Intensity and Distance

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Intensity and Distance

• Intensity dependson luminosity anddistance
• Inverse-square lawdiscovered by Newton
• Magnitude systemused in astronomy

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Parallax

• Parallax is usedto measure thedistance to
  thenearest stars
• d (parsec) 1 / p (arcsec)
• Parsec is 3.26light years

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Stellar Spectra
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Color Related to Temperature
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Temperature Affects Absorption
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Taking a Temperature
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Careful Study of Absorption
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Taking the Temperature of Stars
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Color vs. Temperature

• Cameron Reed in "The Composite Observational-Theor
  etical HR Diagram"The Journal of the Royal
  Astronomical Society of CanadaFebruary/March
  1998 Volume 92 Number 1
• B-V -3.684 log(T) 14.551for log(T) lt 3.961
• B-V 0.344 log(T)2 -3.402 log(T) 8.037for
  log(T) gt3.961

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H-R Diagram

• 90 of the stars appear on the main sequence when
  plotted on the H-R diagram
• Giants have appear to have larger radii
• Supergiants appear to have largest radii
• White dwarfs have peculiar spectra

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Stellar Masses with Binaries

• Binary stars have elliptical orbits about the
  center of mass.
• These orbits obey Newtons form of Keplers 3rd
  law M1 M2 4p2/G (a3/p2)
• The ratio of masses can also be determined
  M1 / M2 a2 / a1
• Three values must be measured to determine
  individual masses

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Visual Binaries

• Visual binary star systems supply all the
  necessary data to compute masses (inclination and
  3 values)

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Spectroscopic Binary
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Eclipsing Binary
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Stellar Masses

• Masses from binary star data
• A sizeable fraction of stars have mass data
• Supergiants have higher average mass than Giants
• White Dwarfs not shown on this plot

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Main Sequence Masses

• There is a relation between mass and luminosity
  for main sequence stars
• This relation holds only for the main sequence
  (plotted with filled circles)
• Simple power-law fit gives L M3.5
• Break in fit at M 0.43
• M gt 0.43, L M 4.0M lt 0.43, L 0.23 M 2.3

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Predicted Radii

• According to the theory of thermal radiationL
  s T4 A 4ps T4 R2
• The luminosity of a star is related to the
  surface temperature and radius
• The observed radii in eclipsing binaries are
  consistent with theory

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