1. ASTR 1020 First Homework Due Today Second Homework due Feb 9
Next Observatory Session Tonight at 7pm
Website

2. Escape Velocity Fall from Large Height
Same Energy Needed to Reverse and Fly Away
Escape velocity is the speed
at which object must be thrown
upward to escape and never come
down.
G=6.7x10-11 in mks units

3. The Sun

4. The Sun Falls into “Disk Stability”
99.9% Ended in Sun (0.1% in Jupiter)
Probably the Same Around All Stars -- Planets are Common
Shoots Planet-Size
Bullets into Space

5. Most of Mass Forms Ball in Center
A Star Is a Hot
Ball of Hydrogen
(plus 11% Helium)
One Million Miles

6. What Stops the Fall? Gravity Gets Stronger As Material Gets More Dense
R smaller implies F greater
The smaller it gets, the faster it falls in!
Why doesn’t it just become a black hole?
Or worse yet, a point-like singularity of mass?

7. Scaling
Scientists often do “scaling” do avoid all those large numbers.
For example, the Sun is ABOUT a million times the mass of the Earth and a hundred times the size.

8. Question
What is the surface gravity of the Moon in gees? R=.01Re, M=.25Me?
a).04
b)4
c)16
d).16

9. Answer d

10. Temperature Temperature is a Measure of the
Random Kinetic Energy per Particle
The faster the atoms move, the higher the temperature.
But we’re talking about random motion.
If they all move together, then the object moves.

11. Thermal Pressure Thermal Gas Pressure Balances Gravitational Pressure
Balloon
Every Time An Bounces Off
Edge of Balloon It Keeps It From
Collapsing
That’s Pressure
Pressure is Force per Unit Area

12. Pressure is Proportional to Temperature
Low Temperature
Atoms Move Slowly
High Temperature
Atoms Move Fast

13. A Star Is Held Up By Thermal Pressure From Below
Gravity
Outer Mass
Individual Atoms Don’t
Orbit Entire Inside of
Star Like This
They Jostle Each Other
But Effect Is The Same

14. Temperature Scales Fahrenheit – Celsius Kelvin
0=salt water freeze 100=human body
Celsius
0=pure water freeze 100=water boil (sea level)
C=(F-32)x5/9
Kelvin
0=absolute zero degrees between freeze and boil
K=C+273
-273C = 0K = Absolute Zero
At Absolute Zero Atoms Stop Moving

15. Thermal Pressure Pressure Is Proportional to Temperature x Density
P Pressure
V Volume
n # moles
R Constant
T Temperature (K)
Ideal Gas Law
Chemistry Style
P Pressure
V Volume
N # atoms
k Constant
T Temperature (K)
Ideal Gas Law
Physics Style
Pressure Is Proportional to Temperature x Density

16. A Star Always Balances Gravitational Pressure with Thermal Pressure
Pressure Balance
A Star Always Balances Gravitational Pressure with Thermal Pressure
At Each Point Inside
Thermal Pressure (Jostling)
Gravity

17. But We Have a Problem The Sun is Luminous Radiates Energy Into Space
Luminosity is Power Radiated -- ergs/second
The Energy Comes From Motion of the Atoms
Temperature Drops
What Happens When T Drops?

18. Luminosity Effect When T Drops Thermal Pressure Can’t Hold Off Gravity
The Sun Shrinks -- Radius Drops
Energy is Released as Gas Falls Deeper Into Gravity Field
Temperature Rises
Note – Loss of Energy Results in
a) Temperature Rise
b) Radius Decrease

19. But Wait A Minute… Isn’t the Sun Stable?
The Sun has been remarkably stable for 4 billion years as evidenced by geological records.
This collapse is the process by which the Sun coalesced.
But then it stopped. Why?
The Sun collapsed until a new source of energy offset the losses
to radiation.
NUCLEAR FUSION IT’S BURNING HYDROGEN
As long as it burns H at this rate, it will be stable.

20. Fusion Increases with T
As T in core of Sun increases so does energy production
Sun shrank steadily, with T rising until, about 10 million years
after it started to form, it reached its current size
There is a VERY fast increase in nuclear energy production
above 1,000,000K.
At 15,000,000K in the core nuclear power generated finally balanced
the luminosity from the surface.
That’s the equilibrium we are still in.

21. The Nuclear Core Envelope 1 Million K core 15x106K
Photosphere 5000K At Surface

22. Cosmic Composition H hydrogen 89% by number He helium 11%
O oxygen 0.1%
C carbon 0.06%
N nitrogen %
Pretty much the composition of the entire universe.
Sun and Jupiter have this composition
Earth does not.

23. Fusion vs. Fission Fusion: Atoms unite and release energy (Fuse)
New atom must be no heavier than iron z=26
Fission: Heavy atoms split to release energy
Initial atom must be heavier than iron
WWII Nukes were fission bombs made of U and Pu
Sun works on FUSION of H into He

24. Proton-Proton Chain Bottom Line: H+H+H+H  He 1H1 +1H1  1H2 + e+ + n
1H2 +1H1  2He3 + g
2He3 +2He3  2He4 + 1H1 + 1H1
5x106 < T < 2x107K

25. CNO Cycle 6C12 +1H1  7N13 + g 7N13  6C13 + e+ + n
7N14 +1H1  8O15 + g
8O15  7N15 + e+ + n
2x107 < T < 108K
7N15 +1H1  6C12 + 2He4
Net: 1H1 +1H1 + 1H1 +1H1  2He4 + 2e+ + 4g +2n
hydrogen -> helium + energy

26. Triple-a Reaction 2He4 +2He4  4Be8 + g 4Be8 +2He4  6C12 + g
T < 108K
4Be8 +2He4  6C12 + g
Must be very dense for this to work
Be8 decays back into helium very quickly
unless struck by another He4
Too low density
in Big Bang
Net: 2He4 +2He4 + 2He4 +2He4  6C12 + 2g
helium -> carbon + energy

27. Solar Schematic

28. Seen by Ancient Persians
Sunspots
Seen by Ancient Persians
(and me!)

29. Groups of Sunspots

30. Solar Corona Visible in Eclipse

31. The Sun Viewed in X-rays

32. X-ray Movie

33. X-ray Loops

34. Magnetic Structure

35. Dynamic Structure

36. Solar Turbulence

37. Differential Rotation
Rotates in 25 days at Equator
28 days Mid Latitude
30 days Poles
Rapidly Twists Up

40. Sunspots Erupt in Groups

41. During mid 1600’s sunspots became non-existent
Sunspot Cycle
During mid 1600’s sunspots became non-existent
Maunder Minimum

42. Solar Wind 5x105K Corona 2x106K Photosphere 5500K
Transition Region 105K
Chromosphere 104K
Photosphere 5500K

43. Solar Wind Passes Earth

44. Summary: Sun as a Star Formed from cloud 4.6x109 years ago
Collapsed to present size
stabilized by nuclear reactions
Emits 4x1026 W
Runs on proton-proton chain and CNO cycle
Now 20% brighter
Turbulent upper envelope
Magnetic Fields from Differential Rotation
Sunspots, Corona, Solar Wind
Activity Cycle 11 years

45. STARS

46. Stars are grouped in Galaxies
Sun and all the stars we see are part of Milky Way Galaxy
We all orbit a common center
Sun is 3x1020m from center of MW
You are here
Each star orbits
center
Disk Stability Again

47. Distances to the Stars
Closest Star, Proxima Centauri is 4x1016m away. (Alpha Cen ~4.3x1016m)
Need a more convenient unit

48. The Light Year
Light Travels at 300,000km/s (186,000miles/s = 3x108m/s)
That’s one foot per nanosecond
One Year is 3.15x107 seconds long
In one year light travels 3.15x107x3x108 = 1016m
This is the definition of a light year. Prox Cen is at 4ly.

49. Question
There’s a big black hole in the Center of the Milky Way at a distance of 3x1020m. How long does it take for its light to reach us?
A) 3years
B) 30 years
C) 300 years
D) 3000 years
E) 30,000 years

50. Question
There’s a big black hole in the Center of the Milky Way at a distance of 3x1020m. How long does it take for its light to reach us?
A) 3years
B) 30 years
C) 300 years
D) 3000 years
E) 30,000 years

51. The Parsec
Astronomers use the parsec as a measure of distance
1pc = 3ly
1pc = 3x1016m
Origin of parsec comes from method of measuring distance

52. Each Star Orbits the Center

53. How Long does that Take?
Takes about a hundred million years to circumnavigate the galaxy

54. Star Names Arabic Names Constellations
Antares, Capella, Mira, etc.
Constellations
a Orionis, b Cygni, … then 49 Ori, 50 Ori, etc.
Catalogues HD80591, SAO , etc
RA and Dec – just position in the sky

55. Proper Motion 2003 All stars move Nearby stars move faster
Appear to move against fixed field
Can Take Many Years
Use Old Photographic Plates
1900

56. Parallax
I year cycle

57. The Parsec 1 parsec 360 degrees in circle 60 arcminutes per degree
1AU
1 arcsecond
360 degrees in circle
60 arcminutes per degree
60 arcseconds per arcminute
200,000AU = 1 parsec = 3x1016m
parsec parallax second

58. Question
Based on the definition of a parsec , if star A has a parallax of 0.5 arcseconds and star B has a parallax of 0.75 arcseconds which one is farther from the Earth?
A. Star B is farther away because it has a higher parallax
B. Star A is farther away because it has a lower parallax
C. All stars are the same distance away from the Earth
D. It is impossible to tell from this information.

59. Question
Based on the definition of a parsec , if star A has a parallax of 0.5 arcseconds and star B has a parallax of 0.75 arcseconds which one is farther from the Earth?
A. Star B is farther away because it has a higher parallax
B. Star A is farther away because it has a lower parallax
C. All stars are the same distance away from the Earth
D. It is impossible to tell from this information.

60. Measure Parallax
distance to a star in parsecs = 1/(parallax in arcseconds)
e.g. measure .04” parallax, then distance is 25pc
Measuring Parallax was first successful way to measure
distances to stars after centuries of trying
Took high speed photography in 1890’s to do it.

61. Question
The parallax of an observed star is 0.1 arcseconds, how many lightyears is it away from Earth?
a. 1 light year
b. 3 light years
c. 10 light years
d. 30 light years
e. 75 light years

62. Question
The parallax of an observed star is 0.1 arcseconds, how many light years is it away from Earth?
a. 1 light year
b. 3 light years
c. 10 light years
d. 30 light years (10parsecs)
e. 75 light years