1. What is a Hertzsprung-Russell diagram?
An H-R diagram plots the luminosity and temperature of stars
Luminosity
Temperature

2. Most stars fall somewhere on the main sequence of the H-R diagram

3. giants and supergiants
Large radius
Stars with lower T and higher L than main-sequence stars must have larger radii:
giants and supergiants

4. Stars with higher T and lower L than main-sequence stars must have smaller radii:
white dwarfs
Small radius

5. Spectral lines of supergiants, giants, and main-sequence stars
Figure 8.9: Differences in widths and strengths of spectral lines distinguish the spectra of supergiants, giants, and main-sequence stars. (Adapted from H. A. Abt, A. B. Meinel, W. W. Morgan, and J. W. Tapscott, An Atlas of Low-Dispersion Grating Stellar Spectra, Kitt Peak National Observatory, 1968)
Figure 8.9: Differences in widths and strengths of spectral lines distinguish the spectra of supergiants, giants, and main-sequence stars. (Adapted from H. A. Abt, A. B. Meinel, W. W. Morgan, and J. W. Tapscott, An Atlas of Low-Dispersion Grating Stellar Spectra, Kitt Peak National Observatory, 1968)
Fig. 8-9, p. 141

6. A star’s full classification includes spectral type (line identities) and luminosity class (line shapes, related to the size of the star):
I - supergiant
II - bright giant
III - giant
IV - subgiant
V - main sequence
Examples: Sun - G2 V
Sirius - A1 V
Proxima Centauri - M5.5 V
Betelgeuse - M2 I

7. Luminosity classes
Figure 8.10: The approximate location of the luminosity classes on the H–R diagram.
The approximate location of the luminosity classes on the H–R diagram.
Fig. 8-10, p. 142

8. Stellar Sizes
A few stars are big enough to resolve in the new telescopes.
Sizes for these can now be calculated directly.
Betelguese is 600X larger than the Sun.

9. Indirect Size determinations
Using the luminosity and the temperature of a star the area and hence the size of a star can be determined.
𝐿=𝜎𝐴 𝑇 4 =4𝜋𝜎 𝑟 2 𝑇 4
𝑟= 𝐿 4𝜋𝜎 𝑇 4

10. Thus, Polaris is 100 times larger than the sun.
Example:
Polaris has just about the same spectral type (and thus surface temperature) as our sun, but it is 10,000 times brighter than our sun.
𝐿=4𝜋𝜎 𝑟 2 𝑇 4
Thus, Polaris is 100 times larger than the sun.

12. H-R diagram depicts: Temperature Color Spectral Type Luminosity Radius

13. Which star is the hottest?B
Which star is the hottest? D
Luminosity A
Temperature

14. Which star is the hottest?B
Which star is the hottest? D
Luminosity A A
Temperature

15. Which star is the most luminous?B
Which star is the most luminous? D
Luminosity A
Temperature

16. C B
Which star is the most luminous? C D
Luminosity A
Temperature

17. C B
Which star is a main-sequence star? D
Luminosity A
Temperature

18. C B
Which star is a main-sequence star? D D
Luminosity A
Temperature

19. C B
Which star has the largest radius? D
Luminosity A
Temperature

20. C B
Which star has the largest radius? C D
Luminosity A
Temperature

21. What is the significance of the main sequence?

22. Main-sequence stars are fusing hydrogen into helium in their cores like the Sun
Luminous main-sequence stars are hot (blue)
Less luminous ones are cooler (yellow or red)

23. Mass measurements of main-sequence stars show that the hot, blue stars are much more massive than the cool, red ones
High-mass stars
Low-mass stars

24. The mass of a normal, hydrogen-burning star determines its luminosity and spectral type!
High-mass stars
Low-mass stars

25. Core pressure and temperature of a higher-mass star need to be larger in order to balance gravity
Higher core temperature boosts fusion rate, leading to larger luminosity

26. For stars
Main regions
Some HR diagrams use the magnitude instead of the luminosity. While it looks like a linear scale remember it’s really a log scale.

27. The Distance Modulus
If we know a star’s absolute magnitude or luminosity, we can infer its distance by comparing absolute and apparent magnitudes:
Distance Modulus
= mV – MV
= log10(d [pc])
Distance in units of parsec
Equivalent:
d = 10(mV – MV + 5)/5 pc

28. Stellar Distances. From parallax outward each new method is calibrated by data from the earlier method. The methods overlap.

29. Stellar Properties Review
Luminosity: from brightness and distance
10-4 LSun LSun
Temperature: from color and spectral type
3,000 K ,000 K
Mass: from period (p) and average separation (a)
of binary-star orbit
0.08 MSun MSun

30. Stellar Properties Review
Luminosity: from brightness and distance
10-4 LSun LSun
Temperature: from color and spectral type
3,000 K ,000 K
Mass: from period (p) and average separation (a)
of binary-star orbit
0.08 MSun MSun
(0.08 MSun)
(100 MSun)
(0.08 MSun)
(100 MSun)

31. The most visible stars Notice the scales are log scales.
The diagonals are lines of constant radius, from the radius-luminosity-temperature relationship.

32. Nearby Stars

33. A random sample of stars

34. There should be nothing special about our local neighborhood
There should be nothing special about our local neighborhood. It therefore is useful as a random sample of stars in the galaxy.
The larger our sample size the better our conclusions. So a complete sample out to greater distances gives a more accurate picture of the galaxy as a whole.

35. A Census of the Stars
Faint, red dwarfs (low mass) are the most common stars.
Bright, hot, blue main-sequence stars (high-mass) are very rare.
Giants and supergiants are extremely rare.

36. Mass & Lifetime Until core hydrogen (10% - 15% of total) is used up
Sun’s life expectancy: 10 billion years
Life expectancy of 10 MSun star:
10 times as much fuel, uses it 104 times as fast
10 million years ~ 10 billion years x 10 / 104
Life expectancy of 0.1 MSun star:
0.1 times as much fuel, uses it 0.01 times as fast
100 billion years ~ 10 billion years x 0.1 / 0.01

37. Stellar Population Distribution Explained
Faint, red dwarfs (low mass) are the most common stars. Long lived
Bright, hot, blue main-sequence stars (high-mass) are very rare. Short lived
Giants and supergiants are extremely rare.
Passing through a stage.

38. Main-Sequence Star Summary
High Mass:
High Luminosity
Short-Lived
Large Radius
Blue
Low Mass:
Low Luminosity
Long-Lived
Small Radius
Red

39. What are giants, supergiants, and white dwarfs?

40. Off the Main Sequence
Stellar properties depend on both mass and age: those that have finished fusing H to He in their cores are no longer on the main sequence
All stars become larger and redder after exhausting their core hydrogen: giants and supergiants
The core of most stars ends up small and white after fusion has ceased: white dwarfs

41. Which star is most like our Sun?D
Luminosity B C
Temperature

42. A
Which star is most like our Sun? D B
Luminosity B C
Temperature

43. A
Which of these stars will have changed the least 10 billion years from now? D
Luminosity B C
Temperature

44. A
Which of these stars will have changed the least 10 billion years from now? D
Luminosity B C C
Temperature

45. A
Which of these stars can be no more than 10 million years old? D
Luminosity B C
Temperature

46. A
Which of these stars can be no more than 10 million years old? D
Luminosity A B C
Temperature

47. Why do the properties of some stars vary?

48. Variable Stars
Any star that varies significantly in brightness with time is called a variable star
Some stars vary in brightness because they cannot achieve proper balance between power welling up from the core and power radiated from the surface
Such a star alternately expands and contracts, varying in brightness as it tries to find a balance

49. Pulsating Variable Stars
The light curve of this pulsating variable star shows that its brightness alternately rises and falls over a 50-day period

50. Pulsating Variable Stars
Most pulsating variable stars inhabit an instability strip on the H-R diagram
The most luminous ones are known as Cepheid variables

51. Cepheid Variable Stars
Cepheid variable’s magnitude will vary between magnitudes over a period from days to months.
They have been show to have a period of variability that depends on luminosity.
This allows Cepheid variables to be used as a standard candle to measure distance.

52. Cepheid Variable Stars
The accepted explanation for the pulsation of Cepheids is called the Eddington valve
Doubly ionized helium is more opaque than singly ionized helium.

53. Eddington valve cont.
The more helium is heated, the more ionized it becomes.
At the dimmest part of a Cepheid's cycle, the ionized gas in the outer layers of the star is opaque, and so is heated by the star's radiation
Due to the increased temperature the fusion rate in the core increase resulting in an increase in radiation pressure causing the star to begin to expand.

54. Eddington valve cont.
As it expands, the ionized helium cools, and becomes less ionized and therefore more transparent, allowing the radiation to escape cooling the star and slowing the fusion rate.
Then the expansion stops, and reverses due to the star's gravitational attraction.
The process then repeats.

55. What have we learned? What is a Hertzsprung-Russell diagram?
An H-R diagram plots stellar luminosity of stars versus surface temperature (or color or spectral type)
What is the significance of the main sequence?
Normal stars that fuse H to He in their cores fall on the main sequence of an H-R diagram
A star’s mass determines its position along the main sequence (high-mass: luminous and blue; low-mass: faint and red)

56. What have we learned? What are giants, supergiants, and white dwarfs?
All stars become larger and redder after core hydrogen burning is exhausted: giants and supergiants
Most stars end up as tiny white dwarfs after fusion has ceased
Why do the properties of some stars vary?
Some stars fail to achieve balance between power generated in the core and power radiated from the surface

57. Star Clusters Our goals for learning
What are the two types of star clusters?
How do we measure the age of a star cluster?

58. What are the two types of star clusters?

59. Open cluster: A few thousand loosely packed stars, typically found is galactic disk, and typically young in age <5 billion yrs.

60. Useful movies to download:
Globular cluster visualization
Stellar collision simulation:
Globular cluster: Up to a million or more stars in a dense ball bound together by gravity, most found in the galactic halo with some in the disk, some of the oldest objects found in the galaxy.

61. How do we measure the age of a star cluster?
Massive blue stars die first, followed by white, yellow,
orange,
and
red stars

62. Main-sequence turnoff point of a cluster tells us its age

63. Pleiades now has no stars with life expectancy less than around 100 million years
Main-sequence
turnoff

64. Detailed modeling of the oldest globular clusters reveals that they are about 13 billion years old

65. What have we learned? What are the two types of star clusters?
Open clusters are loosely packed and contain up to a few thousand stars
Globular clusters are densely packed and contain hundreds of thousands of stars
How do we measure the age of a star cluster?
A star cluster’s age roughly equals the life expectancy of its most massive stars still on the main sequence