Temperature, Color, Radius & Compositioni

Surface Temperature

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

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

Sun’s spectrum Absorption lines The star Vega

Originates at bottom of photosphere. Result of absorption.

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

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

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

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

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

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

Color

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

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

Radius

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

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

Exception: Betelgeuse

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

Chemical Composition

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