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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  19 
  2 
  9 
  

  

  Regarded 
  as 
  the 
  dominant 
  portion 
  of 
  so 
  vast 
  a 
  collection 
  as 
  the 
  larger 
  

   system, 
  the 
  region 
  of 
  maximum 
  stellar 
  concentration 
  is 
  not 
  an 
  impres- 
  

   sive 
  feature; 
  and 
  our 
  instinct 
  for 
  symmetrical 
  arrangements 
  in 
  the 
  

   heavens 
  makes 
  us 
  reluctant 
  to 
  accept 
  this 
  off-sided 
  aggregation 
  as 
  

   the 
  nucleus 
  of 
  the 
  larger 
  system, 
  or 
  the 
  very 
  unsymmetrical 
  curve 
  

   of 
  Figure 
  3 
  as 
  an 
  indication 
  of 
  how 
  the 
  stars 
  in 
  this 
  system 
  are 
  dis- 
  

   tributed. 
  

  

  VII 
  

  

  THE 
  ANALOGY 
  WITH 
  SPIRAL 
  NEBULA 
  

  

  At 
  first 
  sight 
  it 
  seems 
  difTicult 
  to 
  reconcile 
  the 
  improbabilities 
  thus 
  

   brought 
  to 
  light 
  with 
  the 
  symmetry 
  for 
  which 
  we 
  instinctively 
  look. 
  

   Nevertheless, 
  we 
  are 
  not 
  without 
  helpful 
  suggestions. 
  The 
  trend 
  

   of 
  cosmological 
  thought 
  in 
  recent 
  years 
  has 
  been 
  in 
  the 
  direction 
  of 
  

   analogies 
  between 
  the 
  stellar 
  system 
  and 
  the 
  great 
  spiral 
  nebulae 
  

   like 
  Messier 
  33 
  or 
  Messier 
  101 
  (pi. 
  3). 
  In 
  form, 
  there 
  is 
  close 
  resem- 
  

   blance. 
  In 
  both 
  cases 
  the 
  outline 
  in 
  the 
  principal 
  plane 
  is 
  roughly 
  

   circular; 
  and, 
  seen 
  edge-on 
  (pi. 
  4 
  a, 
  b, 
  c), 
  the 
  spirals 
  show 
  the 
  flat- 
  

   tened 
  contour 
  found 
  in 
  our 
  own 
  system. 
  Further, 
  photographs 
  

   made 
  at 
  Mount 
  Wilson 
  by 
  Hubble 
  with 
  the 
  100-inch 
  reflector 
  (pi. 
  4) 
  

   show 
  that 
  at 
  least 
  some 
  of 
  these 
  nebulae 
  are 
  gigantic 
  systems 
  of 
  stars, 
  

   composed 
  of 
  dift'erent 
  classes 
  of 
  objects 
  — 
  diffuse 
  nebulosity, 
  novae, 
  

   Cepheid 
  variables, 
  and 
  ordinary 
  giant 
  stars 
  of 
  different 
  spectral 
  

   types, 
  which, 
  class 
  for 
  class, 
  correspond 
  to 
  those 
  of 
  the 
  system 
  about 
  

   us; 
  and, 
  finally, 
  that 
  the 
  nebula?, 
  if 
  not 
  actually 
  as 
  large 
  as 
  the 
  stellar 
  

   system, 
  are 
  nevertheless 
  of 
  the 
  same 
  general 
  order 
  of 
  dimensions. 
  

  

  Seen 
  broadside 
  (pi. 
  3), 
  the 
  curving 
  arms 
  of 
  the 
  spirals, 
  with 
  their 
  

   irregular 
  knots 
  and 
  condensations 
  of 
  stars, 
  lack 
  the 
  smoothness 
  of 
  

   distribution 
  that 
  counts 
  in 
  our 
  own 
  system 
  seem 
  to 
  suggest; 
  but 
  it 
  

   requires 
  little 
  imagination 
  to 
  realize 
  that 
  were 
  we 
  situated 
  in 
  the 
  cen- 
  

   tral 
  plane 
  of 
  a 
  spiral 
  like 
  Messier 
  33, 
  we 
  should 
  find 
  the 
  scattered 
  

   aggregations 
  of 
  stars 
  blending 
  into 
  an 
  encircling 
  band 
  of 
  Milky 
  Way 
  

   clouds, 
  with 
  irregularities 
  perhaps 
  no 
  greater 
  than 
  those 
  in 
  the 
  star 
  

   clouds 
  of 
  our 
  own 
  galaxy. 
  Again, 
  the 
  conspicuously 
  bright 
  central 
  

   condensation 
  which 
  is 
  characteristic 
  of 
  the 
  spirals 
  makes 
  us 
  wonder 
  

   if 
  the 
  cosmological 
  analogy 
  is 
  complete, 
  for 
  thus 
  far 
  we 
  have 
  looked 
  

   in 
  vain 
  in 
  .our 
  own 
  sj^stem 
  for 
  anything 
  resembling 
  a 
  dominant 
  cen- 
  

   tral 
  nucleus. 
  But 
  even 
  this 
  seemingly 
  well-marked 
  exception 
  falls 
  

   into 
  line 
  when 
  the 
  position 
  of 
  the 
  observer 
  is 
  properly 
  credited 
  with 
  

   its 
  influence 
  on 
  appearances. 
  

  

  With 
  the 
  examples 
  of 
  edge-on 
  spirals 
  (pi. 
  4) 
  before 
  us, 
  imagine 
  

   ourselves 
  again 
  within 
  one 
  of 
  these 
  objects, 
  at 
  some 
  distance 
  from 
  the 
  

   center, 
  with 
  our 
  eyes 
  turned 
  toward 
  the 
  nucleus. 
  Does 
  it 
  seem 
  

   likely 
  that 
  we 
  should 
  then 
  see 
  the 
  central 
  condensation? 
  Apparently 
  

   not, 
  at 
  least 
  not 
  the 
  brightest 
  portion 
  at 
  the 
  .very 
  center. 
  Even 
  

  

  