1. Star Formation Our Sun formed billions of years ago
We see evidence that star formation is a constant process

2. Star Formation Star formation starts with Dark Dust Clouds
Relatively Dense, but very cool
Cloud starts to contract under its own gravity
Contraction causes heating
Nuclear Reactions begin
Many things oppose the gravitational contraction
Heat
Rotation
Magnetism

3. Stage 1: Interstellar Cloud
DDC contracts
Very large (tens of parsecs)
Very massive (thousands of Sun masses)
Very cool (~10K)
Internal cloud pressure initially balances against gravity
Something happens for it to become unstable
Star explosion, formation
Cloud begins to fragment

4. Stage 2: Collapsing Cloud
The fragmented cloud is about 100x larger than our solar system
It is still very cool because it radiates away its heat out into space
The very center is the warmest part ~100 K
Eventually the cloud will contract enough (becoming dense enough) where it will trap most of the heat generated

5. Stage 3: Fragmentation Ceases
At this point the cloud is the size of our solar system
The gas at the center is very dense
Radiation cannot escape
Temperature increases to ~10,000 K
This region is called a Protostar
Protostar is still contracting
Internal pressure cannot yet counteract gravity

6. Stage 4: A Protostar
Protostar’s temperature increases with contraction
Reaches ~1,000,000 K
(10,000,000 K needed for nuclear ignition)
Has a luminosity 1000 times greater than our Sun
No nuclear reactions
Very Large
Releasing gravitational potential energy as it shrinks
Can plot it on H-R diagram

7. Stage 4: A Protostar
Protostar’s temperature increases with contraction
Reaches ~1,000,000 K
(10,000,000 K needed for nuclear ignition)
Has a luminosity 1000 times greater than our Sun
No nuclear reactions
Very Large
Releasing gravitational potential energy as it shrinks
Can plot it on H-R diagram

8. Stage 5: Protostar Evolution
The Protostar is very hot – the gas is at high pressure
But it still isn’t enough to counteract gravity
The Protostar continues to shrink
This is called the “T Tauri” phase

9. Stage 5: Protostar Evolution
The Protostar shrinks to about 10X the size of the Sun
Central Temp = 5,000,000 K
Not enough for fusion
Contraction slows
Harder for gravity to compress hotter gas

10. Stages 1 – 5

11. Stage 6: Newborn Star 10 million years have passed
The Protostar has shrunk down to almost 1 solar radius
Its central temperature reaches 10,000,000 K
Fusion of Hydrogen begins
It is still not on the main sequence

12. Stage 7: The Main Sequence
For 30 – 50 more million years, the new star continues to contract
Eventually gravity is exactly balanced by the outward pressure of the Nuclear reactions
The star also exactly balances the energy it produces by fusion and the energy it radiates out
The core is at 15,000,000 K and the surface is at 6000 K.
It will stay like this for 10 billion years
This takes long? Full grown in 4 months!

13. Other Mass Stars How do stars of other masses evolve?
The basic steps are the same, but they will settle on the main sequence at different points.
But then why is main sequence so broad?
Stars have slightly different compositions

14. Failed Stars
Some of the gas clouds that could form stars don’t. Why not?
They contract under gravity
They get hot
Where does it go wrong?
The gravity crushes the cloud, but not enough to ignite thermonuclear fusion
There just isn’t enough mass to create a large enough gravitational force

15. Failed Stars These failed stars are called Brown Dwarfs
Brown dwarfs are less than 0.08 solar masses,
(80 times the mass of Jupiter)
The lower limit for Brown Dwarfs is 12 Jupiter masses
Anything less massive is just called a planet
These objects eventually become cold balls of gas
We estimate there are 100 billion of these in our Galaxy

16. Very High Mass Clouds
So the lower limit for stars is ~ 0.08 Solar Masses
Is there an upper limit?
We thought it was about 100 solar masses
There is so much gravitational force that the nuclear fusion begins quickly
Then there is so much nuclear fusion that it overcomes the force of gravity holding the star together
These stars blow themselves up early in their lifetime

17. Very High Mass Clouds
In 2010 Astronomers announced that they found a star with a birth weight of about 320 solar masses
Much larger than models predict could be stable
Current mass is about 265 solar masses

18. Main Sequence Stars How do main sequence stars know how big to be?
What properties dictate temperature?
Will the star always remain this way?

19. PVT Thermostat If  then Q  T  P  V     
Stars have P, V, T, , and  values
 is the rate of nuclear reactions
They have a self-regulating feature to balance the internal and external forces
If  then Q  T  P  V     

20. Stellar Evolution
Stars have only two significant regions during their evolution
Core – where the fusion is taking place
Envelope – an inert layer of gas
There is no convection between core and envelope
As Hydrogen gets turned into Helium, the He will be inert (not hot enough to fuse)
Later in the star’s life it runs low on Hydrogen

21. Stellar Evolution

22. The Active Sun The Sun seems steady and predictable
The luminosity (energy output) is nearly constant
The surface activity of the Sun is changing and unpredictable
Doesn’t affect the evolution of the Sun
Greatly affects the Earth

23. Sunspots Sunspots are dark areas on the Sun’s surface
Typically about 10,000 km across
Umbra – dark center
Penumbra – grey surroundings
Due to temperature change

25. Sunspots Sunspots appear randomly, in different numbers
However, there is an 11 year cycle

26. Sunspots
Sunspots are caused by magnetic field lines blocking convection
The magnetic field is locally strong and breaks through the surface
If this happens so we see it at the side of the Sun, it is a prominence

27. The Sun and Earth These factors affect the climate of the Earth
Luminosity actually decreases slightly when sunspots are absent
“Maunder minimum” was a period of solar inactivity from 1645 – 1715
May be linked to the “Little Ice Age” in Europe
We haven’t been studying these things long enough to conclude the effects yet!

28. The Sun and Earth

29. The Sun and Earth

30. The Active Sun Prominences eject hot solar gases
They follow magnetic fields
Gas cools and falls back into the Sun
Releases massive energy
1025 Joules
1 Billion years of Earth’s energy production

31. The Active Sun Solar Flares are more energetic than prominences
Caused by rapid release of magnetic energy
Equivalent to millions of 100-megaton bombs
Gas breaks free of magnetic field lines and escapes into space

32. The Active Sun
Coronal Mass Ejections are a release of a “magnetic bubble” of gas
This gas interferes with the normal solar wind and can interact with the Earth’s magnetic field
Imparts a large amount of energy and disrupts communications

33. Energy = Mass X (speed of light)2
Nuclear Fusion
The Sun (and all other stars) convert mass directly into energy according to Einstein’s equation:
E = mc2
Energy = Mass X (speed of light)2
The mass of individual atoms is very small, and sub- atomic particles (protons, neutrons) is even less
Usually measured in atomic mass units (u) instead of kilograms

34. E = mc2 = (1.66x10-27 kg)(3x108 m/s)2 = 1.5 x10-10 J
Nuclear Fusion
1 atomic mass unit = × kilograms
Converting 1 amu directly to energy:
E = mc2 = (1.66x10-27 kg)(3x108 m/s)2 = 1.5 x10-10 J
Doesn’t seem like a lot, does it?
One kg of mass transformation equals 9x1016 J!
The United States uses approximately 4,000,000 Giga Watt hours per year. (1.5x1019 J)
That is only 167 kg of mass turning into energy!
Hiroshima bomb only turned 1 gram of mass into Energy

35. Nuclear Fusion Similar electrical charges repel
Protons are both positively charged
However, if they have enough energy they can overcome the repulsive force

36. Nuclear Fusion
Protons fuse to Deuterons, Deuterons fuse with protons to form Helium-3, Helium-3 fuses to form Helium-4

37. Nuclear Fusion Mass of a Hydrogen atom (proton): 1.008178u
Mass of Helium-4:
Does the mass of Helium-4 equal the sum of its parts (4 Hydrogen atoms)?
= ?
No. The parts are more than the whole. Where did the mass go?

38. Nuclear Fusion The missing amount of mass is called the Mass Defect
The Mass Defect is converted directly to energy
This energy represents the binding energy of the nucleus
For low atomic masses, a larger nucleus is more stable and therefore lower energy
Fusion releases energy
For large atomic masses a smaller nucleus is more stable
Fission releases energy

39. Nuclear Fusion
By looking at the luminosity of the Sun, we can calculate that it gives off 3.9x1026 W
The mass difference between He and 4 H is amu
This equals an energy of 4.3x10-12 J
At this rate 600 million tons of Hydrogen must be converted into Helium every second

40. Stellar Evolution
Because there is no convection between the core and envelope, there is no Hydrogen re-supply
The Helium builds up in the core
With less nuclear reactions:
The temperature goes down
The pressure goes down
Gravity starts to win vs. pressure
The star contracts

41. Stellar Evolution As the star contracts, it will heat up
(Gravitational PE conversion)
Temp well above 10 Million K
Not hot enough for HE fusion
100 Million K
Hydrogen shell of the core burns fast and furious
Increased energy causes star to “puff” outward

42. Stellar Evolution Stage 8 Star The star’s surface temperature drops
Luminosity increases slightly
Radius increases by ~3x
On its way to becoming a Red Giant
Will take ~100 million years
Envelope expands due to increasing gas pressure while core is shrinking

43. Stellar Evolution Stage 9: The Red Giant Branch
The star grows to about 100x its main sequence size
The surface temperature stays constant
Cooler surface is becoming opaque to interior radiation
Luminosity is very high because of its size
While the outer shell has grown, the core continues to shrink
It is mostly non-burning Helium “ash”
No pressure to counteract gravity
Temperature increases to above 100 Million K

44. Stellar Evolution Stage 10: Helium Fusion
Two Helium atoms collide and create Beryllium-8
Triple Alpha Reaction
Beryllium-8 is very unstable
Usually breaks apart in s
Because of high density in core, Be-8 will fuse with a Helium nucleus to form Carbon-12

45. Stellar Evolution The core is developing layers like an onion
Hydrogen is fusing in outer core shell
Helium is fusing in middle layer
Inert Carbon ash is
building up in its core
Not hot enough to fuse
into another element
Gravity is continuing to
compress the core
The core is getting hotter

46. Stellar Evolution
The core will shrink until it reaches “electron degeneracy”
Ionized electrons are swimming around in the core
At the degeneracy point gravity has contracted the core so much that individual electrons “touch”
At this point gravity can’t contract it any more
The core is supported by this electron “pressure”
Temperature continues to increase

47. Stellar Evolution The core temperature is rising
The PVT Thermostat should result in a pressure increase, increasing the volume of the star
However the pressure is fixed
Pressure of the electron degeneracy
This breaks the PVT Thermostat
Helium fusion is running rampant
Dumps a lot of energy into the core: Helium Flash

48. Stellar Evolution
Helium Flash is an explosion that expands the star’s core outwards
How can the explosion overcome gravity?
The Helium Flash creates a lot of photons
Results in a large “Radiation Pressure”
This pressure overcomes gravity
Core becomes “normal” again, pressure – gravity equilibrium is re-established

49. Stellar Evolution Hydrogen and Helium continue fusing
As a result of the Helium flash the core cools
This reduces energy output
It moves on the H-R diagram
Where it goes depends on mass
20-30% escapes

50. Stellar Evolution The carbon core continues to get larger
Hydrogen and Helium burning gets more intense
Temperatures get hotter and hotter
Stage 11
Star expands back out to the Giant Branch

51. Stellar Evolution
Gravity cannot compress the carbon core enough to raise the temperature enough to fuse Carbon
600 Million K required
Pressure is so high that the electrons are degenerate again
Density is 1010 kg/m3
A grape would weigh 1000 kg
The big problem is that the core no longer produces energy

52. Stellar Evolution
Outer layers are expanding due to heat convection from the core; the core is contracting
As the core runs out of fuel, it heats up and blasts out the envelope forming a Planetary Nebula
The name has nothing to do with planets

53. Stages 13 and 14 Its heat will eventually dissipate away into space
What is left at the center of the nebula is a White Dwarf
It is super-massive, and hot only due to stored energy
About the size of Earth
Mass is about half that of the Sun
Its heat will eventually dissipate away into space
At that point it will be a black dwarf
It is a cold, dark, massive portion of space

54. Death Path of a Star

55. Stellar Evolution

56. Higher Mass Stars
The main difference between a star like our Sun and a star of higher mass (~ 2.5 times greater) is that they can produce higher temperatures
More mass means greater gravity
Greater gravity means greater core compression
Greater compression means hotter temperatures
Hotter temperature means easier fusion

57. Higher Mass Stars
Above 2.5 solar masses helium fusion happens easily, because core does not become degenerate
There is no Helium Flash
Above 8 solar masses the temperature of a contracted core is hot enough to fuse Carbon (and heavier elements)

58. Stellar Explosions Low mass stars end in a fizzle
Higher mass stars can explode dramatically
Novae
Supernova Type I
Supernova Type II

59. Stellar Explosions Novae: Need a white dwarf and Evolving Red Star
Stars revolve around their common center of mass
White dwarf draws mass off of companion star
Accretion disk forms
Friction causes particles to fall inwards
Disk gets hot, gives off radiation

60. Stellar Explosions As gas falls onto surface of white dwarf:
Becomes degenerate
Hydrogen starts to fuse
Heats gas up even more
Causes more reactions
So many photons are created that there is an explosion
Accretion disk gets blown off of white dwarf
Great increase in luminosity
Can re-occur relatively quickly

61. Stellar Explosions Supernova Type I
These detonations are “hydrogen poor”
The process is essentially the same as a Nova
Chandrasekhar Limit is the difference
1.4 Solar masses
Carbon Fusion begins instantly, everywhere
NOTE: Collapse of Core due to gravity
Star detonates
No remnants are left behind

62. Stellar Explosions Supernova Type II
Stars with mass greater than ~8 solar masses
Star evolves as usual
The “onion” layers inside the star build to Iron
Due to high mass, fusion of these heavy elements is “easy”

63. Stellar Explosions Fusion Energy Release
Energy is released by fusing atoms as long as mass is converted to energy
From hydrogen to iron, the mass per nucleon decreases as atomic number increases
Above iron, mass per nucleon decreases
Fusion cannot continue

64. Nuclear Binding Energy

65. Stellar Explosions Making heavy elements
How do the plentiful elements heavier than iron form if fusion is an endothermic process at that point?
Neutrons are a byproduct of other nuclear reactions
Neutrons (no electric charge) are absorbed by a heavy nucleus
Creates an isotope

66. Stellar Explosions Neutron Capture
The isotopes created are usually unstable and decay in time (minutes to years) – Beta decay
When this happens in evolving stars, the process is relatively slow so it is called the s-process
This explains the synthesis of non-radioactive elements

67. Stellar Explosions Back to the Type II
As the high mass star runs out of fuel, the large (3X+ the mass of the Sun) core collapses under the force of gravity
Core shrinks  heats up  becomes degenerate
Core gets so hot that photons get enough energy to destroy an iron nucleus
The core is now a soup of protons, neutrons, and electrons

68. Stellar Explosions Photoelectron Dissociation
Electrons in this soup are moving near the speed of light
Fast enough to smash into a proton and result in a neutron (Reverse Beta decay)
Neutrinos are created in this decay, and there are so many they blast off the envelope of the star
ALL THIS TAKES 1 SECOND

69. Stellar Explosions
The blast can be a billion times brighter than our Sun
It will outshine the galaxy it is in
In 1054 Chinese Astronomers witness the Supernova explosion we know as the Crab Nebula
It outshone Venus
It could be seen in the day
This lasts for ~ a month

70. Stellar Explosions
What happens to the star after the supernova explosion? Is there anything left?
In a Type II Supernova just the envelope is blown off, the core remains
What that core becomes will depend on its mass

71. Stellar Explosions
If the remaining core is between 1.4 and ~3 solar masses, the neutrons are compressed to the degeneracy
The result is a Neutron Star
They are approximately 10 – 20 km in diameter
One teaspoon would weigh a billion tons!

72. Black Hole Sun A single off of Soundgarden’s ’94 album Superunknown
If the star has a core with a mass greater than about 3 solar masses, the Supernova remnant may become a Black Hole
Above about 3 solar masses gravity crushes the degeneracy pressure and the core collapses to infinite density
This will make the gravity immediately near the star huge

73. Black Holes
By compressing a large amount of mass into such a small space, the gravity is so large that:
Light passing by is “gravitationally Doppler shifted”
Space time is significantly warped
The escape velocity is faster than the speed of light

74. Black Holes
Schwarzschild Radius: how small you would have to squeeze a mass down to for it to become a black hole
Event Horizon: the point near the black hole where you cannot escape its gravitational pull
You cannot see “events” beyond this horizon because the light cannot make it to you

75. Black Holes We have to look for the gravitational effects:
If light can’t escape a black hole, then how do we find one?
We have to look for the gravitational effects:
Look for a binary star system where you can only see one visible object
Look for a binary star system where one star is accreting mass to the other one
Measure the effect on other nearby stars
Look for binaries that are black hole-black hole or black hole-neutron star

76. Chapter 16 Test 10 Multiple Choice 5 Free Response
Some from text
5 Free Response
Answer 4
The mass of a single Hydrogen atom is amu. The mass of one Helium atom is amu. Four Hydrogen atoms are fused together to produce one Helium atom.
How much energy is released by the fusion of 4 H atoms into 1 He atom?
The Sun produces 3.9 x 1026 Joules of energy every second by converting Hydrogen to Helium. If an oil tanker carries 4.6x1033 Hydrogen atoms in its oil, then how many tankers would the Sun need to consume every second?

77. Chapter 16 Test
Nuclear reactions occur within the core of a star at tremendous temperatures and pressures. Explain what occurs in the core on an atomic level. What happens to an atom made up of many protons, neutrons, and electrons? What causes the particles to fuse together? How is energy released and where does it come from?
Describe the properties of the Radiative Zone of the Sun. This zone is cooler than the core, how does this affect the atoms found there? How is energy transmitted through this zone?

78. Chapter 16 Test
Describe what happens in the Convective Zone of the Sun. What is the dominant form of heat transfer (and describe its properties)? How quickly does energy transfer through this zone?
What is the significance and main properties of the Photosphere? What is the main method of energy transport?