Stars

1. Stars Other Suns

2. Physical Properties • Luminosity • Mass • Diameter (radius) Must know distance to find out these properties!

3. Physical Properties • Surface temperature • Chemical composition Analyze spectra to infer these properties; distance not required.

4. Distances • Direct: Heliocentric stellar parallaxes (AU as baseline) • Smaller parallax, greater distance • Inverse relation: Distance is inversely proportional to parallax angle • Precise parallaxes of many stars by Hipparcos satellite

5. Luminosities • Measure flux at earth • Imagine sphere with radius equal to distance; area collects star’s luminosity • Inverse-square law: Flux inversely proportional to distance squared

6. Masses • Find: Binary systems (lots!) • Apply: Newton’s version of Kepler’s 3rd • Need: Distance, orbital period & separation, center of mass • Get: Mass of each star

7. Diameters (Radii) • Current techniques can measure angular diameters directly for some stars • Angular diameter inversely proportional to distance • Need distance to find physicaldiameter

8. Diameters (Radii) • Infer: From luminosity, surface temperature • Assume: Radiates like blackbody; temperature gives flux at surface • Luminosity: From surface flux and area => infer radius (area = 4 π R2 for sphere of radius R)

9. Composition • Analyze spectra (most contain absorption lines) • Match dark lines to those for known elements • Gives composition of photosphere only

10. Surface Temperatures • From color: Bluish-white (hottest) to reddish (coolest) • From peak in continuous spectrum or matching continuous spectrum to that of a blackbody • Assume radiate somewhat like blackbodies (Planck curve)

11. Energy • Fusionreactions! (E = mc2)PP Chain, CNO cycle • In high-temperature cores (above ignition temperatures) • Energy flows to surface (radiation, convection ), radiated into space

12. Spectral Classes • Temperaturesequence from hottest (O) to coolest (M) • Based on intensities of certain dark lines of specific elements (especially Balmer series of hydrogen) • Related to colors of stars (continuous spectra)

13. Hertzsprung-Russell Diagram • Graph of stellar luminosities (need distances!) versus surface temperatures (colors or spectral types) • See patterns among stars => different physical features • Main sequence, giants, supergiants, white dwarfs

14. Luminosity Classes • Pattern on H-R digram • Samespectral types (surface temperatures) but differentluminosities! • Infer different surface areas and so different radii: Supergiants, giants, main sequence

15. Mass-Luminosity Relation • Graph luminosities versus masses (from binary systems) • Pattern: Larger masses have much greater luminosities • Luminosity directly proportional to mass to the 4th power (L ~ M4)

16. Lifetimes • Fuel reserve: Directly proportional to mass • Use: Directly proportional to luminosity • Lifetime= Reserve/Use or M/M4 or 1/M3 => more mass, shorter lifetime!

17. Ages • Lifetime: Total span of active life from fusion reactions • Age: Time elapsed since fusion reactions began • Sun’s lifetime: 10 Gy; sun’s age, 5 Gy; when age = lifetime, star dies (no more fusion)