1. The Trigonometric Parallax B p B = 1 AU = 1.496*10 13 cm d = (1/p[arcsec]) parsec d 1 pc = 3.26 LY ≈ 3*10 18 cm

2. The Moving Cluster Method x    vv vrvr v

3. The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects): Sirius (brightest star in the sky): m v = -1.42 Full moon: m v = -12.5 Sun: m v = -26.5

4. Color and Temperature Orion Betelgeuze Rigel Stars appear in different colors, from blue (like Rigel) via green / yellow (like our sun) to red (like Betelgeuze). These colors tell us about the star’s temperature.

5. Blackbody Radiation (I) The light from a star is usually concentrated in a rather narrow range of wavelengths. The spectrum of a star’s light is approximately a thermal spectrum called Blackbody Spectrum.

6. Blackbody Radiation F surf =  T eff 4  = 5.67*10 -5 erg/(cm 2 s K 4 ) Wien’s displacement law : max ≈ 0.29 cm / T K (T K = temperature in Kelvin).

7. The Color Index (I) B band V band The color of a star is measured by comparing its brightness in different wavelength bands: The blue (B) band and the visual (V) band. We define B-band and V-band magnitudes just as we did before for total magnitudes.

8. Optical Wavelength Bands U: 0 ≈ 3650 Å B: 0 ≈ 4400 Å V: 0 ≈ 5500 Å

9. The Color Index We define the Color Index B – V (i.e., B magnitude – V magnitude) The bluer a star appears, the smaller the color index B – V. The hotter a star is, the smaller its color index B – V. B - V Temperature

10. Example: For our sun: Absolute V magnitude: 4.83 Absolute B magnitude: 5.51 => Color index: B – V = 0.68 From standard tables: B – V = 0.68 => T ≈ 5800 K.

11. The Color-Color Diagram B - V U - B -0.51.51.00.50.02.0 1.5 1.0 0.5 0.0 -0.5 Blackbody B0 A0 F0 G0 K0 M0

12. The Hertzsprung-Russell Diagram B - V T eff M bol Log(L) Most stars are found along the Main Sequence Zero-Age Main Sequence (ZAMS)

13. Radii of Stars in the Hertzsprung-Russell Diagram 10,000 times the sun’s radius 100 times the sun’s radius As large as the sun 100 times smaller than the sun RigelBetelgeuze Sun Polaris Giants Supergiants White Dwarfs

14. Extinction and Reddening

15. Interstellar Extinction

16. Wavelength-Dependent Extinction