FE 168 7 
XV. 
ON THE TEMPERATURE OF CERTAIN STARS. 
By W. E. WILSON, D.Sc., F.R.S. 
(Pirate XI.) 
(Read, January 17; Published, Marcu 24, 1905. | 
Numerous attempts have been made to classify the stars on a scale of tempera- 
ture by a study of their spectra. Sir Norman Lockyer has proposed a rising 
and falling scale in which the highest temperature is represented by stars like 
Sirius, which give a brilliant, continuous spectra, extending well into the ultra- 
violet, and only crossed by a few dark lines, principally of hydrogen. Next in the 
order of lower temperature come stars like the Sun and Arcturus, whose spectra 
are darkened by numerous fine metallic lines; and then, lower still, are a class of 
reddish stars called IV-type, and also a very similar class called Wolf-Rayet, which 
‘generally give a spectrum in which the lines of titanium are very dark, as also 
the bands of carbon. It has been lately pointed out that these [V-type stars give 
a spectrum almost quite similar to that of a sun-spot. Lockyer has always main- 
tained that a sun-spot is caused by the falling down of cool material on the Sun’s 
photosphere, thus causing it tolook dark. As these [V-type stars give a spectrum 
almost identically the same as asun-spot, he naturally assumes that they also must 
be at a lower temperature than the Sun. The principal reason given for the 
coolness of a sun-spot is that in its spectrum some of the Fraunhofer’s lines are both 
widened and darkened. I have lately taken a photograph of the spectrum of what 
I call an ‘artificial sun-spot”; and in this the dark lines are widened in exactly 
the same manner as in a real sun-spot, and this with absolutely no difference in 
the temperature of the absorbing gas, but merely by reducing the brilliancy of the 
source which gave the continuous spectrum (see Plate XI). The experiment was 
made in this way: an opal globe surrounding an electric arc represented the Sun’s 
photosphere ; on the globe was fixed a little piece of tissue-paper which stopped, 
say, 50 per cent. of the light ; this was my sun-spot. Beyond this was a glass cell 
containing nitric oxide fumes; this represented the Sun’s chromosphere, and gave 
me the dark-line spectrum. A lens was then arranged to form an image of the 
TRANS. ROY. DUB. SOC., N.S., VOL. VIII., PART XV. 2 Ki 
