﻿198 
  ANNUAL 
  EEPORT 
  SMITHSONIAN 
  INSTITUTION, 
  19 
  2 
  9 
  

  

  collection 
  of 
  very 
  hot, 
  massive 
  stars, 
  mostly 
  brighter 
  than 
  the 
  sixth 
  

   magnitude, 
  having 
  very 
  conspicuous 
  lines 
  of 
  helium 
  in 
  their 
  spectra. 
  

   These 
  bright 
  helium 
  stars 
  lie 
  close 
  to 
  the 
  Milky 
  Way 
  and 
  constitute 
  a 
  

   local 
  cluster, 
  very 
  much 
  flattened 
  — 
  so 
  much 
  so, 
  in 
  fact, 
  that 
  the 
  cluster 
  

   is 
  little 
  more 
  than 
  a 
  thin 
  sheet 
  of 
  stars, 
  extending 
  out 
  a 
  thousand 
  light 
  

   years 
  or 
  so 
  in 
  the 
  general 
  direction 
  of 
  the 
  Milky 
  Way. 
  The 
  sun 
  and 
  

   planets 
  lie 
  a 
  little 
  outside 
  the 
  thin 
  layer 
  of 
  stars, 
  and 
  at 
  a 
  distance 
  of 
  

   about 
  300 
  light 
  years 
  from 
  the 
  center 
  of 
  the 
  collection. 
  The 
  direction 
  

   of 
  the 
  .center 
  is 
  in 
  longitude 
  236°; 
  the 
  tilt 
  of 
  the 
  plane 
  about 
  which 
  

   the 
  helium 
  stars 
  cluster 
  is 
  12°, 
  in 
  longitude 
  160°. 
  These 
  figures 
  are 
  

   nearly 
  those 
  shown 
  by 
  Figure 
  2 
  for 
  the 
  center 
  of 
  the 
  system, 
  and 
  for 
  

   the 
  position 
  of 
  the 
  secondary 
  galaxy 
  derived 
  from 
  counts 
  of 
  all 
  kinds 
  of 
  

   stars 
  to 
  the 
  sixth 
  magnitude. 
  The 
  agreement 
  is 
  too 
  close 
  to 
  be 
  

   simply 
  coincidence, 
  and 
  we 
  conclude 
  that 
  most, 
  if 
  not 
  aU, 
  of 
  the 
  stars 
  

   brighter 
  than 
  the 
  sixth 
  magnitude 
  bear 
  some 
  close 
  relation 
  to 
  the 
  local 
  

   cluster 
  of 
  helium 
  stars. 
  That 
  the 
  bright 
  helium 
  stars 
  do 
  form 
  a 
  

   localized 
  cluster 
  is 
  easily 
  recognized 
  from 
  their 
  physical 
  characteristics, 
  

   which 
  cause 
  them 
  to 
  stand 
  out 
  from 
  their 
  neighbors 
  as 
  a 
  unit. 
  Since 
  

   the 
  stars 
  brighter 
  than 
  the 
  sixth 
  magnitude, 
  as 
  a 
  whole, 
  are 
  symmetri- 
  

   cally 
  distributed 
  about 
  the 
  same 
  plane 
  as 
  the 
  helium 
  stars, 
  the 
  infer- 
  

   ence 
  is 
  that 
  most 
  of 
  them 
  belong 
  to 
  that 
  cluster, 
  and 
  that 
  together 
  

   they 
  constitute 
  a 
  local 
  system 
  of 
  which 
  the 
  helium 
  stars 
  are 
  only 
  the 
  

   nucleus. 
  

  

  Apparently, 
  therefore, 
  we 
  must 
  amplify 
  our 
  picture 
  of 
  the 
  stellar 
  

   system 
  by 
  supposing 
  that 
  a 
  secondary 
  aggregation 
  of 
  stars 
  — 
  the 
  local 
  

   system 
  — 
  exists 
  Vv^ithin 
  the 
  larger 
  system. 
  The 
  local 
  system 
  lies 
  near 
  

   the 
  plane 
  of 
  symmetry 
  of 
  the 
  larger 
  system, 
  but 
  at 
  a 
  great 
  distance 
  

   from 
  the 
  central 
  point. 
  Like 
  the 
  larger 
  system, 
  it 
  is 
  flattened; 
  its 
  

   plane 
  of 
  symmetry 
  is 
  tilted 
  12° 
  to 
  that 
  of 
  the 
  larger 
  system. 
  We 
  our- 
  

   selves 
  are 
  within 
  the 
  local 
  system, 
  300 
  light 
  years 
  from 
  its 
  center 
  

   situated 
  in 
  longitude 
  236°; 
  the 
  far 
  more 
  distant 
  center 
  of 
  the 
  larger 
  

   system 
  seems 
  to 
  be 
  in 
  longitude 
  325°, 
  a 
  little 
  to 
  the 
  east 
  of 
  that 
  indi- 
  

   cated 
  by 
  the 
  stars 
  brighter 
  than 
  the 
  eighteenth 
  magnitude. 
  

  

  Looking 
  out 
  on 
  the 
  sky, 
  we 
  see 
  the 
  intermingled 
  stars 
  of 
  both 
  

   systems. 
  When 
  we 
  count 
  the 
  stars 
  to 
  the 
  sixth 
  magnitude 
  only, 
  

   we 
  deal 
  chiefly 
  with 
  those 
  of 
  the 
  local 
  system, 
  and 
  hence 
  find 
  them 
  

   crowding 
  toward 
  the 
  secondary 
  galaxy 
  marked 
  by 
  the 
  thin 
  stratum 
  of 
  

   bright 
  helium 
  stars; 
  the 
  center 
  appears 
  to 
  be 
  in 
  longitude 
  236°, 
  because 
  

   that 
  is 
  the 
  direction 
  of 
  the 
  center 
  of 
  the 
  local 
  system. 
  When 
  we 
  

   extend 
  the 
  counts 
  to 
  a 
  fainter 
  limit, 
  we 
  add 
  many 
  stars 
  belonging 
  to 
  

   the 
  larger 
  system, 
  and 
  thus 
  introduce 
  the 
  characteristics 
  of 
  that 
  

   system. 
  The 
  resulting 
  distribution 
  is 
  not 
  that 
  of 
  either 
  system 
  alone, 
  

   but 
  something 
  in 
  between; 
  the 
  secondary 
  galaxy 
  is 
  less 
  inclined 
  to 
  the 
  

   Milky 
  Way, 
  while 
  the 
  direction 
  of 
  the 
  center 
  has 
  shifted 
  a 
  little 
  east- 
  

   ward 
  along 
  the 
  Milky 
  Way 
  toward 
  that 
  of 
  the 
  larger 
  system. 
  But 
  

  

  