TEANSACTIONS OF SECTION A. 147 



3. The Progressive Spectra oj Nova Aquila. 1918-i'J. 

 By the Rev. A. L. Cohtie, S.J., F.R.A.Sr 



The present paper gives a general account of the changes in the spectrum 

 of Nova Aquilic, between the dates June 10 and October 23, 1918, and in 

 July, August 1919. iMeasurements and comparisons of the earlier plates lend 

 little, if any, countenance to the view that a spectrum of the G, or solar type, 

 was present in the ultra-violet regions of the spectrum. The broad dark bands 

 observed in this part of the spectrum were mainly due to the doubling of 

 the dark hydrogen series, and the superposition with decreasing wave-length 

 of members of the second series over those of the first, on account of then- 

 relative displacements. The blends so formed probably contained also enliauced 

 lines of titanium, vanadium, and calcium, found in the spectrmn of a Cygni. 



There is more evidence for a spectrum of the Procyon, or F type, in the 

 ultra-violet spectrum of the Nova in its earlier stages. The K line of calcium, 

 and the H^ line of liydrogen, with which possibly is blended a magnesium 

 hue, bear a greater likeness to Procyon than to a Cygni. But, with these 

 exceptions, tue dark line spectiuiiis of the Nova, even in the ultra-violet, 

 exactly matclies that of a Cygni, the great majority of the lines in the 

 spectrum of which are characteristic of the solar chromospheric spectrum. 



Although the increase in brilliancy of the star in its earlier life-stages was 

 rapid, 2-41 magnitudes in 2i hours, yet it was not much greater than that 

 of some variable stars; e.g. R. Ursee Maj., 1-87 mag., and S.S. Cygni 

 1-97 mag. per 24 hours. 



The whole of the hydrogen series of dark lines was represented in the 

 spectrum, which extended very far into the ultra-violet. These dark hydroo-en 

 lines were doubled, and greatly displaced. The displacements were prop°or- 

 tionaJl to wave-length. These dark lines were accompanied on their more 

 refrangible sides by broad bright bands, about 50 A.U. in average width The 

 bright red band of liydrogen was by far the brightest in the early stages of 

 the spectrmn. A noimal spectrum of a Cygni fits the centres of these bright 

 bands. This entails that the dark nydrogen lines were displayed with a velocity 

 of approach of the order of 1700 km/ sec for the first set, and 2400 km/ sec 

 for the second set, twice and thrice respectively the velocity of the quickest 

 moving solar prominences. The dark line a Cvgni spectrum was displaced 

 concomitantly with the first set of hydrogen lines. 



The regularly decreasing widths of the hydrogen bright bands with 

 decreasing wave-length indicates also velocity in the line of sight. These brio-ht 

 bands were very complicated in structure in the earlier spectra, but by June" 15 

 showed a definite triple character, which was maintained until they began 

 to fade away at the end of August 1918. That is, for about five months they 

 maintained their relative widths and displacements, of the order of 1300 km/ 

 sec approach and 1200 km/sec recession. The spectium represented by the 

 middle members of the trip-le bright bands remained stationary. The lono- 

 continuance of the widths and great velocities of these bright bands present 

 a great difhculty for any explanation based on a motion in the line of sight 

 Is it possibly a Zeeman magnetic effect? 



By June 15 the spectrum was almost entirely a bright band spectrum of 

 the a Cygni or A type. The bright bands were in many cases doubled, notably 

 in the band X 4640, which is a nebula band appearing thus early in the 

 spectrum. This band grew in intensity until the end of August 



By the end of July 1918 the bright band a Cygni spectrum had become 

 very taint, which phase was accompanied by the brightening of a bright band 

 7 Ononis, or B type spectrum, chiefly helium lines. The brighter nebula 

 imes were also vei-y prominent. 



After August 23 the predominant type of spectrum was that of a pinnetnry 

 nebula. The chief nebula and 0-type star lines were lelatively much the 

 stronger. But the y Ononis lines remained untH at least October 23- helium 

 oxygen, nitrogen. Some o Cygni lines also still faintly subsisted ; e.g. titanium'. 



= See Ohservatonj, Oct. 1^19, Vol. 42. No. 544, pp. Pm. ZCT. 



