﻿172 
  ANNUAL 
  EEPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  broken-up 
  atoms 
  and 
  electrons 
  at 
  the 
  sun's 
  center. 
  We 
  could 
  allow 
  

   for 
  its 
  dynamical 
  effects 
  by 
  decreasing 
  our 
  assumed 
  mean 
  molecular 
  

   weight 
  by 
  5 
  per 
  cent; 
  but 
  this 
  mean 
  molecular 
  weight 
  is 
  not 
  in 
  

   any 
  case 
  known 
  to 
  within 
  5 
  per 
  cent. 
  In 
  exceptionally 
  massive 
  

   stars, 
  the 
  pressure 
  of 
  radiation 
  assumes 
  somewhat 
  greater 
  impor- 
  

   tance. 
  For 
  example, 
  at 
  the 
  center 
  of 
  a 
  star 
  of 
  some 
  ten 
  times 
  

   the 
  sun's 
  mass, 
  radiation 
  pressure 
  is 
  about 
  equal 
  to 
  gas 
  pres- 
  

   sure. 
  To 
  allow 
  for 
  its 
  effects 
  in 
  this 
  case 
  we 
  should 
  have 
  to 
  sup- 
  

   pose 
  the 
  assumed 
  mean 
  molecular 
  weight 
  halved 
  — 
  reduced 
  perhaps 
  

   from 
  2 
  to 
  1. 
  In 
  every 
  case 
  we 
  shall 
  get 
  a 
  true 
  picture 
  of 
  stellar 
  

   structure 
  if 
  we 
  think 
  of 
  the 
  layers 
  of 
  stellar 
  matter 
  as 
  held 
  up 
  

   against 
  gravitation 
  by 
  the 
  incessant 
  impact 
  of 
  a 
  certain 
  number 
  of 
  

   atomic 
  nuclei 
  or 
  partially 
  stripped 
  atoms, 
  the 
  "molecular 
  weight" 
  

   of 
  which 
  is 
  practically 
  the 
  same 
  as 
  that 
  of 
  the 
  corresponding 
  com- 
  

   plete 
  atoms, 
  together 
  with 
  a 
  far 
  greater 
  number 
  of 
  free 
  electrons 
  of 
  

   standard 
  " 
  molecular 
  weight 
  " 
  0,00055, 
  and 
  a 
  rather 
  small 
  number 
  

   of 
  " 
  molecules 
  " 
  of 
  radiation 
  the 
  molecular 
  weight 
  of 
  which 
  is 
  

   negligibly 
  small. 
  The 
  combined 
  impacts 
  of 
  these 
  three 
  types 
  of 
  pro- 
  

   jectiles 
  prevent 
  the 
  star 
  from 
  falling 
  in 
  under 
  it 
  own 
  gravitational 
  

   attraction. 
  

  

  This 
  gives 
  us, 
  I 
  think, 
  the 
  best 
  snapshot 
  picture 
  of 
  a 
  star's 
  struc- 
  

   ture. 
  The 
  corresponding 
  picture 
  of 
  its 
  mechanism 
  is 
  obtained 
  by 
  

   thinking 
  of 
  the 
  nuclei 
  as 
  a 
  ray 
  particles, 
  of 
  the 
  free 
  electrons 
  as 
  

   /8 
  ray 
  particles, 
  and 
  of 
  the 
  radiation 
  as 
  y 
  rays 
  (although 
  in 
  most 
  

   stars 
  the 
  main 
  bulk 
  of 
  the 
  radiation 
  has 
  the 
  wave 
  length 
  of 
  X 
  rays) 
  ; 
  

   and, 
  precisely 
  as 
  in 
  laboratory 
  work, 
  the 
  ^ 
  rays 
  are 
  more 
  penetrating 
  

   than 
  the 
  a 
  rays, 
  and 
  the 
  y 
  rays 
  are 
  more 
  penetrating 
  than 
  eitlier. 
  

  

  THE 
  TRANSPORT 
  OF 
  ENERGY 
  INSIDE 
  A 
  STAR 
  

  

  In 
  ordinary 
  kinetic 
  theory 
  of 
  gases, 
  conduction 
  of 
  heat 
  is 
  studied 
  

   by 
  regarding 
  the 
  molecules 
  of 
  the 
  gas 
  as 
  carriers 
  of 
  energy; 
  each 
  

   molecule 
  has 
  a 
  carrying 
  power 
  which 
  is 
  jointly 
  proportional 
  to 
  its 
  

   heat 
  energy, 
  its 
  velocity 
  and 
  its 
  free 
  path. 
  In 
  the 
  interior 
  of 
  a 
  star 
  

   there 
  are, 
  as 
  we 
  have 
  seen, 
  three 
  distinct 
  types 
  of 
  carriers 
  — 
  the 
  

   nuclei 
  (or 
  atoms), 
  the 
  free 
  electrons, 
  and 
  the 
  radiation. 
  We 
  can 
  

   compare 
  the 
  relative 
  carrying 
  capacities 
  of 
  these 
  three 
  types 
  of 
  

   carriers 
  by 
  multiplying 
  up 
  the 
  energy, 
  velocity, 
  and 
  free 
  path 
  of 
  each. 
  

  

  The 
  nuclei 
  and 
  the 
  free 
  electrons 
  have, 
  of 
  course, 
  quite 
  definite 
  

   free 
  paths. 
  The 
  same 
  is 
  true 
  of 
  the 
  radiation 
  if 
  this 
  is 
  regarded 
  

   as 
  consisting 
  of 
  discrete 
  quanta; 
  when 
  a 
  quantum 
  is 
  emitted 
  a 
  free 
  

   path 
  begins, 
  and 
  when 
  it 
  is 
  reabsorbed 
  the 
  free 
  path 
  ends. 
  Whether 
  

   we 
  think 
  in 
  terms 
  of 
  undulatory 
  theory 
  or 
  quanta, 
  we 
  may 
  suppose 
  

   that 
  a 
  beam 
  of 
  radiation 
  is 
  reduced 
  in 
  intensity 
  by 
  a 
  factor 
  e~'"'^ 
  

   on 
  passing 
  through 
  a 
  thickness 
  x 
  of 
  matter 
  of 
  density 
  p, 
  where 
  k 
  is 
  

  

  