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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  degrees, 
  while 
  there 
  are 
  intermediate 
  limits 
  for 
  stars 
  of 
  intermediate 
  

   mass. 
  

  

  This 
  is 
  obviously 
  one 
  of 
  the 
  fundamental 
  facts 
  of 
  physical 
  astron- 
  

   omy 
  : 
  What 
  does 
  it 
  mean 
  ? 
  The 
  normal 
  event 
  for 
  a 
  star 
  like 
  V 
  Pup- 
  

   pis, 
  losing 
  mass 
  and 
  capacity 
  for 
  generation 
  of 
  energy 
  together, 
  

   would 
  be 
  a 
  gradual 
  shrinkage 
  of 
  size 
  accompanied 
  by 
  a 
  steady 
  in- 
  

   crease 
  of 
  central 
  temperature. 
  What 
  is 
  it 
  that 
  checks 
  this 
  normal 
  

   course 
  of 
  evolution 
  so 
  soon 
  as 
  the 
  central 
  temperature 
  touches 
  

   300,000,000 
  degrees? 
  

  

  I 
  have 
  recently 
  suggested 
  that 
  the 
  upper 
  limit 
  of 
  temperature 
  for 
  

   any 
  star 
  is 
  simply 
  that 
  at 
  which 
  its 
  central 
  atoms 
  begin 
  to 
  be 
  stripped 
  

   nearly 
  or 
  entirely 
  bare 
  of 
  electrons. 
  This 
  is 
  merely 
  a 
  matter 
  of 
  sim- 
  

   ple 
  calculation, 
  but 
  we 
  have 
  to 
  suppose 
  the 
  atoms 
  at 
  the 
  star's 
  center 
  

   to 
  have 
  the 
  high 
  atomic 
  weights 
  which 
  other 
  considerations, 
  as 
  we 
  

   have 
  seen, 
  assign 
  to 
  them. 
  For 
  example, 
  a 
  temperature 
  of 
  300,000,000 
  

   degrees 
  suffices 
  to 
  strip 
  the 
  last 
  electrons 
  off 
  atoms 
  of 
  atomic 
  Aveight 
  

   300 
  or 
  more. 
  The 
  fall 
  in 
  the 
  critical 
  central 
  temperature 
  as 
  a 
  star 
  

   gets 
  older 
  and 
  less 
  massive 
  is, 
  on 
  this 
  view, 
  a 
  direct 
  consequence 
  of 
  

   the 
  decrease 
  in 
  the 
  atomic 
  weight 
  of 
  the 
  stellar 
  material, 
  which 
  

   occurs 
  as 
  the 
  heaviest 
  atoms 
  gradually 
  annihilate 
  themselves. 
  

  

  It 
  remains 
  to 
  explain 
  why 
  this 
  temperature 
  constitutes 
  an 
  upper 
  

   limit, 
  why 
  a 
  star 
  can 
  not 
  go 
  on 
  getting 
  hotter 
  after 
  its 
  innermost 
  

   atoms 
  are 
  stripped 
  bare 
  of 
  electrons. 
  So 
  far 
  as 
  I 
  can 
  see, 
  only 
  one 
  

   answer 
  is 
  possible 
  : 
  The 
  stripping 
  of 
  the 
  electrons 
  from 
  an 
  atom 
  must 
  

   remove 
  its 
  power 
  of 
  annihilating 
  itself, 
  and 
  so 
  must 
  inhibit 
  its 
  ca- 
  

   pacity 
  for 
  generating 
  radiation. 
  The 
  central 
  atoms 
  of 
  the 
  star 
  now 
  

   act 
  precisely 
  like 
  the 
  governor 
  of 
  a 
  steam 
  engine, 
  regulating 
  the 
  

   generation 
  of 
  energy 
  so 
  that 
  the 
  central 
  temperature 
  is 
  kept 
  close 
  to 
  

   the 
  critical 
  temperature. 
  If 
  the 
  star 
  begins 
  to 
  get 
  too 
  hot, 
  the 
  cen- 
  

   tral 
  atoms 
  become 
  stripped 
  bare 
  of 
  electrons, 
  and 
  so 
  leave 
  off 
  generat- 
  

   ing 
  energy. 
  The 
  star 
  then 
  begins 
  to 
  cool 
  off, 
  and 
  as 
  it 
  does 
  so 
  the 
  

   atoms 
  reform 
  and 
  resume 
  their 
  generation 
  of 
  energy, 
  again 
  heating 
  

   up 
  the 
  star. 
  The 
  mechanism 
  provides 
  a 
  perfect 
  thermostat, 
  and 
  it 
  is 
  

   easily 
  shown 
  that 
  its 
  action 
  is 
  stable. 
  As 
  a 
  star 
  ages 
  the 
  heavier 
  

   atoms 
  at 
  its 
  center 
  are 
  the 
  first 
  to 
  be 
  transformed 
  into 
  radiation 
  and 
  

   so 
  to 
  disappear 
  ; 
  their 
  place 
  is 
  taken 
  by 
  lighter 
  atoms, 
  and 
  as 
  a 
  lower 
  

   temperature 
  suffices 
  to 
  strip 
  these 
  lighter 
  atoms 
  bare 
  of 
  electrons, 
  

   the 
  critical 
  central 
  temperature 
  of 
  the 
  star 
  falls. 
  

  

  An 
  interesting 
  confimation 
  of 
  this 
  hypothesis 
  is 
  provided 
  by 
  the 
  

   components 
  of 
  newly 
  formed 
  binaries. 
  These 
  have 
  the 
  high 
  energy- 
  

   generating 
  capacity 
  per 
  unit 
  mass 
  of 
  young 
  stars, 
  associated 
  with 
  the 
  

   small 
  mass 
  appropriate 
  to 
  much 
  older 
  stars. 
  Clearly 
  the 
  "gov- 
  

   ernor 
  " 
  action 
  ought 
  to 
  be 
  particularly 
  active 
  in 
  checking 
  the 
  genera- 
  

   tion 
  of 
  energy 
  in 
  these 
  stars, 
  so 
  that 
  they 
  ought 
  to 
  have 
  central 
  

  

  