﻿MODERN 
  ASTEONOMY 
  ABBOT 
  

  

  157 
  

  

  It 
  appears 
  that 
  starting 
  with 
  the 
  red 
  giants 
  at 
  less 
  than 
  3,000° 
  

   absokite 
  C. 
  of 
  surface 
  temperature, 
  the 
  successive 
  classes 
  K, 
  G, 
  F, 
  A, 
  

   and 
  B 
  exhibit 
  a 
  scale 
  of 
  constantly 
  rising 
  surface 
  temperatures, 
  cul- 
  

   minating 
  with 
  approximately 
  16,000° 
  absolute 
  C. 
  for 
  the 
  surface 
  of 
  

   such 
  a 
  star 
  as 
  Rigel. 
  Our 
  own 
  sun, 
  at 
  6,000° 
  absolute 
  C, 
  is 
  medium 
  

   in 
  this 
  as 
  in 
  other 
  characters. 
  

  

  Within 
  the 
  stars 
  two 
  forces 
  strive 
  against 
  the 
  enormous 
  com- 
  

   pressive 
  force 
  of 
  gravitation. 
  These 
  are, 
  first, 
  the 
  agitation 
  of 
  the 
  

   electrons, 
  and 
  atomic 
  nuclei, 
  after 
  the 
  fashion 
  of 
  pressures 
  in 
  all 
  

   gases, 
  and 
  second, 
  the 
  pressure 
  of 
  radiation. 
  In 
  the 
  interior 
  of 
  

   stars, 
  the 
  temperatures, 
  

   which 
  may 
  reach 
  40,000,- 
  

   000°, 
  are 
  so 
  exalted 
  that 
  

   the 
  radiation 
  therein 
  ex- 
  

   erts 
  perfectly 
  tremendous 
  

   pressure. 
  At 
  such 
  tem- 
  

   peratures 
  the 
  prevailing 
  

   wave 
  lengths 
  are 
  of 
  the 
  

   order 
  of 
  X 
  rays, 
  rather 
  

   than 
  of 
  visible 
  light. 
  

   Such 
  very 
  short 
  wave 
  rays 
  

   are 
  unable 
  to 
  penetrate 
  

   more 
  than 
  a 
  few 
  centi- 
  

   meters 
  through 
  the 
  ion- 
  

   ized 
  atomic 
  gas 
  of 
  the 
  

   stellar 
  substance. 
  Under 
  

   these 
  circumstances 
  the 
  

   energy 
  of 
  the 
  interior 
  is 
  

   imprisoned, 
  and 
  after 
  

   reaching 
  the 
  blue 
  stage 
  

   the 
  central 
  parts 
  of 
  a 
  star 
  

   grow 
  hotter 
  and 
  hotter; 
  

   they 
  are, 
  though, 
  quite 
  un- 
  

   able 
  to 
  prevent 
  the 
  ex- 
  

   terior 
  from 
  cooling 
  back 
  through 
  the 
  successive 
  spectrum 
  classes 
  A, 
  

   F, 
  G, 
  K, 
  and 
  M, 
  with 
  constantly 
  increasing 
  density. 
  Hence 
  we 
  find 
  

   two 
  kinds 
  of 
  red 
  and 
  yellow 
  stars, 
  one 
  sort 
  very 
  bulky 
  and 
  rare, 
  the 
  

   other 
  relatively 
  very 
  small 
  and 
  dense. 
  Our 
  sun 
  belongs 
  to 
  the 
  dwarf 
  

   class, 
  having 
  passed 
  the 
  acme 
  of 
  brightness. 
  

  

  Until 
  recent 
  years 
  the 
  source 
  of 
  energy 
  which 
  maintains 
  the 
  tre- 
  

   mendous 
  outward 
  flow 
  of 
  stellar 
  radiation 
  was 
  unexplained. 
  Now, 
  

   however, 
  the 
  atoms 
  are 
  shown 
  to 
  consist 
  entirely 
  of 
  groups 
  of 
  equal 
  

   electric 
  charges, 
  positive 
  and 
  negative, 
  supposed 
  to 
  remain 
  separate 
  

   by 
  virtue 
  of 
  orbital 
  motion. 
  It 
  is 
  conceived 
  that 
  under 
  the 
  cir- 
  

  

  FiG. 
  2. 
  — 
  The 
  arrangement 
  of 
  atoms 
  as 
  it 
  is 
  in 
  com- 
  

   mon 
  salt, 
  whose 
  molecules 
  each 
  contain 
  one 
  atom 
  

   of 
  chlorine 
  and 
  one 
  of 
  sodium. 
  Each 
  of 
  these 
  

   atoms 
  in 
  its 
  turn 
  is 
  made 
  up 
  of 
  a 
  family 
  of 
  elec- 
  

   tric 
  charges, 
  positive 
  and 
  negative 
  in 
  equal 
  num- 
  

   bers. 
  The 
  running 
  together 
  of 
  these 
  charges 
  

   annihilates 
  the 
  atoms, 
  and 
  gives 
  up 
  energy 
  to 
  

   maintain 
  the 
  radiation 
  of 
  sun 
  and 
  stars 
  

  

  