﻿200 
  

  

  ANNUAL 
  EEPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1929 
  

  

  ward; 
  they 
  thin 
  out 
  least 
  rapidly 
  when 
  we 
  move 
  toward 
  the 
  center, 
  

   faster 
  when 
  we 
  travel 
  in 
  the 
  opposite 
  direction, 
  and 
  fastest 
  of 
  all 
  

   when 
  we 
  proceed 
  toward 
  the 
  poles 
  of 
  the 
  Milky 
  Way. 
  The 
  signifi- 
  

   cant 
  detail 
  is 
  the 
  behavior 
  in 
  the 
  direction 
  of 
  the 
  center 
  of 
  the 
  larger 
  

   system, 
  which 
  turns 
  out 
  to 
  be 
  just 
  the 
  opposite 
  of 
  that 
  to 
  be 
  expected 
  

   were 
  the 
  local 
  system 
  not 
  present. 
  We 
  thus 
  conclude, 
  not 
  only 
  

   that 
  a 
  local 
  system 
  exists, 
  but 
  that 
  it 
  dominates 
  the 
  situation 
  to 
  such 
  

   an 
  extent 
  that 
  the 
  characteristic 
  distribution 
  within 
  the 
  larger 
  

   system 
  which 
  we 
  expected 
  to 
  find 
  is 
  totally 
  obscured. 
  How 
  completely 
  

   this 
  is 
  the 
  case 
  is 
  illustrated 
  by 
  the 
  uppermost 
  curve 
  of 
  Figure 
  3, 
  

   which 
  shows 
  the 
  numbers 
  of 
  stars 
  per 
  unit 
  volume 
  at 
  different 
  dis- 
  

   tances 
  from 
  the 
  sun 
  in 
  two 
  different 
  directions, 
  one 
  (left) 
  toward 
  

   the 
  center 
  of 
  the 
  larger 
  system, 
  the 
  other 
  (right) 
  in 
  the 
  direction 
  

   diametrically 
  opposite. 
  Distances 
  of 
  points 
  on 
  the 
  curve 
  above 
  the 
  

   bottom 
  of 
  the 
  diagram 
  represent 
  numbers 
  of 
  stars. 
  Even 
  toward 
  

   the 
  center, 
  the 
  stars 
  thin 
  out 
  so 
  rapidly 
  that 
  at 
  2,000 
  parsecs 
  (6,500 
  

  

  / 
  

  

  10000 
  5000 
  5000 
  10000 
  PARSECS 
  

  

  Figure 
  3. 
  — 
  Variation 
  in 
  the 
  number 
  of 
  stars 
  per 
  unit 
  volume 
  at 
  difierent 
  distances 
  from 
  the 
  sun 
  (figures 
  at 
  

   bottom) 
  in 
  the 
  direction 
  of 
  the 
  center 
  of 
  the 
  stellar 
  system 
  (toward 
  the 
  left) 
  and 
  in 
  the 
  opposite 
  direction. 
  

   The 
  upper 
  curve 
  includes 
  all 
  stars 
  together. 
  This 
  can 
  be 
  resolved 
  into 
  two 
  other 
  curves, 
  one, 
  nearly 
  

   symmetrical, 
  representing 
  the 
  local 
  system, 
  and 
  another 
  representing 
  the 
  larger 
  system. 
  Distances 
  in 
  

   parsecs 
  may 
  be 
  expressed 
  in 
  light 
  years 
  by 
  multiplying 
  by 
  3.2e 
  

  

  light 
  years) 
  the 
  density 
  is 
  only 
  one-half 
  that 
  near 
  the 
  sun, 
  while 
  at 
  

   5,000 
  parsecs 
  (16,250 
  light 
  years) 
  it 
  is 
  only 
  one-fifth. 
  The 
  great 
  

   concentration 
  of 
  density 
  near 
  the 
  sun 
  represents 
  the 
  influence 
  of 
  the 
  

   local 
  system. 
  

  

  However 
  we 
  approach 
  the 
  matter, 
  therefore, 
  the 
  larger 
  system, 
  in 
  

   our 
  own 
  vicinity 
  at 
  least, 
  seems 
  to 
  sink 
  into 
  a 
  position 
  of 
  relative 
  

   unimportance, 
  and, 
  when 
  we 
  attempt 
  to 
  learn 
  more 
  about 
  it, 
  we 
  

   meet 
  with 
  great 
  difficulties. 
  

  

  VI 
  

  

  SEPARATION 
  OF 
  THE 
  LOCAL 
  AND 
  LARGER 
  SYSTEMS 
  

  

  To 
  proceed, 
  we 
  must 
  try 
  to 
  get 
  rid 
  of 
  the 
  local 
  system 
  by 
  removing 
  

   its 
  members 
  from 
  our 
  counts. 
  This 
  is 
  a 
  hazardous 
  undertaking, 
  

   because, 
  in 
  general, 
  we 
  can 
  not 
  specify 
  the 
  system 
  to 
  which 
  any 
  given 
  

   star 
  belongs; 
  and 
  we 
  are 
  thus 
  obliged 
  to 
  make 
  an 
  assumption, 
  namely, 
  

  

  