intensity and distance intensity depends on luminosity and distance inverse-square law discovered by...
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Intensity and Distance
• Intensity dependson luminosity anddistance
• Inverse-square lawdiscovered by Newton
• Magnitude systemused in astronomy
Parallax
• Parallax is usedto measure thedistance to thenearest stars
• d (parsec) =1 / (arcsec)
• Parsec is 3.26light years
Color vs. Temperature
• Cameron Reed in "The Composite Observational-Theoretical 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) < 3.961
• B-V = 0.344 [log(T)]2 -3.402 log(T) +8.037for log(T) >3.961
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
Stellar Masses with Binaries:
• Binary stars have elliptical orbits about the center of mass.
• These orbits obey Newton’s form of Kepler’s 3rd law: M1 + M2 = 42/G (a3/p2)
• The ratio of masses can also be determined: M1 / M2 = a2 / a1
• Three values must be measured to determine individual masses
Visual Binaries• Visual binary star systems supply all the necessary
data to compute masses (inclination and 3 values)
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
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 = 0.43M > 0.43, L = M 4.0
M < 0.43, L = 0.23 M 2.3
Predicted Radii
• According to the theory of thermal radiation:L = T4 A = 4 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