. Section A . 



PRESIDENTIAL ADDRESS. 



VARIABLE STAR RESEARCH. 



By Alexander W. Robeihs, D.8c., F.RA.S., F.R.S.E. 



It is custoinary for your presitlent to open the work of the session 

 by an address on some subject pertinent to the eonsiileration of the 

 members of the section. For certain very evident reasons the several 

 presidents have ahnost always taken as the subject-matter of their dis- 

 course a topic in which their ignorance was on the whole less thaii that 

 of the bulk of their audience. I do not intend to depart in the present 

 instance from this excellent tradition, and so I have taken as the sub- 

 ject of my address that interesting branch of astronomical research 

 which deals with the changes certain stars undergo in brightness. 



Apart from the fact that it is a study in which I have been 

 personally and constantly interested for nearly twenty years, the 

 subject of variable stars, as these bodies are called wh<ise brightness 

 fluctuates through a clearly discernible range, has sprung into the very 

 forefront of astronomical science during the past decade, and conse- 

 quently commands our attention. 



Variable stars are no longer looked upon as a side field in which 

 the amiable amateur may browse about at his leisure without much 

 harm to himself or good to astronomy. They were certainly considered 

 a very subordinate branch of astronomical research forty or fifty years 

 ago : so much so that any astronomer of the first rank who looked into 

 the subject of variable stars did so with the same apologetic air as he 

 would exhibit were he found examining goods in a second-hand 

 clothes shop. 



To-day the study of variable stars, and of all the allied problems 

 connected with them, claims the attention of all sections of astronomers — 

 of the mathematician, of the physicist, of the chemist and of the prac- 

 tical observer. A present day investigation of the light changes of 

 certain stars calls to its aid, as auxiliary forces, the most refined 

 n)athematical analysis, the most recent discoveries in the domain of 

 astro-physics, and the most recondite advances in stellar chemistry. 



How has this change come about in the position of the study 1 

 Why lias the study of variable stars become such an important section 

 of astronomical research ? Mainly because it has been discovered that 

 the variation of certain stars have an important relation to their 

 physical conditions — past, present and future ; or, to put it otherwise, 

 when we investigate, define and interpret the light changes of certain 

 stars, we are straightway dealing with theii' shape, density, movements 

 and history. We are indeed at the threshold of the problem how stars 

 are born, grow old, decay and die. 



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