The Sun 

1. The Sun 

2. Astro 129: Chapter 1a

3. Astro 129: Chapter 1a

4. The Solar Constant What is the Solar flux arriving at Earth? Use relation between luminosity and flux L = 3.9×1026 W, 1AU = 150×106 km What is the source of the Sun’s energy? Astro 129: Chapter 1a

5. The Source of the Suns Energy Kelvin-Helmholzcontraction doesn’t work Chemical reactions don’t work. To produce the observed solar flux from chemical reactions the sun would have burned out in about 10,000 years. (a chemical reaction releases roughly 1x10-19 Joules per atom) We need a process that can liberate more energy per unit mass than what can be achieved by chemical reactions. Albert Einstein discovered such a process. Astro 129: Chapter 1a

6. The Source of the Suns Energy There are two avenues by which fusion of hydrogen proceeds in stars. T ≈ T(T= 16 million K) proton – proton chain (example: 4 1H → 4He + neutrinos + gamma-ray photons) T >> TCNO cycle (a carbon nucleus absorbs protons and finally emits a helium nucleus) Astro 129: Chapter 1a

7. The Source of the Suns Energy Proton-Proton Chain Astro 129: Chapter 1a

8. The Proton-Proton Chain The proton-proton chain has four branches PP1, PPII, PPIII, and PPIV. The PPI and PPII branches produce about 86% and 13.9%, respectively, of the Sun’s energy. Proton-Proton Chain PPI Branch PPII Branch Astro 129: Chapter 1a

9. The Proton-Proton Chain We can summarize the thermonuclear fusion of hydrogen as follows: 4 1H 4He + neutrinos + gamma-ray photons Mass of 1H proton = mp = 1.672×10-27 kg Mass of 4He = mHe = 6.645×10-27 kg Dm = 4mp – mHe = 0.0435×10-27 kg How much energy is released for every PPI reaction? Hint use E = mc2 Proton-Proton Chain Astro 129: Chapter 1a

10. Theoretical Model of the Sun We expect fusion to occur mostly near the center of the sun where the temperature is high enough for fusion reactions to occur (T > 10 million K) We need to address the issue of the propagation of energy produced in the center to its surface. To learn about the interior of the sun scientist have followed the following approach: Create a model of the sun which contains as much of the physics as we know and fit this model to the observations. Proton-Proton Chain Astro 129: Chapter 1a

11. Theoretical Model of the Sun Model Assumption: Hydrostatic Equilibrium (A balance between the weight of a layer in a star and the force from pressure that supports it. Thermal Equilibrium (A balance between the input and outflow of heat in a system). As a consequence while the temperature in the solar interior is different at different depths, the temperature at each depth remains constant in time. Energy Transportvia radiative diffusion and convection) Proton-Proton Chain (FP = P/A, where P is pressure, A is area and FP is the force from this pressure) Astro 129: Chapter 1a

12. Theoretical Model of the Sun Proton-Proton Chain Convection is the movement of molecules within fluids (i.e. liquids, gases). In the suns convection zone hotter gas rises outward and cooler gas descends inwards thus facilitating the bulk transfer of energy outward. Radiative Diffusion is the process by which the gamma rays released in fusion reactions are absorbed in only a few millimeters of solar plasma and then re-emitted again in random direction and at slightly lower energy. It takes a long time for radiation to reach the Sun's surface. Estimates of the photon travel time range between 10,000 and 170,000 years.

13. Fitting Models of the Sun to Observations Observations: Surface Temperature (T = 5,800 K) Solar Luminosity (L = 3.9×1026 W) Gas pressure and density at the Sun’s surface are almost zero. Proton-Proton Chain The result is a model of how, T,L, pressure, mass and density vary as a function of distance from the Sun’s center. Astro 129: Chapter 1a

14. The Sun’s Oscillations The sun vibrates (oscillates) at certain frequencies. We can learn about the interior of the sun by studying oscillations on the surface of the sun. The science studying wave oscillations in the Sun is called helioseismology. The solar oscillations are thought to be produced by the motion of fluid (turbulence) in the convection zone. Proton-Proton Chain A simulated sound wave resonating in the Sun. The regions that are moving outward are colored blue, those moving inward, red. As the cutaway shows, these oscillations are thought to extend into the Sun’s radiative zone. Astro 129: Chapter 1a

15. The Sun’s Oscillations Waves in the sun are divided into three different types: Acoustic or pressure (p) modes, driven by internal pressure fluctuations within the sun; their dynamics being determined by the local speed of sound. Gravity (g) modes, driven by buoyancy, Surface gravity (f) modes, akin to ocean waves along the stellar surface. Proton-Proton Chain Different oscillation modes penetrate to different depths inside a star. Astro 129: Chapter 1a

16. The Sun’s Oscillations Discoveries made by helioseismology 1. Set limits on the amount of helium in the Sun’s core and convective zone. 2. constrained the thickness of the transition region between the radiative zone and convective zone. 3. The convective zone and the radiative zone rotate at different speeds, which is thought to generate the main magnetic field of the Sun by a dynamo effect. Proton-Proton Chain A simulated sound wave resonating in the Sun. The regions that are moving outward are colored blue, those moving inward, red. As the cutaway shows, these oscillations are thought to extend into the Sun’s radiative zone. Astro 129: Chapter 1a

17. Photon Transport from Sun’s Core Energy radiated by photons from the surface of the sun originates from the suns core. To get to the surface, however, energy is transported via various mechanisms thus making it difficult to tell how it was actually produced in the core. Photon transport from the core to the surface: Photons created in the Suns core from hydrogen fusion diffuse towards the Suns surface (~ 170,000 years to get out). Beyond the radiative zone energy is transported to the surface through convection (bulk transfer of plasma). At the surface photons are mostly radiated out through blackbody radiation. How do we obtain direct evidence of hydrogen fusion in the Suns core? Proton-Proton Chain

18. Solar Neutrinos A direct method of proving that hydrogen fusion is occurring in the Suns core relies on detecting neutrinos that are released during nuclear fusion. 4 1H 4He + n+ g A neutrino is a subatomic particle with no charge and very little mass. It can travel through ordinary matter almost undisturbed. More than 50 trillion solar electron neutrinos pass through the human body every second. Proton-Proton Chain The Sudbury Neutrino Observatory

19. Solar Neutrino Detections On a rare occasion a neutrino will strike a neutron and convert it to a proton. Using this process Raymond Davis designed and built a neutrino detector that used 100,000 gallons of perchloroethylene (C2Cl4). Detection Method: Occasionally a neutrino will strike the nucleus of one of the chlorine atoms (37Cl) in the cleaning fluid (C2Cl4) and convert one of its neutrons into a proton, creating a radioactive atom of argon (37Ar). Atomic number of Cl = 17. Atomic number of Ar = ? Proton-Proton Chain

20. Solar Neutrino Detections Proton-Proton Chain John Bahcall calculated the expected detection rate of neutrinos but Davis's experiment turned up only one third of this figure. The experiment was the first to successfully detect and count solar neutrinos, and the discrepancy in results essentially created the solar neutrino problem. The Davis Experiment

21. Solving the Solar Neutrino Problem Where were the neutrinos detected in the Davis experiment coming from? The Kamiokande experiment in Japan led by physicist Masatoshi Koshibameasured the direction of the incoming neutrinos and confirmed that they emanated from the Sun. Proton-Proton Chain Kamiokande Observatory

22. Solving the Solar Neutrino Problem The Kamiokande experiment consisted of a large underground tank containing 3000 tons of water surrounded by 1100 light detectors. A solar neutrino would from time to time interact with an electron in the water molecule. The electron recoils in roughly the direction that the neutrino was travelling, so the electrons "point back" to the sun. About half of the predicted neutrinos were detected in the Kamiokande experiment. Proton-Proton Chain Kamiokande Observatory

23. Solution to the Solar Neutrino Problem There are actually three types of neutrinos (ne, nm, nt) but only ne is produced in the sun. The Davis and Kamiokande experiments could only detect ne. It turns out that the ne can change into nm,ntduring its flight from the Sun to the Earth. The change between neutrinos is called neutrino oscillations. Proton-Proton Chain The Sudbury Neutrino Observatory

24. Solution to the Solar Neutrino Problem The solution to the solar neutrino problem came from the Sudbury Neutrino Observatory (SNO) in Canada. SNO could measure all three types of neutrinos. Instead of regular water SNO used heavy water. In heavy water each of the hydrogen nuclei contain a proton and a neutron. Process: neutrinos may knock a neutron out of one of the 2H nuclei of the heavy water molecule. When the neutron is recaptured by another nucleus it creates a flash of light that is recorded. Proton-Proton Chain The Sudbury Neutrino Observatory

25. The Sun’s Structure Proton-Proton Chain 1. Core 2. Radiative zone 3. Convective zone 4. Photosphere 5. Chromosphere 6. Corona 7. Sunspot 8. Granules 9. Prominence

26. Photosphere The Sun is gaseous throughout its volume. Below the photosphere the Sun is opaque to visible light. The photosphere is heated from below by energy streaming outward from the solar interior. Proton-Proton Chain The photosphere is the layer in the solar atmosphere from which the Sun’s visible light is emitted.

27. Photosphere Proton-Proton Chain Question: Why is the “thin” photosphere opaque? The photosphere is made primarily of hydrogen and helium, and has a density of about 10-4 kg/m3. Despite its low density the photosphere is opaque and we can only see through ~400 km of gas in the photosphere. The main reason for the photosphere being very opaque is the presence of negative H ions which are H atoms with an additionally bound electron. Answer: Negative H ions in the photosphere are efficient light absorbers.

28. Photosphere The high-altitude gas we observe at the Sun’s limb is not as hot (about 4,000 K) and thus does not glow as brightly as the deeper, hotter gas (5,800 K) seen near the disk center. The Sun’s spectrum is close to a blackbody with a temperature of 5,800 K. Superimposed on this spectrum are absorption lines produced by the cooler gas (4,000 K) in the outer photosphere. Proton-Proton Chain The edge or limb of the Sun looks darker because we are seeing the upper photosphere which is cooler. This effect is called limb darkening.

29. Photosphere Proton-Proton Chain The Photosphere has a blotchy pattern called granulation. A granule is about 1,000 km wide. Granulation is caused by convection currents. Where gas rises it is hotter and looks brighter and when gas sinks into the photosphere it is cooler and looks darker.

30. Photosphere Proton-Proton Chain Superimposed on the pattern of granulation are even larger convection cells called supergranules. A supergranule is about 35,000 km wide. Gases rise upward in the middle of a supergranule, move horizontally outward toward its edge, and descend back into the Sun. Supergranules detected in a Doppler image.

31. Chromosphere The chromosphere is a thin layer of the Sun's atmosphere above the photosphere, ~ 2,000 km deep. The chromosphere’s spectrum is dominated by the red Ha emission line at 656 nm and also contains emission lines of highly ionized He, Ca, and ionized metals. Tphotosphere: 6,000K(bottom) – 4,000K(top), over 400 km Tchromosphere : 4,000K(bottom) – 25,000K(top), over 2000 km Proton-Proton Chain During a total solar eclipse, the Sun’s glowing chromosphere can be seen around the edge of the Moon. The expanded area above shows spicules, jets of chromospheric gas.

32. Spicules Spicules are jets of gas that rise from the top of the chromosphere and are located near the edges of the supergranules. Each spiculelasts for about 15 min and together all spicules cover about 1% of the sun's surface. Proton-Proton Chain A spicule rising from the chromosphere.

33. Corona The corona is the outermost region of the Sun's atmosphere and extends from the chromosphere to several million kilometers. The corona is very faint compared to the photosphere but can be viewed during a solar eclipse. Proton-Proton Chain An image of the corona taken during an eclipse shows a number of streamers extending millions of km above the sun.

34. Corona Properties of the solar corona: Temperature ≈ 1 − 2×106 K Spectrum: emission-line spectrum of highly ionized gases (G corona). The corona is 10−12 times as dense as the photosphere. Example: One of the prominent lines found in the sun is at 530 nm produced by Fe XIV (has lost 13 e− of its 26 e−). Proton-Proton Chain Because there are so few atoms in the corona, despite its high temperature, the total amount of energy in these moving atoms of the corona is rather low.

35. Corona Proton-Proton Chain

36. Corona Because of the large temperature of the corona many ions and electrons escape the suns gravitational pull and form the solar wind. The corona’s high temperature results in intense UV and X-ray emission. One of the major surprises was the discovery that the temperature of the chromosphere and corona increase with increasing distance from the sun. Proton-Proton Chain UV observations of the corona made by SOHO show coronal holes that emit much less UV and have lower temperature and density than their surrounding areas. The solar wind is stronger near these coronal holes.

37. Sunspots Proton-Proton Chain An isolated sunspot, (b) a group of sunspots Sunspots are temporary cool regions in the solar photosphere. Because of their relatively low temperature they appear dark. They have typical sizes of ~ 10,000 km across and have a lifetime of hours to years. TUmbra ~ 4,300 K and TPenumbra ~ 5,000 K. Estimate the ratio of the flux from the umbra to that from the photosphere assuming a similar emitting area.

38. Astro 129: Chapter 1a

39. The sun's rotation can be tracked by observing the rotation of sunspots. Observations show that a sunspot near the equator takes ~ 25 days to go around once whereas sunspots at larger latitudes take longer. The sun does not rotate as a rigid body. Different parts of the sun rotate at different velocities. This is referred to as differential rotation. Proton-Proton Chain Sunspotting

40. Astro 129: Chapter 1a

41. Sunspot Cycle Proton-Proton Chain The average number of sunspots varies with a period of ~ 11 years. The periodic change in the number of sunspots is called the sunspot cycle.

42. Sunspot Latitudes Proton-Proton Chain At the beginning of each sunspot cycle, most sunspots are found near latitudes 30° north or south. As the cycle goes on, sunspots typically form closer to the equator.

43. Zeeman Effect When an atom is placed in a magnetic field, the interaction between the field and the electron current shifts the atom’s energy. According to the principles of quantum mechanics, only certain orientations are allowed. An energy level is split into different energy levels, corresponding to the different orientations of a particular electron orbit. Proton-Proton Chain Zeeman showed that a spectral line splits when the atoms are subjected to an intense magnetic field. The more intense the magnetic field, the wider the separation of the split lines.

44. Sunspots have strong magnetic fields Proton-Proton Chain When Hale focused a spectroscope on sunlight coming from a sunspot, he found that many spectral lines appear to be split into several closely spaced lines. Hale’s discovery showed that sunspots are places in the photosphere that have strong magnetic fields.

45. Magnetograms Proton-Proton Chain A magnetogram is an image of the sun showing variations in strength of a magnetic field. It is based on the Zeeman effect. Hale also discovered that the Sun’s polarity pattern completely reverses itself every 11 years. The Sun’s magnetic pattern repeats itself only after two sunspot cycles, which is why astronomers speak of a 22-year solar cycle.

46. Preceding and Following Members of Sunspot Groups Proton-Proton Chain As a given sunspot group moves with the Sun’s rotation, the sunspots in front are called the “preceding members” of the group. The spots that follow behind are referred to as the “following members.” Preceding members in one solar hemisphere all have the same magnetic polarity, while the preceding members in the other hemisphere have the opposite polarity.

47. The Magnetic-Dynamo Model Proton-Proton Chain Magnetic field lines tend to move along with the plasma in the Sun’s outer layers. Because the Sun rotates faster at the equator than near the poles, a field line that starts off running from the Sun’s north magnetic pole (N) to its south magnetic pole (S) ends up wrapped around the Sun. Sunspots appear where the magnetic field protrudes through the photosphere.

48. The Magnetic-Dynamo Model Proton-Proton Chain Preceding members of sunspots move towards the equator and following members of sunspots move towards the poles. When preceding members from the two hemispheres meet at the equator they cancel and when following members of sunspots reach the pole they initially cancel and then reverse the polarity of the pole. This explains the magnetic field reversal.

49. Rotation of the Solar Interior Proton-Proton Chain The solar rotation period (shown by different colors) varies with depth and latitude. The surface and the convective zone have differential rotation. Deeper within the Sun, the radiative zone seems to rotate like a rigid sphere. Rotation of the Solar interior

50. Magnetic Reconnection Proton-Proton Chain Plasma can easily flow along magnetic field lines but takes more time to diffuse across field lines. Occasionally the magnetic field lines protrude through the suns surface (near the edges of supergranules) forming giant coronal loops. Plasma flows along these loops. Whenever magnetic field lines reconnect a large amount of energy is released that heats up the surrounding plasma to high temperatures.