1. Temperature, Color, Radius
& Compositioni

2. Surface Temperature

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

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

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

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

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

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

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

16. Radius

17. 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

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:
 = o

19. Exception: Betelgeuse

20. 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

21. Chemical Composition

22. 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%