﻿168 
  ANNUAL 
  REPORT 
  SMITHSONIAN" 
  INSTITUTION, 
  1929 
  

  

  of 
  millions 
  of 
  years. 
  In 
  each 
  case 
  there 
  is 
  an 
  exception 
  to 
  " 
  prove 
  the 
  

   rule". 
  In 
  the 
  former 
  case, 
  it 
  is 
  provided 
  by 
  the 
  spectroscopic 
  binaries 
  

   which 
  are 
  so 
  compact 
  that 
  their 
  constituents 
  can 
  defy 
  the 
  pulling- 
  

   apart 
  action 
  of 
  gravitation; 
  in 
  the 
  latter 
  case 
  it 
  is 
  provided 
  by 
  the 
  

   5- 
  type 
  stars 
  which 
  are 
  so 
  massive, 
  possibly 
  also 
  so 
  young, 
  that 
  the 
  

   gravitational 
  forces 
  from 
  lesser 
  stars 
  have 
  not 
  greatly 
  affected 
  their 
  

   motion. 
  

  

  This 
  and 
  other 
  lines 
  of 
  evidence, 
  when 
  discussed 
  in 
  detail, 
  agree 
  in 
  

   suggesting 
  that 
  the 
  general 
  age 
  of 
  the 
  stars 
  is 
  probably 
  between 
  five 
  

   and 
  ten 
  million 
  million 
  years. 
  It 
  may 
  even 
  be 
  possible 
  to 
  fix 
  the 
  age 
  

   of 
  the 
  sun 
  within 
  the 
  narrower 
  limits 
  of 
  seven 
  and 
  eight 
  million 
  

   million 
  years. 
  

  

  THE 
  ORIGIN 
  OF 
  SOLAR 
  URANIUM 
  

  

  We 
  now 
  have 
  all 
  the 
  data 
  for 
  discussing 
  the 
  origin 
  of 
  the 
  radioactive 
  

   atoms 
  in 
  the 
  sun 
  and 
  stars. 
  Thorium, 
  the 
  longest-lived 
  of 
  all 
  radio- 
  

   active 
  substances, 
  is 
  reduced 
  to 
  half 
  its 
  original 
  amount 
  after 
  15,000,- 
  

   000,000 
  years 
  of 
  spontaneous 
  disintegration. 
  A 
  mass 
  of 
  pure 
  thorium 
  

   equal 
  to 
  the 
  sun 
  (2 
  X 
  10^^ 
  gm.) 
  would 
  be 
  reduced 
  to 
  a 
  single 
  atom 
  within 
  

   three 
  million 
  million 
  years. 
  For 
  uranium, 
  with 
  a 
  half-value 
  period 
  of 
  

   5,000,000,000 
  years, 
  the 
  corresponding 
  time 
  is 
  less 
  than 
  a 
  million 
  

   million 
  years. 
  When 
  the 
  earth 
  was 
  born 
  the 
  sun's 
  age 
  was 
  greater 
  

   than 
  either 
  of 
  these 
  times, 
  so 
  that 
  the 
  earth's 
  portion 
  of 
  radioactive 
  

   matter 
  must 
  have 
  been 
  generated 
  during 
  the 
  sun's 
  life 
  in 
  the 
  sun 
  

   itself. 
  

  

  The 
  only 
  possible 
  escape 
  from 
  this 
  conclusion 
  would 
  seem 
  to 
  lie 
  in 
  

   the 
  supposition 
  that 
  the 
  lives 
  of 
  atoms 
  of 
  uranium 
  and 
  thorium 
  are 
  in 
  

   some 
  way 
  enormously 
  prolonged 
  by 
  intense 
  heat 
  and 
  fierce 
  bombard- 
  

   ment 
  such 
  as 
  occur 
  in 
  the 
  sun 
  's 
  interior. 
  We 
  can 
  not 
  absolutely 
  rule 
  

   such 
  a 
  possibility 
  out, 
  but 
  it 
  is 
  difficult 
  to 
  see 
  any 
  single 
  consideration 
  

   which 
  could 
  be 
  adduced 
  in 
  its 
  favor 
  from 
  the 
  side 
  either 
  of 
  experi- 
  

   mental 
  or 
  of 
  theoretical 
  physics, 
  and, 
  in 
  the 
  present 
  state 
  of 
  our 
  

   knowledge, 
  it 
  would 
  seem 
  reasonable 
  to 
  disregard 
  it. 
  

  

  Assuming 
  that 
  these 
  atoms 
  were 
  born 
  in 
  the 
  sun, 
  the 
  problem 
  of 
  

   the 
  manner 
  of 
  their 
  birth 
  takes 
  us 
  to 
  the 
  very 
  heart 
  of 
  present-day 
  

   theoretical 
  physics. 
  

  

  Let 
  us 
  consider, 
  in 
  some 
  detail, 
  two 
  processes 
  which 
  occur 
  on 
  

   earth: 
  The 
  change 
  of 
  atomic 
  make-up 
  through 
  a 
  readjustment 
  of 
  

   electrons, 
  and 
  the 
  change 
  of 
  nuclear 
  make-up 
  through 
  spontaneous 
  

   disintegration. 
  

  

  At 
  first 
  sight 
  the 
  two 
  processes 
  seem 
  very 
  dissimilar. 
  The 
  radio- 
  

   active 
  transformation 
  of 
  the 
  nucleus 
  is 
  spontaneous, 
  in 
  the 
  sense 
  

   that 
  nothing 
  that 
  we 
  can 
  do 
  either 
  expedites 
  or 
  hinders 
  it. 
  Each 
  

   atom 
  of 
  uranium 
  carries 
  its 
  own 
  future 
  history 
  written 
  inside 
  it. 
  It 
  

  

  