STELLAR SPECTROSCOPY FOR BEGINNERS. 



By PROFESSOR A. W. BICKERTON, A.R.S.M. 



I. INTRODUCTORY HISTORIC SKETCH. 



The arming of the telescope with the prism and 

 photographic plate has produced another bound 

 forward in astronomical discoveries as great as that 

 made three hundred years ago, when Galileo armed 

 the eye with his wonderful optic tube. The tele- 

 scope extended our knowledge into such abysses of 

 space that the millions of miles to the distant Sun 

 became almost too short a unit with which to 

 measure them. It showed us that the filmy light of 

 the Galaxy was a seed-bed of millions of suns, as 

 Democritus so clearly described it two thousand 

 years before. Some of these suns proved to be so 

 vast as altogether to dwarf our majestic orb. The 

 telescope showed us also that the distant inhabitants 

 of space possess as great a diversity of structure as 

 immensity of mass. 



The cipher messages unfolded by the prism were 

 to reveal the most unexpected truths : these 

 rainbow-tinted strips of light we call spectra, told us, 

 not merely the structure, but also the chemical 

 composition of the many members of those distant 

 regions. 



The year 1642, when Galileo died, was that 

 in which Newton was born. Amid his many 

 wondrous discoveries of appliance and of principle 

 it had been the especial privilege of Galileo to apply 

 the telescope to the reaping of a rich and varied 

 celestial harvest. Newton continued to garner this 

 wonderful harvest, but he also sowed the seed of 

 another crop of results scarcely to be begun to be 

 reaped before two and a half centuries had elapsed. 

 This fertile experiment of Newton's was to place a 

 prism of glass in a beam of sunlight that streamed 

 through a hole in the shutters of a window ; thus he 

 analysed and decomposed the light into its constitu- 

 ents. It was a wonderful but crude experiment, for 

 the lovely strip of light he so produced was what we 

 call an impure spectrum. Newton never much 

 improved on his beautiful basic experiment ; it did 

 not occur to him to use a narrow slit instead of a 

 round hole, to prevent his colours from overlapping, 

 and so render his tints pure. It was reserved for 

 Wollaston and Fraunhofer to do that, and so 

 discover that the spectrum of the Sun (as Newton 

 called the coloured strip of light) was broken up by 

 black bands. Wollaston saw these bands, but 

 thought too little of them to take much trouble to 

 perfect his experiments. Not so Fraunhofer : he 

 carefully plotted them, and named them with the 

 letters of the alphabet to distinguish them, and so 



we still call these revealing ciphers Fraunhofer lines' 

 By-and-by it was found that the light of burning 

 metals gave coloured lines instead of a rainbow- 

 tinted strip, and soon, in the hands of Bunsen and 

 Kirchhoff, this furnished a means of detecting the 

 chemical elements : an instrument was constructed 

 to do this which was called the spectroscope, and 

 spectrum analysis was born. 



Spectrum Analysis. 



It had long been known that the colour of these 

 flames was characteristic of many elements, and 

 these varied colours were used to give beauty to 

 fireworks and as aids in chemical analysis. If we 

 examine a set piece in a pyrotechnic display with a 

 spectroscope, the instrument spreads each colour out 

 into a series of separate lines of light. The yellow 

 in a firework display is caused by sodium, and this 

 element gives a brilliant yellow band. If an 

 exceedingly narrow slit be used in a good spectro- 

 scope, the band separates into two parts, and if the 

 light be passed through several prisms, the two 

 constituents may be obtained at quite a distance 

 from one another. Kirchhoff noticed that the exact 

 position of these two bright yellow sodium bands 

 was occupied by two black bands in the spectrum 

 of sunlight. He afterwards discovered that the 

 characteristic bright line of other glowing metals 

 corresponded with other sets of black lines in the 

 solar spectrum. Further experiments showed that 

 the bright bands given by a soda flame became 

 reversed when the electric light was allowed to shine 

 through it. The continuous spectrum produced 

 by the electric arc was crossed with black lines 

 when it shone through the yellow flame. The whole 

 mystery of the Fraunhofer lines was cleared up, the 

 sun was a blazing ball of fire shining through an 

 atmosphere of flaming metals ; hence we had the 

 basic discovery that in the radiant beams coming 

 from the Sun we were able to read its composition 

 and character (see Figure 422). With added refine- 

 ments, the same method was applied to the suns 

 that shine at night; the songs of the singing 

 atoms were read, and told us the story of the 

 stars. A great principle had been revealed to 

 man : the same elements that have been used 

 to build this Earth go to make up the structure 

 of the heavens. Gradually, as revelation after 

 revelation has come to us, we realise that " all are 

 but parts of one stupendous whole," that Nature is a 



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