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

Stellar Spectra

Color Related to Temperature

Temperature Affects Absorption

Taking a Temperature

Careful Study of Absorption

Taking the Temperature of Stars

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)

Spectroscopic Binary

Eclipsing Binary

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