1. Intensity and Distance Intensity depends on luminosity and distance Inverse-square law discovered by Newton Magnitude system used in astronomy

2. Parallax Parallax is used to measure the distance to the nearest stars d (parsec) = 1 /  (arcsec) Parsec is 3.26 light years

3. Stellar Spectra

4. Color Related to Temperature

5. Temperature Affects Absorption

6. Taking a Temperature

7. Careful Study of Absorption

8. Taking the Temperature of Stars

9. Color vs. Temperature Cameron Reed in "The Composite Observational- Theoretical HR Diagram" The Journal of the Royal Astronomical Society of Canada February/March 1998 Volume 92 Number 1 B-V = -3.684 log(T) + 14.551 for log(T) < 3.961 B-V = 0.344 [log(T)] 2 -3.402 log(T) +8.037 for log(T) >3.961

10. 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

11. Stellar Masses with Binaries: Binary stars have elliptical orbits about the center of mass. These orbits obey Newton’s form of Kepler’s 3 rd law: M 1 + M 2 = 4  2 /G (a 3 /p 2 ) The ratio of masses can also be determined: M 1 / M 2 = a 2 / a 1 Three values must be measured to determine individual masses

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

13. Spectroscopic Binary

14. Eclipsing Binary

15. 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

16. 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 = M 3.5 Break in fit at M = 0.43 M > 0.43, L = M 4.0 M < 0.43, L = 0.23 M 2.3

17. Predicted Radii According to the theory of thermal radiation: L =  T 4 A = 4  T 4 R 2 The luminosity of a star is related to the surface temperature and radius The observed radii in eclipsing binaries are consistent with theory