1. E2 Stellar radiation and stellar types

2. Fusion

3. Life-cycle of a star

4. Equilibrium between radiation pressure and gravity

5. Luminosity (symbol L)
Luminosity is defined as the amount of energy radiated by the star per second (The power radiated by the star) Measured in Watts (J.s-1)

6. Black-body radiation

7. Black-body radiation
Need to “learn” this!

8. Black-body radiation
Black Body - any object that is a perfect emitter and a perfect absorber of radiation
object does not have to appear "black"
Stars behave approximately as black bodies

9. Black-body radiation
The amount of energy per second (power) radiated from a star (its luminosity) depends on its surface area and absolute temperature according to
L = σAT4
where σ is the Stefan-Boltzmann constant (5.67 x 10-8 W.m-2.K-4)

10. Example From L = σAT4 and A = 4πR2 we find T = (L/σ 4πR2)¼ = 5800 K
The sun (radius R = 7.0 x 108 m) has a luminosity of 3.9 x 1026 W. Find its surface temperature.
From L = σAT4 and A = 4πR2 we find
T = (L/σ 4πR2)¼ = 5800 K

11. Wien’s law – Finding the temp of a star
λmaxT = constant (2.9 x 10-3 mK)

12. Example
The sun has an approximate black-body spectrum and most of its energy is radiated at a wavelength of 5.0 x 10-7 m. Find the surface temperature of the sun.
From Wien’s law
5.0 x 10-7 x T = 2.9 x 10-3
T = 5800 K

13. Apparent brightness (symbol b)
Apparent brightness is defined as the amount of energy per second per unit area of detector b = L/4πd2 where d is the distance from the star (in m) L is the luminosity (in W)

14. Apparent brightness - CCD
Apparent brightness is measured using a charge-coupled device (used also in digital cameras) Read the final paragraph of page 495.

15. Apparent brightness and Luminosity
Note that the apparent brightness b and luminosity L are proportional
b = L/4πd2
b α L α T4

16. You need to remember the classes and their order
Spectral Class
Colour
Temperature/K O
Blue – B
Blue - white – A
White
7 500 – F
Yellow - white
6 000 – 7 500 G
Yellow
4 500 – 6 000 K
Yellow - red
3 000 – 4 500 M
Red
2 000 – 3 000
You need to remember the classes and their order

17. Oh be a fine girl….kiss me!
Spectral classes
Oh be a fine girl….kiss me!

18. More information from spectra
The spectrum of a star can have dark absorption lines across it. Each dark line represents the absorption of light at a specific frequency by a chemical element in the star

19. At school they called me “Bohr the Bore”!
Niels Bohr
In 1913, a Danish physicist called Niels Bohr realised that the secret of atomic structure lay in its discreteness, that energy could only be absorbed or emitted at certain values.
At school they called me “Bohr the Bore”!

20. The Bohr Model
Bohr realised that the electrons could only be at specific energy levels (or states) around the atom.

21. The Bohr Model
We say that the energy of the electron (and thus the atom) can exist in a number of states n=1, n=2, n=3 etc. (Similar to the “shells” or electron orbitals that chemists talk about!)
n = 1
n = 2
n = 3

22. The Bohr Model
The energy level diagram of the hydrogen atom according to the Bohr model
n = 1 (the ground state)
n = 2
n = 3
n = 4
n = 5
High energy n levels are very close to each other
Energy eV
Electron can’t have less energy than this
-13.6

23. The Bohr Model
An electron in a higher state than the ground state is called an excited electron.
n = 1 (the ground state)
n = 2
n = 3
n = 4
n = 5
-13.6
Energy eV
High energy n levels are very close to each other
electron

24. Atomic transitions
If a hydrogen atom is in an excited state, it can make a transition to a lower state. Thus an atom in state n = 2 can go to n = 1 (an electron jumps from orbit n = 2 to n = 1)
n = 1 (the ground state)
n = 2
n = 3
n = 4
n = 5
-13.6
Energy eV
Wheeee!
electron

25. Atomic transitions
Every time an atom (electron in the atom) makes a transition, a single photon of light is emitted.
n = 1 (the ground state)
n = 2
n = 3
n = 4
n = 5
-13.6
Energy eV
electron

26. Atomic transitions
The energy of the photon is equal to the difference in energy (ΔE) between the two states. It is equal to hf. ΔE = hf
n = 1 (the ground state)
n = 2
n = 3
n = 4
n = 5
-13.6
Energy eV
electron
ΔE = hf

27. Atomic transitions
An electron can also absorb a photon of the same energy and jump to a hjgher level.
n = 1 (the ground state)
n = 2
n = 3
n = 4
n = 5
-13.6
Energy eV
electron
ΔE = hf

28. More information from spectra
The absorption spectrum thus gives us information about a star’s chemical composition

29. Very hot stars
Very hot stars do not show an absorption spectrum as all the gas is ionised so there are no bound electrons orbiting around the nuclei in the star. Thus absorption spectrums can also tell us something about the temperature of a star.

30. Doppler effect on spectra

31. Radial velocity

32. Rotation

33. Different types of stars

34. Binary stars

35. Spectroscopic binaries

36. Eclipsing binaries

37. Eclipsing binaries

38. Cepheids
A type of variable star whose luminosity changes with time (more later!)

39. Red giants and red supergiants
Large in size and red in colour.
Large luminosity
Since they are red, they are comparatively cool.
The source of energy is the fusion of some elements other than hydrogen.

40. White dwarfs Small and white in colour.
Since they are white they are comparatively hot.
Fusion is no longer taking place, and a white dwarf is just a hot remnant that is cooling down.

41. Hertzsprung – Russell diagram

42. Hertzsprung – Russell diagram
The point of classifying the various types of stars is to see is any patterns exists. A useful way of making the comparison is the H-R diagram. Each dot on the diagram represents a different star.
The vertical axis is the luminosity of the star. It should be noted that the scale is not a linear one.
The horizontal axis is the spectral class of the star in the order OBAFGKM. This is the same as a scale of decreasing temperature. Once again the scale is not a linear one.
The result of such a plot is shown on the next slide

43. Cepheids!

44. Hertzsprung – Russell diagram
A large number of stars the fall on the line that goes from the top left to bottom right. This line is known as the MAIN SEQUENCE and stars that are on it are known as the main sequence stars. Our sun is a main sequence star. These stars are ‘normal’ stable stars-the only difference between them is their mass. They are fusing hydrogen to helium. The stars that are not on the main sequence can also be put into categories.

45. Questions
Page 504 Questions 1, 2, 3, 4, 5, 6, 7, 9.