1. Stars
Luminous gaseous celestial body – spherical in shape held by its own gravity

2. How do we study stars?
Light!!

3. Stellar Radiation
H fusion occurs in star’s interior converting mass to E (mass deficit).
T must be ~ 107 K, for nuclei to overcome Coulomb force & fuse.
Interior of the star is so hot it is plasma.

4. BE of He higher than BE 4H.
He - 4.
H isotopes.

5. Excess E is carried away by g photons & neutrinos n
Excess E is carried away by g photons & neutrinos n. Some E gets absorbed in star heats interior more & exerts outward pressure.

6. Stellar Equilibrium- outward P from radiation balances gravity inward in stable stars.

7. Stable Stars maintain size. The sun is stable

8. Ex 1. The sun is losing mass at 4. 26 x 109 kg/s
Ex 1. The sun is losing mass at 4.26 x 109 kg/s. At what rate does the sun emit energy?
Assuming the mass is converted to E.
E = mc2.
(4.26 x 109 kg/s)(3 x 108m/s)2.
3.83 x J each second.

9. Luminosity (L) = total power output of a star W or J/s.
Star Power
Luminosity (L) = total power output of a star W or J/s.
As we just calculated the sun converts mass to Energy
Sun L = 3.9 x 1026 W.

10. Luminosity (W) depends on:
Surface Area
Temperature
Which equation relates power to A & T?

11. Stars are regarded as black bodies
L = sAT4.
L = s4pr2T4.
L – Watts J/s. L = Power
A surface A m2
T Kelvin
s = 5.67 x 10-8W/ m2 K4.

12. Apparent Brightness (b): how bright stars appear
Apparent Brightness (b): how bright stars appear. What we see from Earth depends on L & distance from Earth

13. Apparent brightness
radiation from star that is incident on the Earth per m2. It’s like Intensity

14. Calculation of Apparent Brightness (b):
L = luminosity in W
d = distance to Earth m
b = apparent brightness W/m2.
Intensity

15. Ex 2: The apparent brightness of a star is 6. 4 x 108 W/m2
Ex 2: The apparent brightness of a star is 6.4 x 108 W/m2. If its distance to Earth is 50 LY, find its luminosity.

16. b4pd2 = L (6.4 x 108 W/m2) (4p)(4.73 x 1017 m)2. 1.8 x 10 45 W
d = (9.46 x m/LY)(50 LY) = 4.73 x 1017 m
b4pd2 = L
(6.4 x 108 W/m2) (4p)(4.73 x 1017 m)2.
1.8 x W

17. Finding Star Temperature Remember Black Bodies?

18. Wein’s Displacement Law relates peak l & surface temp for black body.
T in Kelvin
l in meters
Star’s spectra similar to black body.

19. as T inc.
Tot intensity increase for all l.
Peak changes to shorter l higher f.

20. Ex 3: A star has a surface temp of 17 000 K and L = 6. 1 x 10 29 W. a
Ex 3: A star has a surface temp of K and L = 6.1 x W. a. What is the peak l? b. Find its radius.

21. Use Stephen Boltzmann to find R.

22. Solar Spectrum Some radiation l absorbed by outer layers.
Can identify elements in outer layers.
If H is present, H will absorb l = to dif between Bohr orbit levels. Form black lines.

23. Absorption of l

24. Motion & Speed of Stars Doppler Effect/Red or Blue shift gives info.
Absorption lines shift toward longer or shorter l, depending on motion.

25. Red Shift Spectrum – stars moving away from us show dark line shift.
Find v, direction by shift of line spectra.

26. Blue Shift – moving toward us Amount of Shift relates to speed of motion

27. List 3 observations we can make using light to get information about stars. State what we can learn from each type of observation.

28. Use Spectrum to find: Chemical composition surface
(absorption spectrum)
Motion toward or away from Earth
Red/blue shift
Surface temp
Peak l (color)

29. Ex 4: Our sun has T = 6000 K and L = 3. 9 x 1026 W
Ex 4: Our sun has T = 6000 K and L = 3.9 x 1026 W. If star Z has T = 4000 K, &L = 5.2 x W, would it be:   larger or smaller to our sun?   Calculate its radius in terms of our sun’s radius.
Larger
26 x Rsun.

30. Early Star Classification
Spectral Class
Color Temperature Composition.

31. Sun

32. Stellar spectra
From 4:48

33. Star Classification

34. Spectral Classes.
Stars characterized by temperature, absorption lines & color.
Old Classification O – M.

36. Oh be a fine girl – kiss me. Then subdivided in 10 smaller groups 0-9.
OBAFGKM
Oh be a fine girl – kiss me.
Then subdivided in 10 smaller groups 0-9.
Sun – G2.

37. H-R diagram graphs temp against luminosity – Not Linear
Be able to identify general regions of star types on the H-R diagram
90% Stars on Main sequence.

38. H-R Diagram Cool, Super-Large Cool, Large Small, Hot MS High Mass
Fast Burners
Cool, Large
Small, Hot
MS Low Mass
Long Lives

39.
HR Diagram start at 1:24

40. Black body radiation 12 min