﻿COSMICAL 
  PHYSICS 
  JEANS 
  175 
  

  

  must 
  diminish 
  by 
  250,000,000 
  tons 
  every 
  minute. 
  After 
  millions 
  of 
  

   millions 
  of 
  years 
  this 
  rate 
  of 
  wastage 
  produces 
  an 
  effect 
  even 
  on 
  the 
  

   gigantic 
  mass 
  of 
  the 
  sun. 
  To 
  trace 
  the 
  changes 
  in 
  the 
  radius 
  and 
  tem- 
  

   perature 
  of 
  an 
  actual 
  star 
  we 
  must 
  study 
  the 
  sequence 
  of 
  configura- 
  

   tions 
  assumed 
  in 
  turn 
  as 
  the 
  mass 
  and 
  the 
  rate 
  of 
  generation 
  of 
  energy 
  

   change 
  together. 
  In 
  this 
  way 
  I 
  have 
  found 
  (Mon. 
  Not. 
  E,. 
  A. 
  S., 
  

   January, 
  1925) 
  that 
  a 
  normal 
  star 
  would 
  first 
  decrease 
  in 
  size 
  and 
  

   get 
  hotter, 
  but 
  would 
  ultimately 
  expand 
  and 
  get 
  cooler 
  again. 
  This 
  

   result 
  provides 
  a 
  simple 
  dynamical 
  interpretation 
  of 
  the 
  sequence 
  of 
  

   " 
  ascending 
  and 
  descending 
  temperatures 
  " 
  which 
  was 
  first 
  suggested 
  

   'by 
  Lockyer, 
  and 
  formed 
  the 
  outstanding 
  feature 
  of 
  Russell's 
  1913 
  

   theory 
  of 
  stellar 
  evolution 
  — 
  although 
  our 
  physical 
  interpretation 
  is 
  

   very 
  different 
  from 
  that 
  suggested 
  by 
  Russell. 
  

  

  THE 
  ATOMIC 
  WEIGHT 
  OF 
  STELLAR 
  MATTER 
  

  

  In 
  the 
  simplest 
  case, 
  in 
  which 
  energy 
  is 
  generated 
  uniformly 
  

   throughout 
  a 
  star's 
  mass, 
  the 
  surface 
  temperature 
  T 
  assumed 
  by 
  a 
  

   star 
  of 
  mass 
  M 
  and 
  of 
  given 
  luminosity 
  (or 
  rate 
  of 
  generation 
  of 
  

   energy) 
  is 
  given 
  by 
  the 
  equation 
  — 
  

  

  Star's 
  luminosity 
  = 
  c/^'V"' 
  V^V'-'/C 
  ^1^) 
  

  

  Here 
  6^ 
  is 
  a 
  known 
  constant, 
  N 
  and 
  A 
  are 
  the 
  atomic 
  number 
  and 
  

   atomic 
  weight 
  of 
  the 
  stellar 
  atoms, 
  and 
  ju. 
  the 
  effective 
  molecular 
  

   weight 
  (about 
  2) 
  of 
  the 
  broken-up 
  stellar 
  material; 
  T 
  is 
  the 
  tem- 
  

   perature 
  of 
  the 
  star's 
  surface 
  and 
  / 
  (J/) 
  is 
  a 
  quantity 
  I 
  have 
  calcu- 
  

   lated 
  and 
  tabulated, 
  which 
  depends 
  only 
  on 
  the 
  star's 
  mass 
  (Mon. 
  

   Not. 
  R. 
  A. 
  S., 
  85, 
  p. 
  395). 
  

  

  The 
  quantity 
  N-/A 
  necessarily 
  occurs 
  in 
  the 
  foregoing 
  formula, 
  

   because 
  the 
  coefficient 
  of 
  opacity, 
  by 
  which 
  the 
  star's 
  whole 
  structure 
  

   is 
  determined, 
  is 
  proportional 
  to 
  N-/A. 
  If 
  a 
  Maxwell 
  demon 
  could 
  

   cut 
  every 
  atomic 
  nucleus 
  in 
  a 
  piece 
  of 
  matter 
  into 
  two 
  equal 
  halves, 
  

   he 
  would 
  halve 
  both 
  N 
  and 
  A 
  and 
  so 
  also 
  N-/A^ 
  with 
  the 
  result 
  

   that 
  the 
  substance 
  would 
  become 
  twice 
  as 
  transparent 
  as 
  before. 
  

   This 
  shows 
  that 
  a 
  large 
  clot 
  of 
  matter 
  in 
  the 
  form 
  of 
  a 
  massive 
  

   nucleus 
  is 
  far 
  more 
  effective 
  in 
  absorbing 
  X 
  radiation 
  than 
  a 
  large 
  

   number 
  of 
  small 
  clots 
  of 
  equal 
  total 
  mass. 
  It 
  is 
  for 
  this 
  reason 
  that 
  

   the 
  physicist 
  and 
  surgeon 
  both 
  select 
  lead 
  as 
  the 
  material 
  with 
  which 
  

   to 
  screen 
  their 
  X-ray 
  apparatus; 
  a 
  ton 
  of 
  lead 
  is 
  far 
  more 
  effective 
  

   in 
  stopping 
  unwanted 
  X 
  rays 
  than 
  a 
  ton 
  of 
  wood 
  or 
  of 
  iron. 
  If 
  

   we 
  knew 
  the 
  strength 
  of 
  an 
  X-ray 
  apparatus, 
  and 
  the 
  total 
  weight 
  of 
  

   shielding 
  material 
  round 
  it, 
  we 
  could 
  form 
  a 
  very 
  fair 
  estimate 
  of 
  

   the 
  atomic 
  weight 
  of 
  the 
  shielding 
  material 
  by 
  measuring 
  the 
  amount 
  

   of 
  X 
  radiation 
  which 
  escaped 
  through 
  it. 
  

  

  