1. Devil physics The baddest class on campus IB Physics
Teacher of the Week
Devil physics The baddest class on campus IB Physics

2. Tsokos Lesson E-2 Stellar radiation

3. IB Assessment Statements
Topic E-2, Stellar Radiation and Stellar Types
Energy Source
E.2.1. State that fusion is the main energy source of stars.
E.2.2. Explain that, in a stable star (for example our Sun), there is an equilibrium between radiation pressure and gravitational pressure.
Luminosity
E.2.3. Define the luminosity of a star.
E.2.4. Define apparent brightness and state how it is measured.

4. IB Assessment Statements
Topic E-2, Stellar Radiation and Stellar Types
Wien’s Law and the Stefan-Boltzmann Law
E.2.5. Apply the Stefan-Boltzmann Law to compare the luminosities of different stars.
E.2.6. State Wien’s (displacement) Law and apply it to explain the connection between the colour and temperature of stars.
Stellar Spectra
E.2.7. Explain how atomic spectra may be used to deduce chemical and physical data for stars.
E.2.8. Describe the overall classification system of spectral classes.

5. IB Assessment Statements
Topic E-2, Stellar Radiation and Stellar Types
Types of Stars
E.2.9. Describe the different types of stars.
E Discuss the characteristics of spectroscopic and eclipsing binary stars.
The Hertzsprung-Russell Diagram
E Identify the general regions of star types on a Hertzsprung-Russell (HR) diagram.

6. Objectives
Understand that a star is in equilibrium under the action of two opposing forces, gravitation and radiation pressure of the star
Appreciate that nuclear fusion provides the energy source of the star

7. Objectives
Give the definition of luminosity as the power radiated into space by a star and apparent brightness as the power received per unit area on the earth

8. Objectives State the Wien displacement law and solve problems using it
Appreciate the kind of information a stellar spectrum can provide

9. Objectives
State the main properties of main sequence stars, red giants, white dwarfs, and binary stars
Describe the structure of an HR diagram and place the main types of stars on the diagram

10. Energy Source of Stars
Stars are composed of a core and a dense “atmosphere” of extremely hot gasses
The core is a dense mass that creates gravitational force that tries to draw the atmosphere into the core
In the atmosphere, nuclear fusion reactions are taking place between hydrogen atoms that release energy that generates heat

11. Energy Source of Stars
High temperatures overcome electrostatic repulsion between hydrogen protons
High pressure allow atoms to come close enough to collide and fuse

12. Energy Source of Stars
Each fusion cycle turns 4 hydrogen atoms into 1 helium atom with the release of 2 electrons, 2 photons, and 2 neutrinos

13. Energy Source of Stars
The heat creates a radiation pressure that keeps the star from collapsing under the gravitational pressure
To remain stable, the gravitation and radiation forces must be in equilibrium

14. Luminosity
Luminosity is the amount of energy radiated by a star per second, which is the same as saying it is the amount of power radiated by the star
σ is the Stefan-Bolzmann constant, equal to 5.67x10-8 W/m2K4

15. Luminosity
Luminosity depends on 2 things: the surface temperature and the surface area of the star
Think of the star as a sphere and consider that it radiates power in all directions from the surface of the sphere

16. Apparent Brightness
Imagine that you could put a detector somewhere on the surface of the star
The detector would measure the energy at that spot
That spot represents a fraction of the total surface area and the energy it measures would be the same fraction of the total energy generated by the star

17. Apparent Brightness
This is called the apparent brightness whose units are W/m2
If we can measure the apparent brightness, which is the energy at a spot, we can then calculate the Luminosity

18. Apparent Brightness Measured using a charge-coupled device or CCD
It has a photosensitive silicon surface that releases an electron when hit by a photon
The number of electrons released is proportional to the number of incident photons
We then measure the charge released and that becomes a direct measure of the brightness of the object

19. Black Body Radiation
Black-body Law - A body of surface area A and absolute temperature T, radiates energy in waves according to the Stefan-Bolzman law (Section 7-2, pages )

20. Black Body Radiation
Therefore, luminosity is the amount of energy radiated per second by a star of surface area A and absolute surface temperature T, and is given by the equation,

21. Black Body Radiation A is the surface area of a sphere
T is absolute surface temperature in Kelvin
And σ is the Stefan-Bolzman constant 5.67 x 10-8 W/m2K4
So the units for luminosity will be watts

22. Black Body Radiation
Using the previous equation for apparent brightness, we now find this equation,

23. Wavelength
The energy radiated by a star is in the form of electromagnetic radiation that covers an infinite range of wavelengths
This figure gives the radiation profiles for different temperatures

24. Wavelength
This is also called the spectrum of a black body and is the energy radiated per second per wavelength interval from a unit area of the body

25. Wavelength
The horizontal axis represents the wavelength in micrometers
The vertical scale is relative intensity in W/m3

26. Wavelength
Majority of emitted energy centers around the peak wavelength, λ0
Colour of a star is determined by this wavelength
Total power, L, is area under the curve

27. Wien Displacement Law
Relates the wavelength to surface temperature by,
Implies that the higher the temperature, the lower the peak wavelength and vice versa

28. Stellar Spectra - Temperature
Using Wien’s Law, stellar temperature determined by peak wavelength

29. Stellar Spectra – Chemical Composition
Chemical composition obtained from emission spectra
Typical composition is,
70% hydrogen
28% helium
2% other heavier elements

30. Stellar Spectra – Chemical Composition
Most stars have the same composition, but different spectra caused by temperature
In hot stars the hydrogen is ionized. Atoms cannot absorb and re-emit photons thus lines will not show
Cooler stars will have most hydrogen electrons in state n=2 and will show spectra for transitions to n=3 and n=4
‘Cold’ stars will have most of its electrons in the ground state and will only absorb and re-emit ultraviolet photons

31. Stellar Spectra – Spectral Classes
Stars are divided into spectral classes based on colour/temperature
Oh Be A Fine Girl/Guy Kiss Me

32. Stellar Spectra – Rotation
As a star rotates, one side is moving toward the observer and one side is moving away from the observer
Side moving toward the observer will be blue- shifted (shorter wavelength, higher frequency)
Side moving away from the observer will be red- shifted (longer wavelength, lower frequency)

33. Stellar Spectra – Magnetic Fields
In a magnetic field, a spectral line may split into two or more lines (Zeeman effect)
Measurement of the amount of splitting in the spectra provides information about the magnetic field of the star

34. Hertzsprung-Russell Diagram
Correlations between luminosity and temperature
Between temperature and size
Between absolute magnitude and spectral class
Result was the HR Diagram

35. Hertzsprung-Russell Diagram
Important Notes:
Luminosity along the left vertical axis is in terms of our sun’s luminosity
Surface temperature along lower horizontal axis goes from right-to-left
Scales on the axes are not linear

36. Hertzsprung-Russell Diagram
Features:
Most stars fall along a diagonal from upper left to lower right – main sequence stars
Top right, large reddish cool (in temp) stars – red giants
Bottom left, small, bright, hot (in temp) stars – white dwarfs

37. Hertzsprung-Russell Diagram
Features:
90% of all stars are main sequence
9% are white dwarfs
1% are red giants

38. Hertzsprung-Russell Diagram
Features:
As you move from bottom right to top left on main sequence, luminosity and mass increase
Bottom right = red dwarfs
Top left = blue giants

39. Types of Stars Main Sequence Stars Our sun is a main sequence star
Luminosity increases as mass increases
Produce enough energy in the core to balance gravitational force

40. Types of Stars Red Giants Very large, cool, reddish appearance
Luminosity much greater than main sequence stars of the same temperature – up to a billion times greater
Mass as much as 1000 times our sun but low density
Hot core surrounded by envelope of gas

41. Types of Stars White Dwarfs Sirius A and B most well known
Faint and hard to detect
Mass similar to the sun but size similar to earth = density 106 times that of earth
Formed when collapsing stars stabilize as a result of electron degeneracy pressure

42. Types of Stars White Dwarfs
Electron degeneracy pressure occurs when forced into the same quantum state
Pauli exclusion principle says they will acquire large kinetic energy which allows the star to resist gravitational pressure to collapse

43. Types of Stars Variable Stars
Luminosity varies with time – light curve
Periodic or non-periodic
Mainly due to changes in internal structure of the star
As the core becomes more dense, outer gas envelope expands
Mass ejected from outer layers as nebula or supernovas

44. Types of Stars Variable Stars
When mass ejected, luminosity increases by a factor of a million
Matter can also be transferred from one star to another which will heat up and radiate further increasing luminosity

45. Types of Stars Variable Stars Cepheids
Most prominent of the periodic variables
Periods of 1-50 days
Relationship between period of light curve and peak luminosity
Peak luminosity compared to apparent brightness yields distance to stars

46. Types of Stars Variable Stars Cepheids
Study of variable stars provides information about internal structure of stars and testing ground for theories

47. Types of Stars Binary Stars Two stars that orbit a common center
Important because they allow determination of stellar masses

48. Types of Stars Visual Binary
Formula for common period of rotation given below
Measurement of the separation distance and period gives the sum of the two masses making up the binary

49. Types of Stars Eclipsing Binary
Orbit plane in relation to the earth such that the light of one is periodically blocked by the other

50. Types of Stars Spectroscopic Binary
Detected by analyzing the Doppler shift of the light from each

51. Summary Review
Do you understand that a star is in equilibrium under the action of two opposing forces, gravitation and radiation pressure of the star?
Do you appreciate that nuclear fusion provides the energy source of the star?

52. Summary Review
Can you give the definition of luminosity as the power radiated into space by a star and apparent brightness as the power received per unit area on the earth?

53. Summary Review
Can you state the Wien displacement law and solve problems using it
Do you appreciate the kind of information a stellar spectrum can provide

54. Summary Review
Can you state the main properties of main sequence stars, red giants, white dwarfs, and binary stars
Can you describe the structure of an HR diagram and place the main types of stars on the diagram

55. IB Assessment Statements
Topic E-2, Stellar Radiation and Stellar Types
Energy Source
E.2.1. State that fusion is the main energy source of stars.
E.2.2. Explain that, in a stable star (for example our Sun), there is an equilibrium between radiation pressure and gravitational pressure.
Luminosity
E.2.3. Define the luminosity of a star.
E.2.4. Define apparent brightness and state how it is measured.

56. IB Assessment Statements
Topic E-2, Stellar Radiation and Stellar Types
Wien’s Law and the Stefan-Boltzmann Law
E.2.5. Apply the Stefan-Boltzmann Law to compare the luminosities of different stars.
E.2.6. State Wien’s (displacement) Law and apply it to explain the connection between the colour and temperature of stars.
Stellar Spectra
E.2.7. Explain how atomic spectra may be used to deduce chemical and physical data for stars.
E.2.8. Describe the overall classification system of spectral classes.

57. IB Assessment Statements
Topic E-2, Stellar Radiation and Stellar Types
Types of Stars
E.2.9. Describe the different types of stars.
E Discuss the characteristics of spectroscopic and eclipsing binary stars.
The Hertzsprung-Russell Diagram
E Identify the general regions of star types on a Hertzsprung-Russell (HR) diagram.

58. Questions?

59. Homework
#1-26