Using the HR Diagram to Determine Distance: Spectroscopic “Parallax”•(1) Determine temperature from color(2) Determine luminosity based on main sequence position(3) Compare luminosity with flux (apparent brightness)(4) Use inverse square law to determine distance•Flux = luminosity / 4pid^2•d = sqrt[L / 4piF]•What if the star doesn’t happen to lie on the main sequence? Maybe it is a red giant or white dwarf?•We determine the star’s luminosity class based on its spectral line widths•These lines get broader when the stellar gas is at higher densities – indicating a smaller starStellar Masses: Visual Binary Stars•With Newton’s modifications to Kepler’s laws, the period and size of the orbits yield the sum of the masses, while the relative distance of each star from the center of mass yields the ratio of the masses•The ratio and sum provide each mass individually•Why is mass so important? Together with the initial composition, mass defines the entire life cycle and all other properties of the star!•Luminosity, radius, surface temperature, lifetime, evolutionary phases, end result . . .•Example: On the main sequence: Luminosity proportional mass^2•More mass means: more gravity, more pressure on core, higher core temperatures, faster nuclear reaction rates, higher luminosities•How does mass effect how long a star will live? Lifetime proportional fuel available / how fast fuel is burned•So for a star: lifetime proportional mass / luminosity•How long a star lives is directly related to the mass!•Big stars live shorter lives, burn their fuel faster . . .
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