POPULAR SCIENCE 617 



new stars, we find a general common likeness, consisting in a 

 sudden great increase in light and then a gradual waning, the 

 latter phase accompanied in some cases, notably in that of 

 Nova Persei (1901), and to a less extent in that of Nova Aquilse 

 (191 8), by a series of periodic fluctuations. These fluctuations 

 of luminosity in the case of Nova Persei (1901) were accom- 

 panied by an alternating spectrum. As Father Sidgreaves 

 showed, from the Stonyhurst spectrograms of the star, when 

 the apparent magnitude was above 4*5, the spectrum was the 

 bright radiation spectrum characteristic of the second stage in 

 the progressive spectra of a new star. But when the luminosity 

 fell below 4*5, then the nebular spectrum emerged as strength- 

 ened. Father Sidgreaves adopted an ingenious explanation to 

 account for these fluctuations in brilliancy and concomitant 

 changes in spectrum, first suggested by Sir William Huggins 

 in 1866, and which is possibly correct. " The sudden expansion 

 of a vast mass of mixed gases would naturally become oscillatory 

 for a length of time, the temperature oscillating with it ; and 

 when the mean temperature of the oscillations was near the 

 combining temperature of some of the mixed gases, the spec- 

 trum would also alternate with the temperature." ^ This sup- 

 poses, presumably, that the expansion results in the formation 

 of a planetary nebula. Some observers are inclined to believe 

 that the nebula exists independently of the star, and is not 

 formed from, but illuminated by, its outburst. But the diffi- 

 culty remains that the appearance of the nebular radiation, 

 near the sixth radiation of h3^drogen (H^), was concomitant 

 with each waning of the star's light below a critical magnitude. 

 This would indicate a true expansion of the materials forming 

 the light-giving envelope of the star. 



In its initial stages a new star is visually of a brilliant white 

 or bluish-white colour, which changes into an orange or reddish 

 tint, and finally into a greenish hue. These colours are con- 

 nected with the progressive changes which occur in the spectra, 

 as the nova waxes and wanes in luminosity. 



Several observers of repute have stated that the spectrum 

 of a new star, when nearing its maximum brilliancy, is con- 

 tinuous, without any bright radiations or dark absorptions. 

 Such, for instance, was the earliest spectrum reported in the 

 case of Nova Persei (1901) and Nova Aquilae (191 8). The next 

 stage in the spectral changes is the appearance of a series of 

 dark absorptions on a strong continuous background, repre- 

 sentative of hydrogen, iron, calcium, titanium, chromium, 

 silicon, and other metallic vapours, and in the state, and pre- 

 sumably at the corresponding temperature, at which they are 



» Monthly Notices, R.A.S., Ixii. 148. 



