Stellar Evolution

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Main Sequence Stars 

  • High-luminosity main sequence stars have hot surfaces and low-luminosity main-sequence stars have cooler surfaces

  • Just as the Sun does, all stars along the main sequence are fusing hydrogen into helium in their cores.  As a result, the relationship between surface temperature an luminosity arises because the rate of hydrogen fusion depends strongly on a star's mass

  • Stellar masses decrease downward along the main sequence

  • At the upper end of the main sequence, the hot, luminous O stars can have masses as high as 300 times that of the Sun (300 Msun).  On the lower end, cool, dim M stars may have as little as 0.08 times the mass of the Sun (0.08 Msun).  Many more stars fall on the lower end of the main sequence than on the upper end, which tells us that lower-mass stars are much more common than high-mass stars

  • We can estimate a main-sequence star's mass just by knowing its spectral type, based on the fact that mass, surface temperature, and luminosity are all related.

Lifetime of a Main Sequence Star

  • Stars spend the vast majority of their lives as main-sequence stars.
  • Massive stars near the upper end of the main sequence have shorter lives than less massive stars near the lower end
  • A star's lifetime depends on both its mass and its luminosity.  Its mass determines how much hydrogen fuel the star initially contains in its core and its luminosity determines how rapidly the star uses up its fuel.  
  • Though massive stars start their lives with a larger supply of hydrogen, they fuse it into helium so rapidly that they end up with shorter life spans.