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ASTRONOMERS AND PHYSICISTS AT THE CONFERENCE AT YERKES OBSERVATORY, MAY 23, 1925.

Front Row. Left to right: Van der Linden, Frost, Le Maitre, Lassovsky, Van Biesbroeck, Wilson, Miss Douglass, Miss Calvert, Ross, Comstock, Moulton, Stebbins, Barrett. Back Row: Spence, Comrie, Bobrovnikoff, Crew, MacMillan, Urie, Struve, Lee, Bartky, Kunz, Fath, Fox, Hussey, Jewell.

POPULAR ASTRONOMY, No. 327.

Vol. XXXIII, No. 7

AUGUST-SEPTEMBER, 1925

Whole No. 327

SPECTRUM OF COMET c 1925 (ORKISZ).

By NICHOLAS T. BOBROVNIKOFF.

At the suggestion of Professor Frost some spectrograms of the comet were taken by the author with the ultra-violet Zeiss camera, aperture 14.5 cm, and 81.4 focal length. Eastman Speedway plates were exclusively used and the time of exposure ranged from 15 minutes to 5 hours. During the period from April 28 to May 29, 1925, twelve measurable plates were obtained, eight of them with a 30°, one with a 15° prism, and two with a combination of both prisms. Although the comet was favorably situated in the sky, being circumpolar during the whole period of observation, its brightness of between 8th and 9th stellar magnitude was too low for spectrographic observation. Plates were measured on the small Gaertner machine in both directions. As the scale of the spectra is very small, the distance between Hẞ (14861) and H0 (13798) being 3.1 mm, 6.3 mm, 9.4 mm, respectively for the plates taken with 15°, 30°, and the combination of the two, only a slight degree of accuracy can be expected from the measurements. In fact the probable error of the mean for 14693 was found to be 2.6 A and for the unit weight for the same band was 7.5 A. For the rest of the bands these figures are undoubtedly too small.

Settings were made on the centers of the knots as in many cases the edges of the knots were not well defined. The measures were reduced by a curve based on the hydrogen Balmer series in the spectrum of the comparison stars (8 Cassiopeiae or Vega). Assuming A3883 as the wave-length for the best defined cyanogen band in the violet part, the following wave-lengths were derived in the preliminary study.

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The wave-length of the band A5036 was determined by an extrapolation which introduced an additional uncertainty.

Strong

The cut (Plate XXIII) represents the spectrum of the comet taken on May 17-18 with four hours exposure, from 22h 25m to 2h 25m C. S. T. It shows a row of knots which are monochromatic images of the comet's head. Each of the three strongest knots is sharper on the side of the longer wave-lengths and shades off toward the violet. The knot 13883 shows an extension toward the tail about 3' which gives evidence of the presence of cyanogen not only in the nucleus but also in the tail. An extension into the tail may also be seen at the band 14693.

Above and below the comet's spectrum was placed the spectrum of the comparison star & Cassiopeiae, class A5. Ten hydrogen lines and the calcium line K can be seen in it on the original plate. The star was brought on the thread of the guiding telescope after shifting the whole field in RA. Any unavoidable error in setting the star on the thread will affect the absolute wave-lengths by its full amount.

Nearly all the light of the comet was concentrated in the bands 13883 (Cyanogen) and 14693 (Carbon Monoxide). Next following in brightness was the carbon band at A5036, representing thus the ordinary distribution of light in comets. During the whole time of observation a pronounced continuous spectrum was present, even on plates taken with the combination of the two prisms. This reveals a considerable amount of reflected sunlight in the comet as compared with the emission of the comet itself. No striking changes in the comet's spectrum were noted during the time covered by these photographs.

Yerkes Observatory, June, 1925.

NOTE ON THE EXTENT OF THE STELLAR SYSTEM.

By J. G. PORTER.

I find it stated in an article by Professor Shapley, quoted in the Literary Digest, that up to about ten years ago estimates of the greatest diameter of the Milky Way varied from a thousand to thirty thousand light years. During all the forty years that I have been giving instruction in astronomy in the University of Cincinnati I have taught my students that the probable diameter of our stellar system is of the order of a million light years. This is easily arrived at in the following simple way. Assuming, according to Dr. Elkin's investigations, that the average distance of first magnitude stars is 36 light years, we get from the light ratio of the different magnitudes an average distance of 360 light years for sixth magnitude stars, 3,600 light years for eleventh magnitude stars, 36,000 light years for sixteenth magnitude stars, and 144,000 light years for eighteenth magnitude stars. Here at once we arrived at a diameter of nearly 300,000 light years for the Galaxy. But many of the stars of each magnitude are much more.

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