1. Chapter 14
Our Star

2. p. 111

3. Ideas About the Sun’s Energy Production
Pre-19th century
Fire due to wood or coal
Ruled out after we knew the actual size and distance to the Sun
19th Century
Gravitational Contraction
Could maintain energy output for 25 million years
Ruled out when we discovered that the earth was far older than 25 million years

4. So Why Does the Sun shine?
E=mc2
Conversion of matter into energy through nuclear fusion
For fusion to occur, the Sun must generate extreme temperature internally without flying apart

5. Nuclear Fission vs. Nuclear Fusion
Big nucleus splits into smaller pieces.
(Example: nuclear power plants)
Fusion
Small nuclei stick together to make a bigger one.
(Example: the Sun, stars)

7. The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus.

8. The energy comes from the fact that the Helium nucleus has slightly less mass (0.7%) than the sum of two protons and two neutrons.
It appears as kinetic energy and the energy of the gamma-ray photons

9. 4 protons have 0.048*10-27 kg (= 0.7 %) more mass than 4He.
Proton-Proton Cycle
Need large proton speed ( high temperature) to overcome Coulomb barrier (electromagnetic repulsion between protons).
Basic reaction:
4 1H  4He + energy
4 protons have 0.048*10-27 kg (= 0.7 %) more mass than 4He.
T ≥ 107 K = 10 million K
Energy gain = m*c2
= 0.43*10-11 J
per reaction.
Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second, to resist its own gravity.

10. Thought Question
What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy?
A. The core would expand and heat up slightly.
B. The core would expand and cool.
C. The Sun would blow up like a hydrogen bomb.
The solar thermostat keeps burning rate steady.

11. Gravitational Equilibrium
Contraction of the sun due to its own gravity provided the energy that heated the core as Sun was forming.
Once nuclear fusion began, it generated pressure that halted the contraction.
The Sun exists is a state of gravitational equilibrium (also called hydrostatic equilibrium

12. Solar Thermostat
Decline in core temperature causes fusion rate to drop, so core contracts and heats up.
Rise in core temperature causes fusion rate to rise, so core expands and cools down.

13. We will investigate what happens after that in a later chapter
Calculations show that the Sun has enough hydrogen in its core to maintain gravitational equilibrium for 10 billion years.
We will investigate what happens after that in a later chapter
Good Sun movies to download:
Images compiled from Owen Gingerich’s copy of the first edition of Istoria e Dimostrazioni
Wave_fade.mpg photosphere:chromosphere:corona, optical:UV:X-ray
C2_1mth.mpg

14. Sun’s Basic Structure
Insert TCP 6e Figure 14.3

15. Core:
Energy generated by nuclear fusion
~ 15 million K

16. Radiation Zone:
Energy transported upward by photons until they reach a region where the temperature has dropped to about 2 million K. Photons get absorbed there.

17. Convection Zone:
Absorbed photons heat the gas at the bottom of the convection zone. Energy is transported upward by rising hot gas

18. Photosphere:
Visible surface of Sun
~ 5800 K

19. Chromosphere:
Middle layer of solar atmosphere
~ 104–105 K

20. Corona:
Outermost layer of solar atmosphere
~1 million K

21. Solar wind:
A flow of charged particles from the surface of the Sun

22. Radiative Diffusion
Most of the energy from the Sun works its way out from the core as photons. They execute a random walk to make their way out.
It takes thousand of years for energy liberated in the core to get to the top of the radiation zone.

23. Energy Transport in the Sun
Follow a single gamma ray as it heads toward the surface.
The ray is scattered by electrons in the core until it reaches cooler gas
The cooler gas absorbs the gamma photon and emits two x-ray photons, each of less energy than the gamma photon
The x-rays migrate toward cooler regions where they are absorbed and emitted as still longer-wavelength photons.
This process repeats until the single gamma in the core has resulted in about 1800 photons of lower energy.

24. Convection Zone
Photons absorbed at the plasma near the surface heats the plasma and
results in a convection zone (rising hot gas) taking energy to surface.

25. The Photosphere: Granules
Bright central regions are hotter than the edges so they appear brighter.
Doppler shifts show that hot gas is rising in the center and sinking at the edges
Each granule is about the size of Texas
They last about minutes
Figure 7.2: (b) This model explains granulation as the tops of rising convection currents just below the photosphere. Heat flows upward as rising currents of hot gas and sinking currents of cool gas. The rising currents heat the solar surface in small regions that we see as granules.

26. Supergranules
Supergranules which are a little over twice Earth’s diameter, include about 300 granules each.
These supergranules are regions of very slowly rising currents that last a day or two.
They appear to be produced by larger currents of rising gas deeper under the photosphere.

27. Photosphere Spectrum
Below the photosphere, the gas is dense and hot and therefore radiates a continuous spectrum of light.
Atoms in the photosphere absorb photons of specific wavelengths—producing the absorption lines you see.

28. Chromospherre The chromosphere lies above the photosphere.
Visible to unaided eye only during a total eclipse
Pink color due to red, blue, and violet Balmer emission lines of hydrogen.

29. Chromosphere Spectrum
The chromosphere produces an emission spectrum.
Atoms in the lower chromosphere are ionized, and atoms in the higher layers of the chromosphere are even more highly ionized.
Can be used to determine the temperature of various parts of the chromosphere.

30. Temperature Profile
Just above the photosphere, the temperature falls to a minimum of about 4,500 K and then rises rapidly to the extremely high temperatures of the corona.

31. The Solar Corona
The outermost part of the sun’s atmosphere is called the corona, after the Greek word for crown.
You can see the inner parts of the corona during a solar eclipse.

32. Chronograph
Observations made with specialized telescopes called coronagraphs on Earth or in space can block the light of the photosphere and record the corona out beyond 20 solar radii.
Such images reveal that magnetic fields link the sunspots with features in the chromosphere and corona.

33. Coronal Spectrum Complex coronal spectrum
Sunlight reflected from dust particles produces a spectrum with absorption lines just like the sun
When sunlight from the photosphere is scattered off free electrons in the ionized coronal gas, it produces a continuous spectrum without absorption lines.
Fast moving electrons produce photons with a large Doppler shift smearing out the absorption lines
Emission lines of highly ionized gas are superimposed on this continuous spectrum.
Higher in the corona means more ionization – implies the temperature rises

34. Coronal Temperature Profile
Just above the chromosphere, the temperature is about 500,000 K.
In the outer corona, it can be as high as 2 million K or more.

35. Heating the Corona
Magnetic fields extend from the photosphere to the corona.
Turbulence in the photosphere whips the field around
As the gas of the chromosphere and corona has a very low density, it can’t resist movement in the magnetic fields.
This motion heats the gas

36. How we know what is happening inside the Sun?

37. We learn about the inside of the Sun by …
making mathematical models
observing solar vibrations
observing solar neutrinos

38. Mathematical Models
Use basic physics to develop equation to describe the measured properties of the Sun
Using a computer we can calculate the temperature, density, and pressure at any depth within the Sun
From these we predict the rate of fusion can predict many measurable properties of the Sun.
If these models correctly predict the known properties, then we gain confidence that we really do understand what is going on in the solar interior.

39. Helioseismology
Random motions in the sun constantly produce vibrations like resonance in organ pipes.
These are sound waves of very long wave period.
These vibrations are observed by Doppler shifts.
Interpretation requires a lot of data to reconstruct the subsurface features.
GONG, Global Oscillation Network Group, uses telescopes spread around the world to observe the sun continuously.

40. Data on solar vibrations agree very well with mathematical models of solar interior.
Download a movie to illustrate solar oscillations from:

41. Solar Neutrinos
Neutrinos created during fusion fly directly through the Sun.
Observations of these solar neutrinos can tell us what’s happening in core.
John Updike poem - permissions needed ?
Neutrinos interact very rarely with matter. Trillions pass through you every second. So they are very difficult to detect.

42. Searching for Neutrinos
Raymond Davis filled a 100,000-gallon tank with the cleaning fluid perchloroethylene (C2Cl4).
Theory predicted that, about once per day, a solar neutrino would convert a chlorine atom in the tank into radioactive argon.
This could be detected later by its radioactive decay.
Results showed only one event every 3 days
There were two possible explanations
We didn’t correctly understand how the sun and stars make their energy.
There was something about neutrinos that we did not understand.

43. Solar Neutrino Problem
Early searches for solar neutrinos failed to find the predicted number, but they were looking only for electron neutrons, the kind produced by fusion in the Sun
We discovered that some neutrinos change into other types (muon and tau neutrinos) in a process called oscillation.
More recent observations find the right number of neutrinos
Illustrates the great synergy between astrophysics and subatomic physics

44. This solution to the solar neutrino problem is exciting because neutrinos can’t oscillate unless they have mass.
Neutrinos were long thought to be massless.
However, if they have even a small mass, they are so common their gravity could affect the evolution of the universe as a whole.

45. Solar Activity

46. Solar activity is like “weather”.
Spicules
Sunspots
Solar flares
Solar prominences
All these phenomena are related to magnetic fields.
They can at times affect our daily lives.

47. Spicules are small plasma burst
from the photosphere into the chromosphere.
They are created by periodic
sound waves leaving the sun.
The plasma is concentrated to a small thread like jet due to strong magnetic fields.
that last 5 to 15 minutes

48. Sunspots Cooler than other parts of the Sun’s surface (4000 K)
Regions with strong magnetic fields
They appear dark by comparison

49. Observing the Sun
In the early 17th century, Galileo observed the sun and saw spots on its surface.
Day by day, he saw the spots moving across the sun’s disk.
These are sunspots.
He rightly concluded that the sun was rotating.

50. Sunspot Rotation

51. The TRACE satellite can detect the hot gas trapped in the magnetic fields arching above the sunspot group. (NASA/TRACE
Sunspots tend to occur in pairs connected by magnetic fields, represented as lines in this drawing.

52. Magnetic Loops Sunspots tend to occur in pairs resembling a bar magnet
Sunspots tend to occur in pairs resembling a bar magnet
Polarity different on opposite sides of the solar equator
At the end of an 11-year cycle, the new spots appear with reversed magnetic polarity.
Magnetic field lines

54. Zeeman Effect
We can measure magnetic fields in sunspots by observing the splitting of spectral lines.

55. Solar Prominences
Gas in the chromosphere and corona can become trapped in the magnetic fields of sunspots resulting in large prominences that can rise far above the surface.

56. Relatively cool gas (60,000 – 80,000 oK)
Prominences
Relatively cool gas (60,000 – 80,000 oK)
May be seen as dark filaments against the bright background of the photosphere
Looped prominences: gas ejected from the sun’s photosphere, flowing along magnetic loops

57. Prominence in UV
Hot plasma trapped in the magnetic field loop above the chromosphere into the lower corona.
In the visible region they look pink due to the three visible Balmer lines. From above these look dark against the surface and are called filaments

58. Filaments

59. Solar Flares
Magnetic fields can become ‘twisted.’ At some point the field will suddenly reorganize itself and in the process release excess energy heating the plasma to 100 million K releasing a burst of X-rays and accelerating charged particles to nearly the speed of light

60. The Solar Wind Some magnetic field lines extend far out into space
Gas from the solar atmosphere follows along the magnetic fields that point outward and flows away from the sun in a breeze called the solar wind.
This X-ray photograph shows dark regions called coronal holes from which much of the solar wind escapes.

62. Auroras
Solar winds, guided by the Earth’ magnetic field, and low density gases at ~130 km above the surface create conditions like the gas discharge tubes.

64. Coronal Mass Ejections
Flares and other solar storms sometimes eject large numbers of charged particles from the solar corona.
Called a coronal mass ejection
If aimed toward Earth, they can create a geomagnetic storm,
Particularly strong aurora
Disrupt power delivery
Interfere with communications
Damage satellites

65. Variation in Sunspot Activity with Time: The Sunspot Cycle
Averages 11 years, min to min, but varies from 7 to 15 years
Sunspot minimum
Few, if any, sunspots visible
We just emerged from one a year or so ago
Sunspot maximum
May see dozens of sunspots simultaneously
The frequency and intensity of solar flares, prominences, and CME’s, follow the sunspot cycle

66. Maunder Butterfly Diagram
Sunspots appear at higher latitudes (farther from the equator) early in the cycle, and at lower latitudes later in the cycle.

67. Maunder Minimum

68. The Sun’s Magnetic Cycle
The Babcock mode
As the electrons in the ionized gas carry any magnetic field. The field is is ‘frozen’ into the gas.

69. Where these magnetic tubes burst through the sun’s surface, sunspot pairs occur.
After about 11 years the field is so tangled that it begins to rearrange itself into a simpler structure, but with opposite polarity