﻿MODEKN 
  ASTEONOMY 
  ABBOT 
  

  

  161 
  

  

  In 
  order 
  to 
  understand 
  this 
  majestic 
  march 
  of 
  the 
  celestial 
  hosts, 
  

   we 
  must 
  first 
  review 
  the 
  recent 
  studies 
  by 
  Hubble 
  of 
  the 
  spiral 
  

   nebulae 
  and 
  their 
  relation 
  to 
  our 
  galaxy 
  of 
  stars. 
  Hubble 
  has 
  been 
  

   able, 
  in 
  photographs 
  by 
  the 
  100-inch 
  reflector, 
  to 
  resolve 
  several 
  

   spiral 
  nebulse, 
  including 
  the 
  Great 
  Nebula 
  of 
  Andromeda, 
  into 
  hosts 
  

   of 
  separate 
  stars 
  of 
  excessive 
  faintness. 
  Some 
  of 
  these 
  stars 
  he 
  finds 
  

  

  -300 
  

  

  Fig. 
  4. 
  — 
  The 
  march 
  of 
  stars 
  as 
  compared 
  to 
  the 
  place 
  of 
  the 
  siin. 
  If 
  all 
  the 
  groups, 
  

   I 
  to 
  XII, 
  of 
  the 
  stars 
  were 
  collected 
  at 
  the 
  sun, 
  at 
  the 
  intersection 
  of 
  the 
  lines 
  0, 
  

   then 
  after 
  a 
  certain 
  time 
  the 
  several 
  groups 
  would 
  all 
  seem 
  to 
  have 
  moved 
  along 
  

   tlie 
  general 
  direction 
  B 
  A, 
  and 
  to 
  have 
  spread 
  out 
  to 
  fill 
  the 
  figures 
  shown. 
  This 
  

   great 
  celestial 
  way 
  lies 
  nearly 
  toward 
  the 
  star 
  Antares 
  

  

  (o 
  be 
  of 
  the 
  Cepheid 
  type 
  of 
  variable 
  brightness. 
  Shapley 
  had 
  shown 
  

   that 
  these 
  variables 
  are 
  standard 
  objects 
  whose 
  periods 
  of 
  variation 
  

   are 
  truly 
  indicative 
  of 
  their 
  absolute 
  brightness. 
  Hence 
  Hubble 
  

   was 
  able 
  to 
  determine, 
  from 
  the 
  periods 
  of 
  variation, 
  the 
  real 
  bright- 
  

   ness 
  of 
  these 
  Cepheids 
  found 
  within 
  the 
  spiral 
  nebulse. 
  This 
  done, 
  

   their 
  distances 
  followed 
  at 
  once, 
  from 
  a 
  knowledge 
  of 
  their 
  apparent 
  

   magnitudes. 
  

  

  