Temperature, Color, Radius & Compositioni

1. Temperature, Color, Radius & Compositioni

2. Surface Temperature

3. Most light comes from photosphere – thin layer at bottom of atmosphere.. Sun’s visible “surface” is photosphere.

4. Hot & dense gas – emits continuous spectrum Cooler, thinner gas – absorbs continuous light

5. Sun’s spectrum Absorption lines The star Vega

6. Originates at bottom of photosphere. Result of absorption.

7. 12,000 K 6000 K Intensity 3000 K Wavelength 1. Location of peak of blackbody (continuous emission) curve yields temperature.

8. Real Stars Intensity 50,000 K 10,000 K 6000 K (Sun) Wavelength

9. Line Strength 10,000 K Temp. • Appearance (“Strength”) of the spectrum • lines reflects temperature. e.g., Hydrogen (“Balmer”) lines

10. Appearance (“Strength”) of the spectrum • lines reflects temperature. Temp Wavelength

11. Balmer Thermometer Balmer line transitions. •  High temp: most H is • ionized. • Low temp: most H is in ground state.

12. Lines of many elements respond to temperature. Line Strength Temp.

13. Color

14. 12,000 K 6000 K Intensity 3000 K Wavelength * Star color is determined by temperature.

15. Taurus (“The Bull”) T  13,000 K T  3500 K

16. Radius

17. D  d (AU)  (o) D = 865,000 mi Sun d 0.50 0.25 0.10 0.05 0.005 1 2 5 10 100

18. In general . . . d &   D Sun: d = 1 AU,  = 0.5o D = 865,000 mi For other stars, it’s not so easy to get diameter . . .  Sun at 4 ly from Earth:  = 0.000002o

19. Exception:Betelgeuse

20. R 1 m2 E Another way to get radius . . . Stars radiate like blackbodies. From each sq meter, energy E radiated: E  T4 Total energyemitted by star = total surface area  E

21. Chemical Composition

22. Appearance of lines of element implies presence of element. • But, absence of lines does not imply absence of element. • e.g., Hydrogen •  Astronomers find: most stars dominated by • hydrogen & helium. • e.g., Sun: H 74% • He 25%