with care 

 it is not difficult to fix the place of a comet within 30" or 40". 



OBITUARY. — The Council of the British Astronomical 

 Association has had another loss by death, that of Dr. David 

 Smart. He was an indefatigable cometary computer, and 

 gave invaluable assistance in the laborious work of carrying 

 back the motion of Halley's Comet for two thousand years, 

 and in discussing the extensive series of observations of its 

 recent return that were sent in from all parts of the world. 



* Since there are 63,360 inches in a mile we have the curious relation exactly satisfied that on a scale of 1 inch for the distance 



Earth-Sun a Light-Year is 1 mile. 



179 



