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  ANNUAL 
  REPOKT 
  SMITHSONIAN 
  INSTITUTION, 
  1921 
  

  

  THE 
  INTERPRETATION 
  OF 
  STELLAR 
  SPECTRA 
  

  

  There 
  is 
  only 
  one 
  method 
  available 
  to 
  this 
  end, 
  namely, 
  a 
  study 
  

   of 
  the 
  radiation 
  emitted 
  by 
  different 
  stars. 
  Apart 
  from 
  unscientific 
  

   speculations, 
  physical 
  astronomy 
  may 
  be 
  said 
  to 
  have 
  come 
  to 
  birth 
  

   in 
  1863, 
  when 
  Huggins 
  attached 
  a 
  spectroscope 
  to 
  a 
  telescope 
  and 
  

   found 
  that 
  certain 
  lines 
  in 
  the 
  spectra 
  of 
  the 
  stars 
  were 
  identical 
  

   with 
  lines 
  emitted 
  in 
  the 
  laboratory 
  by 
  the 
  known 
  chemical 
  elements. 
  

   The 
  early 
  stellar 
  spectroscopists 
  believed 
  that 
  they 
  were 
  investigating 
  

   " 
  the 
  chemistry 
  of 
  the 
  stars," 
  although 
  we 
  know 
  now 
  that 
  they 
  were 
  

   merely 
  opening 
  up 
  a 
  fundamental 
  problem 
  of 
  the 
  physics 
  of 
  the 
  stars. 
  

   The 
  spectrum 
  of 
  Sirius, 
  for 
  example, 
  was 
  found 
  to 
  exhibit 
  hydrogen 
  

   lines 
  very 
  stronglj^ 
  and 
  calcium 
  lines 
  very 
  weakly; 
  in 
  the 
  solar 
  

   spectrum 
  the 
  relative 
  strength 
  of 
  these 
  two 
  sets 
  of 
  lines 
  was 
  reversed, 
  

   calcium 
  being 
  strong 
  and 
  hydrogen 
  weak. 
  They 
  concluded 
  that 
  

   hydrogen 
  was 
  specially 
  prominent 
  in 
  the 
  constitution 
  of 
  Sirius 
  and 
  

   calcium 
  in 
  that 
  of 
  the 
  sun. 
  Believing 
  that 
  Sirius 
  must 
  one 
  day 
  

   develop 
  into 
  a 
  star 
  similar 
  to 
  our 
  sun, 
  they 
  conjectured 
  that 
  its 
  sub- 
  

   stance 
  must 
  graduall}^ 
  cliange 
  from 
  hydrogen 
  into 
  calcium 
  and 
  other 
  

   more 
  complex 
  elements, 
  thus 
  finding 
  support 
  for 
  the 
  long-established 
  

   hjq^othesis 
  that 
  the 
  more 
  complex 
  elements 
  were 
  formed 
  by 
  gradual 
  

   evolution 
  out 
  of 
  the 
  simplest. 
  

  

  The 
  true 
  interpretation 
  of 
  these 
  early 
  observations, 
  as 
  the 
  investi- 
  

   gations 
  of 
  Saha, 
  K. 
  H. 
  Fowler, 
  and 
  Milne 
  have 
  abundantly 
  proved, 
  

   is 
  merely 
  that 
  the 
  surface 
  of 
  Sirius 
  is 
  at 
  a 
  temperature 
  at 
  which 
  

   hydrogen 
  is 
  specially 
  active 
  in 
  emitting 
  and 
  absorbing 
  radiation, 
  

   while 
  the 
  sim's 
  surface 
  is 
  at 
  a 
  lower 
  temperature 
  at 
  which 
  hydro- 
  

   gen 
  is 
  comparatively 
  inert; 
  calcium, 
  iron, 
  etc., 
  having 
  become 
  active 
  

   in 
  its 
  place. 
  Just 
  as 
  the 
  laboratory 
  physicist 
  can 
  produce 
  dif- 
  

   ferent 
  spectra 
  from 
  the 
  same 
  vacuum 
  tube 
  by 
  varying 
  the 
  mode 
  and 
  

   conditions 
  of 
  excitation, 
  so 
  nature 
  produces 
  different 
  spectra 
  from 
  

   the 
  same 
  stellar 
  material 
  by 
  varying 
  its 
  temperature. 
  

  

  Clearly 
  this 
  circumstance 
  robs 
  stellar 
  spectra 
  of 
  all 
  direct 
  evolu- 
  

   tionary 
  significance. 
  The 
  spectra 
  of 
  the 
  stars 
  merely 
  tell 
  us 
  their 
  

   present 
  surface 
  temperatures, 
  so 
  that 
  even 
  if 
  we 
  could 
  arrange 
  the 
  

   stars 
  in 
  order 
  of 
  age, 
  a 
  comparison 
  of 
  their 
  spectra 
  would 
  only 
  show 
  

   whether 
  their 
  surfaces 
  were 
  becoming 
  hotter 
  or 
  cooler 
  ; 
  it 
  would 
  give 
  

   no 
  information 
  as 
  to 
  chemical 
  changes 
  occurring 
  in 
  their 
  substance. 
  

  

  THE 
  SIZES 
  OF 
  THE 
  STARS 
  

  

  The 
  Imowledge 
  of 
  a 
  star's 
  surface 
  temperature 
  nevertheless 
  opens 
  

   the 
  door 
  to 
  further 
  valuable 
  knowledge. 
  The 
  hotter 
  a 
  surface 
  is, 
  the 
  

   more 
  energetically 
  it 
  radiates 
  heat, 
  and 
  from 
  a 
  laiowledge 
  of 
  a 
  star's 
  

   surface 
  temperature 
  it 
  is 
  easy 
  to 
  calculate 
  its 
  radiation 
  per 
  square 
  

  

  