1. The Hitchhiker's Guide to the Galaxy
“We demand rigidly defined areas of doubt and uncertainty.” Vroomfondel the philosopher.
The Hitchhiker's Guide to the Galaxy

2. Notices:
HW1 is on-line now and due Tuesday.
Reading: Chapters 3 and 1

3. The 5 basic physical properties of stars:
Luminosity,
Mass, Radius (combined as surface gravity),
Surface Temperature (Teff)
Surface chemical composition (X,Y,Z)
Distance, via parallax;
Radial velocity via Doppler shift; and
Proper motion,

4. So far..
Teff=2.9x106/lmax
Lap=4pR2sT4/d2

5. (need M, probably from catalog of spectral types)
Distance (3 ways)
Parallax: d = 1/p (for distance in pc and parallactic angles in arcseconds.)
Lap = L/d2 (need Teff and R)
Distance modulus:
(need M, probably from catalog of spectral types)

6. Radius (4 ways) If distance is known: Eclipsing binaries
Directly: Interferometry. R=(a/2)d.
If distance is known:
Eclipsing binaries
Get L from luminosity class and spectral type: R=[L/(4psT4)]1/2

7. Mass (1 method)
From binaries only.

8. Surface composition: X, Y, Z X + Y + Z = 1
X is the mass fraction of H
Y is the mass fraction of He
Z is the mass fraction of everything else

9. Spectral Classification
This is based on what is observed in the spectrum.

10. OBAFGKM Spectral types:
This is a temperature sequence- as can be seen from the continuous portion of the spectra.

11. Spectral Sequence. Numbers were added to complete the sequence:
F: 0 – 9
G: 0 – 9
M: 0 – 9 (76% of local stars are M class stars)

12. Other classifications:
L: Metal hydrides and alkali metals. No TiO or VO. These have Teff of 1,300-2,000K
T: Brown dwarfs with Teff of 700-1,300K. Methane dominates in the IR.
W: Wolf-Rayet stars. Helium dominated supergiants. WN or WC (or WO even)
C: Carbon stars. Red giants.
S: have zirconium oxide and TiO. Also red giants.
D: degenerate (for white dwarfs only). DA, DB,

13. Major components of each spectral class.
These are good to know.

14. Major components of each spectral class.
These are good to know.
Remember these are not composition, just lines that are formed.

16. Luminosity class. Determined by spectral line broadening.

17. Luminosity class. Determined by spectral line broadening.
Under higher pressure, randomized motions within stellar gas is increased, creating Doppler broadening due solely to pressure.
This is called pressure broadening and is a measure of surface gravity.

18. Luminosity class. Determined by spectral line broadening.
Under higher pressure, randomized motions within stellar gas is increased, creating Doppler broadening due solely to pressure.
This is called pressure broadening and is a measure of surface gravity.
So large, low-density stars have lower surface gravity, and narrower lines.
Small, condensed stars have high density, high gravity and broader lines.

20. Luminosity classes.
What are missing?

21. Luminosity classes.
What are missing?
Any stars below the main sequence.

22. Luminosity classes VI and VII are seldom written
Luminosity classes VI and VII are seldom written. Usually it is sd and D.
sdO, sdB, sdM
DA, DB

23. Stellar classifications
To completely describe a star, you include both its spectra type and its luminosity class.
Our Sun: G2 V
Betelguese: M2 Iab
Sirius: A1 V
Vega: A0 V
Polaris: F7 Ib-II

24. Then the true space velocity is:
Stellar Motions
There are two additional motions we have not discussed: Radial velocity (Doppler shift) and proper motion.
Then the true space velocity is:

25. Radial Velocity

26. Radial Velocity
In this formula, positive velocity values are moving away (redshifted) and negative velocities are approaching (blueshifted).
Note that the formula is set up such that positive values are moving away from us.

27. Radial Velocity
Example, Our Sun orbits within the Milky Way at 250 km/s. What is the shift for a line with a rest wavelength of nm (Hydrogen alpha)?

28. Radial Velocity
Example, Our Sun orbits within the Milky Way at 250 km/s. What is the shift for a line with a rest wavelength of nm (Hydrogen alpha)?
Dl=lv/c = (250)/(3x105) = 0.55 nm.

29. Here are spectra of our Sun (presumably at rest compared to us) and Arcturus. How fast is Arcturus moving? Towards or away from us?

30. How fast is Arcturus moving. Towards or away from us. Dl= 0
How fast is Arcturus moving? Towards or away from us? Dl= 0.15nm so v = 50 km/s (positive, so away from us) redshifted.

31. Proper motion
Stellar motion across our line of sight. The actual stellar motion is vpm=dm where m is the angular rate (converted from radians/year) and d is their distance from us.

32. Proper Motion Example. Work out on paper.
The star WISEP J shows a proper motion of 2.1”/yr and is estimated to be at a distance of 17pc. What is its proper motion in km/s? vpm=dm

33. The star WISEP J191239. 9-361516. 4 shows a proper motion of 2
The star WISEP J shows a proper motion of 2.1”/yr and is estimated to be at a distance of 17pc. What is its proper motion in km/s? vpm=dm
To answer this, you have to convert d to km and m to rad/sec. Sec/yr~px107, I get
170 km/s

35. Non-constant stars: Variable stars
Extrinsic- binary stars.
Intrinsic-
Regular: pulsating stars.
Irregular: nova, supernova, and giant pulsators.

36. Extrinsic- binary stars- eclipsing

37. Variable binary stars: Ellipsoidal variable- at least 1 star is not round.

39. Reflection-effect binary: A hot star is heating part of a cool star.

41. Pulsating stars: We will have an entire section on this!!!
Cepheid variables:
Red supergiants

43. d Scuti variables: Main Sequence A-F Stars.

44. RR Lyrae pulsators: Horizontal branch stars

45. Solar-like pulsators.

46. There are pulsating stars everywhere!
This is good since seismology can probe their insides.

47. Irregulars: SN, eruptive, spots
Supernova

48. Flare stars.

49. Additionally, the spots themselves are not permanent.
Spotted stars
As the stars spin, spots leave or come into our line-of-sight, changing the brightness.
Additionally, the spots themselves are not permanent.

50. Recap: Observable properties
M: Only using gravity: binary stars and/or planets.
Teff using Wien’s law (spectroscopic): Teff = 2.9x106/
Distance: 1) Parallax: d=1/p
2) Luminosity: Lap = L/d2
3) Distance modulus:
Radius: 1) Interferometry: R=(/2)d where = angular size of the star
2) Eclipsing binary stars.
3) Luminosity: Lap=L/(4d2) and L=4R2T4.
Brightness: L = AT4 , Lap = L/d2,

51. Recap: Observable properties
Composition: X (H), Y (He), Z (‘metals’: O,C,N,...) measured spectroscopically.
Spectral type: indicator of temperature
Luminosity class: indicator of radius (gravity)

52. Things we will presume when examining stars…..

53. Stars are giant balls of gas. Let’s start there.
Where does that lead us for describing the physics of stars?

54. Stars are giant balls of gas. Let’s start there.
Where does that lead us for describing the physics of stars?
Gas has mass, so gravity will try to compress/collapse it.
Something must resist gravity: for a non-degenerate gas, this must be thermal pressure.
These two things must be in balance.
Since stars are luminous, they are losing heat energy. This must be replenished, requiring an energy source.
Spherical shape.

55. Using our Sun as a ‘generic’ star, what other conditions can we obtain/presume?

56. 92% H by particle number- a monatomic ideal gas.
Using our Sun as a ‘generic’ star, what other conditions can we obtain/presume?
92% H by particle number- a monatomic ideal gas.
Stars are isolated- space exerts no pressure, external torques, or substantial gravitational fields (can we prove this last one?).
Can spherical symmetry be broken? Not by spin (slow rotation), not by magnetic fields (many dex too weak).
Over the course of human history, the Sun has not perceptibly changed: stars are static.

57. And yet… stars are emitting (and therefore producing) energy
And yet… stars are emitting (and therefore producing) energy. So over very long time scales, they must evolve.
When we look at stars in the sky, two things dominate: brightness and length of time at an evolutionary stage.

58. Chapter 1: Our Sun.
Read this chapter.

59. The photosphere

60. This is the visible surface of the Sun.
The photosphere
This is the visible surface of the Sun.
However, we have to remember that the Sun is gaseous, and therefore we do not see a surface at all.

62. Effective Temperature
Since light comes from different depths of the photosphere, it also comes from different temperatures. The top of the photosphere is about 4500K while the base is about 9000K. The majority of the light comes from between 5800 and 6000K.

63. Effective Temperature
Since light comes from different depths of the photosphere, it also comes from different temperatures. The top of the photosphere is about 4500K while the base is about 9000K. The majority of the light comes from between 5800 and 6000K.
Will have to keep this in mind when we discuss spectral line forming regions. Different lines will form in different regions.

64. The visible portion of our Sun is about 500km deep, whereas the Sun itself is nearly 700,000km deep!

65. We can see 0.07% into the Sun. Roughly the same is true for all stars.