Objectives

2. Objectives • Source of the Sun’s energy. • Internal structure of the Sun. • How do we find out the properties of the Sun’s interior? • Evidence for Thermonuclear reactions. • What is solar wind? • Sunspots and their relationship with magnetic field. • Eruptions in the atmosphere of the Sun.

4. Structure of Sun • Size: about 100 times diameter of Earth • Inner parts • core • radiative zone • convective zone • Outer parts • photosphere • chromosphere • corona • The Sun as a Star: the Sun is a typical star, in terms of mass, size, surface temperature, chemical composition.

5. Sun’s Energy Source • The Sun is the main source of light and heat in the solar system. • Without the light (energy) from the Sun, there would be no life on Earth. • The Sun closely approximates a blackbody with a surface temp. of 5800K. • Emits radiation of all wavelengths, with peak emission in the visible region of the EM spectrum.

6. Sun’s Energy Source • Sun’s size and its extremely high surface temperature helps explain this tremendous output of energy. • Sun’s luminosity: L= 3.29 x 1026 watts • i.e. the Sun produces 3.26 x 1026 Joules of energy per second • A typical reading bulb produces 100 watts, (i.e. 102 joules of energy per sec).

7. Sun’s Energy Source • How does the Sun keep its surface so hot? • And how does it keep shining, day after day, year after year, century after century? • what is the fundamental source of Sun’s energy? • For centuries, this was one the greatest mysteries in science. • It was complicated by the discovery in the 19th century that the Sun is at least 100 million yrs. old (current data: Sun is 4.5 billion yrs. old)

8. Sun’s Energy Source • Possible mechanisms: • Kelvin-Helmholtz contraction? • Sun’s high temperature is due to the compression of its interior gases caused by the gravitational contraction. • Calculations show this is viable only if the Sun is less than 25 million yrs. old • This answer does not work!

9. Sun’s Energy Source • Possible mechanisms: • Can we explain Sun’s energy as being produced by a process similar to ordinary burning - i.e a chemical reaction? • In this scheme the Sun will run out of stuff to burn in less than 10,000 yrs. • This answer does not work either! We need a “burning” process that produce much more energy per atom!

10. Sun’s Energy Source • 1905: Albert Einstein discovered the key to solving this century old mystery! • His special theory of relativity predicted that matter can be converted to energy according to the equation: • where m is the mass in kg and c = 3 x 108 m/s is the speed of light in empty space.

11. Sun’s Energy Source : Thermonuclear Fusion • What type of process will convert mass into energy? • Thermonuclear fusion: fusing together of two light nuclei to form a heavier nuclei. • nucleus1 + nucleus2  nucleus3 + energy • In such a process: • mass(nucleus1) + mass(nucleus2) > mass(nucleus3) • Missing mass is converted to energy according to Einstein’s mass-energy equation: E = m c2

12. Thermonuclear Fusion • Thermonuclear fusion can take place only at extremely high temperature and pressure: • Under these conditions atoms are completely ionized (i.e. stripped of all their electrons, and only the nucleus remain) • These conditions (high temp. and high press. are required for the positively charged nuclei to overcome the repulsive forces and fuse together.

13. The proton-proton chain • Under the extreme conditions at the center of the Sun, Hydrogen nuclei fuse together to form Helium nuclei, and in the process convert a small amount of mass into a large amount of energy. • Such extreme conditions exist at the Sun’s center.

14. The proton-proton chain • This nuclear reaction is called the proton-proton chain or Hydrogen burning. • These reactions affect the nucleus of atoms - hence the name nuclear reaction, as opposed to chemical reactions (ex: burning), that affect the electrons of atoms.

15. The proton-proton chain • 1H + 1H  2H +  +  (gamma ray photons)

16. The proton-proton chain • 2H + 1H  3He +  (gamma ray photons)

17. The proton-proton chain • 3He + 3He  4He + 1H + 1H

18. The proton-proton chain • We can summarize the thermonuclear reaction of hydrogen as follows: 4 H  He + 2 neutrinos + gamma ray photons. • Neutrinos() are subatomic particles with no charge and little or no mass. (We will neglect the mass of the neutrino). • Most of the energy released in the thermonuclear fusion is in the form of gamma-ray photons.

19. The proton-proton chain 4 H  He + 2  + -rays . • Amount of energy produced, (i.e. the energy of the gamma ray photons produced) is given by: • E = m c2 (note:  and photons are massless) • where m is the mass lost in one reaction: m = mass of 4 H nuclei- mass of 1 He nucleus • mass lost in one reaction = 4.8 x 10-29 kg. • 0.7% of the mass of the 4 H nuclei is lost • Energy produced E = 4.3 x 10-12 joule.

20. The proton-proton chain • Burning 1 kg of Hydrogen will produce 6.3 x 1014 joules of energy. • To produce the observed luminosity of the Sun 6 x 1011kg of Hydrogen is consumed per sec. • At this rate the Sun has enough Hydrogen to keep “burning” for 5 billion years more. • The Sun has existed for 4.5 billion yrs. • The Sun is a middle aged star!

21. A theoretical model of the Sun • For thermonuclear fusion to take place the temperature has to be greater than 107 K (T >10 million Kelvin). • The temp. of the Sun’s visible surface is 5800K. • H. burning must take place in the interior. • Where does it take place? • How does the energy produced in the interior make its way to the surface?

22. A theoretical model of the Sun • To answer these questions we need to understand the conditions of the Sun’s interior. • Since we cannot send a probe into the Sun, astronomers use laws of physics to construct theoretical models of the Sun. • The main ingredient that go into building this model is that - the Sun is not undergoing any Dramatic changes • it is not expanding, or collapsing. • nor is it significantly cooling or heating up.

23. Pump it up…Hydrostatic equilibrium • The Sun has very strong gravity, but does not collapse upon itself due to a balance of inward and outward pressures. This balance is called hydrostatic equilibrium. • inward: gravity • outward: pressure from being hot. • heated gases expand.

25. Pump it up…Hydrostatic equilibrium • From previous picture we can see that the pressure must increase with increasing depth. • Hydrostatic equilibrium also tells us that the density of the gas has to increase with depth. • Also, since the pressure increases when you go deeper into the interior, so does the temperature. • because when you compress a gas the temperature tends to rise.

26. Thermal equilibrium • At a given depth the temperature is constant. • it does not change with time. • This principle is called Thermal Equilibrium. • Since the Sun is in thermal equilibrium, then all the energy generated in the interior must be transported by some mechanism(s) to the surface, where it is emitted into space. • If too much or too little energy is transported, the Sun will get either hotter or colder with time.

27. Energy transport in the Sun There are two mechanisms by which energy is transported in the Sun: • Convection: Circulation of gases (fluids) between hot and cold regions. • Hot gases rises to the surface and the cooler gases sink to the interior.

28. Convection

29. Energy transport in the Sun • Radiative diffusion: Photons created in the core diffuse outwards. • In and near the core, the atoms are stripped off their electrons because of extremely high temperature. • They can’t capture photons. The deep interior is relatively transparent to radiation. • The result is a slow migration of the photons towards the surface

30. A Theoretical Model of the Sun • To develop a model of the Sun’s interior: • write down the physical ideas: hydrostatic equilibrium, thermal equilibrium and energy transport as a set of equations. • Solve these equations using computer simulations. • Check the answers with observed data (ex: Sun’s surface temperature, luminosity, etc.) to test the model. • Make other predictions.

31. Core temp. greater than 107 K T.N. fusion can take place

32. Sun’s Interior

33. Inner parts of the Sun • Core - where energy is produced (Thermonuclear fusion). • Temperature ~ 15 million kelvin. • Density ~ 160,000 kg/m3 ~ 14 times as dense as lead. • Pressure ~ 3.4 x 1011 atm ( 1atm = air pressure at sea level). • Sun’s energy is produced inside a region of 200,000 km (or 1/4th of the radius). • Outside this region the temperature is too low for thermonuclear fusion reactions to take place.

34. Inner parts of the Sun • Radiative zone • This region is comparatively transparent to EM radiation. • energy is carried away from core as electromagnetic radiation (photons) by the radiative diffusion mechanism. • However light has a tough time traveling through this region since the solar material in this region is very dense. • Therefore, it takes light 170,000 years for the energy created at the core to travel through the radiative zone (696,000 km) at a rate of 50cm per hour (20 times slower than a snails pace)

35. Inner parts of the Sun • Convective zone – • In this region the temperature is low enough for nuclei to join with electrons and form hydrogen atoms, and these absorb light very efficiently. • Gases are opaque to light, thus convection is the transportation mechanism. • Therefore, radiative diffusion is not an efficient method of energy transport in this region. • material(gas) convects energy (heat) to surface. • Hot gas goes up & cooler gas comes down.

36. Methods of probing the interior of the Sun • Helioseismology: measuring vibrations of the Sun as a whole. • The Sun vibrates at a variety of frequencies like a ringing bell. • These vibrations can be observed at the surface. • Studying these vibrations give scientists valuable information about the Sun’s interior.

37. Methods of probing the interior of the Sun • Solar Neutrinos: The only direct evidence of the thermonuclear reaction at the core. • Only the neutrino () survives the journey through the solar interior. • The  has energy but no charge an almost no mass. • Travels at the speed of light and interacts with nothing – goes right through the Earth. • With knowledge of neutrino physics scientists have built neutrino detectors to study these particles.

38. Methods of probing the interior of the Sun • Neutrino telescope: Super Kamiokande (Japan) • 3000 tons of purified water in a large underground tank. • 1000 light detectors to detect flashes of light that are emitted during rare neutrino collisions with electrons.

39. Outer parts of the Sun: The Solar Atmosphere • Photosphere - surface of Sun that we see. Radiates energy as continuous spectrum (5800K) • Chromosphere - low density gases form “atmosphere” - red color comes from hydrogen emission line. • Corona - outer part of atmosphere - extremely hot .

41. The Solar Atmosphere The Photosphere • Lowest of the of the 3 layers. • All the visible light that we see is emitted by this layer.

42. The Solar Atmosphere The Photosphere • Photosphere shines(emit radiation) like a nearly perfect blackbody at a temperature of 5800K. • The photosphere is heated from below by the energy streaming out from the solar interior. • Therefore, the temperature should decrease as you go upwards in the photosphere. • Spectral studies show that the temperature decreases to a “cool” 4800K.

43. The Solar Atmosphere The Photosphere • All the absorption lines in the Sun’s spectra are produced by atoms in this relatively cool layer absorbing photons with various wavelengths. • Photosphere consists of very low density gas, primarily Hydrogen & Helium. • Density ~10-4 kg/m3(.01% of Earth’s avg. density) • Although it is low density it is opaque to visible light. • We can only see 400km into the photosphere.

44. The Solar Atmosphere The Photosphere • When observing with a telescope (fitted with a special filter) we can see a blotchy pattern in the photosphere called granulation. • Light colored granules surrounded by dark colored boundaries. • Caused by convection.

45. The Solar Atmosphere The Chromosphere • The Chromosphere has a density 1/10,000th that of the Photosphere • This is the reason why we cannot see it. • It can only be seen during a total Solar Eclipse, or by using special filters, where the Photosphere is blocked from view. • Unlike in the Photosphere the temperature rises with altitude in the Chromosphere, from 4000K - 25,000K.

46. The Solar Atmosphere The Chromosphere • Photograph taken during a total solar eclipse. • It shows the Chromosphere as a pinkish glowing region around the Sun. • Spicules: Stream of gases pulled upward.

47. The Solar Atmosphere The Chromosphere • Unlike the photosphere, the chromosphere has a spectrum dominated by emission lines. • Emission lines are light emitted when electrons in atoms of thin hot gases fall to lower orbits. • The dominant emission line in the chromosphere’s spectrum is due to the single electron in Hydrogen atoms falling from the 3rd orbit to the 2nd orbit - H emission line (656.2 nm - Red region). • Gives the characteristic pinkish color

48. The Solar Atmosphere The Corona • Outer most region of the Sun’s atmosphere. • Extends to several million kilometers and one millionth as bright as the Photosphere • Can be seen only if we block the Photosphere • Using filters or during a total solar eclipse • Corona is not a spherical shell of gas but numerous streamers extending in different directions. • Displays emission line spectrum.

49. The Corona • Spectral studies show that the temperature in the Corona reaches 2 million kelvin. • However, it s not very hot due to its low density.

50. The Solar Atmosphere The Solar Wind • Sun’s gravity keeps the atmosphere from escaping to space (just like on Earth) • To escape a body like the Sun, air molecules have to acquire an escape velocity. • But, due to the Corona’s high temperature, air molecules have extremely high speeds. • As a result some gas from the Corona gets ejected to space - Solar Wind. • The Sun emits ~ a million tons of material to space every second.