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

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

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 = Full moon: m v = Sun: m v = -26.5

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.

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.

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).

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.

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

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

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.

The Color-Color Diagram B - V U - B Blackbody B0 A0 F0 G0 K0 M0

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

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