The Sun and its properties

1. The Sun and its properties Astronomy 115

2. Stars are a basic unit of a galaxy • The Milky Way galaxy (ours) contains about 100 billion stars. • The Milky Way is one of billions of galaxies. • The glowing “arms” of the Milky Way are illuminated partly by the glow of numerous stars.

3. Our Sun is a star, and a conveniently close star to observe Most photos of the Sun, such as this one, are taken using a filter; otherwise, the light would be too intense for the camera. In this image, a hydrogen-alpha filter was used, which blocks all light but the red light from glowing hydrogen. The features on the right limb of the Sun are called solar prominences.

4. Red R Orange O Yellow Y Green G Blue B Indigo I Violet V Isaac Newton in Opticks (1704) demonstrated that sunlight could be dispersed into various colors by a prism, and then re-combined by another prism to yield sunlight again.

5. Fraunhofer’s Surprise In 1813, Joseph von Fraunhofer, the owner of a glass manufacturing firm in Munich, made an interesting discovery. Using a precision dispersing prism, he discovered that the `solar blackbody` was cut by thousands of dark bands.

6. Fraunhofer’s Surprise Fraunhofer tried to test whether this effect was real. 1) He tested with different optics. 2) He tested by looking at different objects (moon and planets).

7. Iron Bunsen and Kirchhoff`s solution: Robert Bunsen (Univ. of Heidelberg) turned pyromania into one of the great discoveries of modern physics.Bunsen set fire to things in order to figure out their elemental composition. A colleague there, Gustav Kirchhoff, suggested using a prism to break the light apart. They quickly discovered (1860) that burning substances produced light in narrow bands with unique patterns.

8. Blueprint to Composition: Bunsen and Kirchhoff`s trick was the key to finding out the composition of anything from the light it produced. Many of the lines they found had the same wavelength as those of Fraunhofer`s dark bands. They were seeing the composition of the Sun!

9. Kinds of Spectra: Bunsen found that he could identify the signature of different elements in the Fraunhofer spectrum of the Sun. Why were Bunsen`s heated gas spectra composed of bright lines while Fraunhofer`s exhibited a continuous spectrum with dark bands? Bunsen`s fires were stimulating light emissions in the hot gas. So what are Fraunhofer`s bands? Absorption by (and re-emission from) a cooler gas!

10. Types of Spectra Continuous: black body radiation continuous Absorption: requires a cool object in front of a hot background (ex: Fraunhofer) discrete Emission: requires a hot object with a cool background (ex: Bunsen) discrete

11. Basic definitions: • Element: a substance that cannot be broken down by chemical means (defined by number of protons) • Atom: the smallest piece of matter that is still an element • Molecule: two or more atoms that are bound together by chemical bonds • Nucleus: the protons and neutrons bound together at the center of an atom

12. Atoms and light Why do elements have the `discrete` interactions that Bunsen saw? Why do different elements (and molecules) have different interactions? The answers have to do with the nature of atoms and how they are put together.

13. A brief history of the atom First discussion of the nature of matter (~400 BCE): • Leucippus, a Greek philosopher, all matter consisted “tiny and indivisible bodies called atoms”. • The word atom comes from the Greek word `atomos` (not divisible). • Democritus, another Greek philosopher, these atoms were not all alike, but had different shapes and sizes to make different matter. • Opposed (Aristotle):The prevailing view that everything was made up of four basic elements: earth, fire, air, and water, not atoms. • Views such as Aristotle`s dominated science for many centuries, until the Renaissance.

14. Dividing the ‘indivisible’:The plum pudding model • J. J. Thomson discovers cathode rays are made of electrons (he called them ‘corpuscles’ – 1897). Electrons are shown to have a negative charge • Thomson proposes model of the atom (1904): • Atom has smaller components • Negatively charged corpuscles/electrons (plums) • Positive ‘soup’ to balance negative charge (pudding)

15. Discovering “nothing” • Meanwhile, Ernest Rutherford (Cambridge) discovers two new types of radiation emitted by uranium (1899): • Alpha particles (): later found to be the helium nucleus • Beta particles (): later called the “electron” by Thomson In 1909, Rutherford fires alpha particles at gold foil. Expected only small angle scattering due to gold atoms’“plum pudding”. Saw mostly no scattering with occasional back scattering. Positive Nucleus Matter is mostly empty space!!!! Negative electron

16. Rutherford’s atom • Mass is highly concentrated in the positively-charged nucleus at the center of the atom. • Electrons (negatively charged) “orbit” the nucleus. • Lots of empty space in-between • Similar to today’s atom • Number of protons determine the element identity • Number of electrons determine the chemical properties of the atom Positive Nucleus Negative electron

17. The nucleus is not uniform Rutherford (1918):Discovers the proton. The proton is about 2000 times as massive as the electron and has a positive charge, exactly the same magnitude as the electronic charge. James Chadwick (1932):Discovers the neutron. Neither positive nor negative, it has about the same mass as a proton. Nuclei are made up of protons and neutrons.

18. Atoms, elements, and isotopes Atoms (below - periodic table (Mendeleev, Meyer, 1867)) • Nucleus • Protons – number determines the element (atomic #) • Neutrons – number determines the isotope (mass #) • Electrons – number determines the chemical properties

19. Atoms, elements, and isotopes Isotopes: Atoms with the same number of protons but different numbers of neutrons are called isotopes. Isotopes have the same chemical properties, but different masses, different emission spectra, and participate in different nuclear reactions. helium (4He) hydrogen (1H) deuterium (2H) tritium (3H) e- p+ n p+ p+ p+ n n n p+ n e- e- e- e- A stable isotope of hydrogen – 99.98% natural abundance A stable isotope of hydrogen – 0.02% natural abundance New element; not an isotope of H Radioactive isotope of hydrogen

20. Bohr atomic model Energy States: Niels Bohr (Copenhagen Univ.), based on Rutherford’s work, suggested a quantized structure of electronic orbits in an atom (1913) Bohr and Werner Heisenberg later (1926) modify structure to account for the wave properties of electrons. p+ E1 e- Electron distances and energies are discrete (quantized) values E2 E3

21. Atoms and light Electrons exist in `orbits` (much like planets in the solar system) that are stable at specificseparations from the nucleus. Energy States: The distance from the nucleus determines the energy of the electron (lower E is closer). p+ E1 e- The spacing of these energy levels is not even. E2 E1E2 > E2E3 > E3E4 etc… E3

22. Atoms and light So what does all of this have to do with Bunsen and Fraunhofer lines?

23. Atoms and light Energy States: If you heat the atom up to high enough temperatures, the electron will jump to higher orbits (higher energy state). p+ E1 e- E2 E3

24. Atoms and light Energy States: If you heat the atom up to high enough temperatures, the electron will jump to higher orbits (higher energy state). p+ How does heating do this? Collisions with other atoms E1 e- E2 E3

25. Atoms and light After a time, the electron falls back to the lowest energy state. Energy States: A photon is given off. The energy of the photon is exactly equal to the energy difference between the two energy states. p+ E1 e- E2 E3

26. Atoms and light: absorption Process of emission is fully reversible. Energy States: Electron can absorb a photon and jump to a higher energy level. The energy of the photon must be exactly equal to the energy difference between the two energy states. p+ E1 e- E2 E3

27. Atoms and light: absorption Process of emission is fully reversible. Energy States: Electron can absorb a photon and jump to a higher energy level. The energy of the photon must be exactly equal to the energy difference between the two energy states. p+ E1 e- E2 E3

28. Conservation of energy The energy difference between electron orbital states is exactly equal to the energy of the photon emitted or absorbed. E2 – E1 = h f Where E1 and E2 are the energies associated with the electronic orbital states, f is the frequency of light, and h is Planck’s constant = 6.62 × 10–34 J•s

29. hydrogen energy level diagram

30. Iron Quantized energy • Different frequencies are perceived as different colors • Atoms of different elements have different allowable energy level transitions and thus emit and absorb different discrete colors. • Example: Each line in the spectrum of iron is different energy level transition

31. Types of spectra Continuous: black body radiation continuous Absorption: requires a cool object in front of a hot background (ex: Fraunhofer) discrete Emission: requires a hot object with a cool background (ex: Bunsen) discrete

32. Ionization: Energy States: e- What happens if a veryenergetic photon interacts with an atom? Such a photon can give enough energy to the electron that it can escape the atom. p+ e- The amount of energy necessary to do this is called the binding energyof the atom.

33. Ionization: When an atom absorbs light (or thermal) energy greater than the binding energy, the electron escapes. Energy States: e- The atom is left with a positive charge and is called an ion. p+ e- e- Together, ions and free electrons are called plasma. Plasma is found in stars, space, and parts of our atmosphere.

34. What about molecules? Electronic Energy States: Molecules are atoms that are connected by bonds (electrons). At a basic level a molecule will behave similarly to an atom. e- e- Molecules also have discrete electron energy levels. p+ p+ p+ p+ - Like atoms, electrons in molecules can absorb a photon and move to a higher energy level

35. What about molecules? Electronic Energy States: e- A photon with enough energy can free an electron by overcoming the binding energy. This produces a molecular ion. p+ p+ p+ p+ e- Plasmas can also contain molecular ions.

36. Photo-dissociation Electronic Energy States: Or overcome the molecular binding energy and break the molecule up (photo-dissociation). e- p+ e- p+ p+ e- e- p+

37. Vibrational modes p+ p+ p+ p+ p+ p+ ElectronicEnergy States: With molecules there`s an additional complexity. In addition to electronic energy levels, molecules have vibrationalenergy levels. e- t1 p+ p+ t2 e- t3

38. So the Sun’s composition was determined and turned out to be relatively simple

39. Though the composition is relatively uniform, the Sun is layered by density Working outward, the dense core (where fusion takes place) goes from 0 to 0.25 Rsun, followed by the radiative zone (0.25 to 0.7 Rsun), topped by the convective zone (0.7 to 1 Rsun). The visible surface of the sun is called the photosphere, and the “atmosphere” of the Sun has two layers, the relatively thin chromosphere and the thicker, but less dense, corona.

40. Energy transfer = moving the energy of fusion at the core to the Solar System • As the names of the layers imply, it is not the composition of the sun that is interesting, but the manner in which energy is transmitted from layer to layer. • This difference in manner of energy transfer will be a direct result of the lessening density of the Sun outwards; in fact, the outer edge of the convective zone (the photosphere) is far less dense than the Earth’s atmosphere!

41. Conduction is heat transfer by… • Direct contact • Material flow • Photons • Phase change

42. Convection is heat transfer by… • Direct contact • Material flow • Photons • Phase change

43. The second law of thermodynamics governs energy (heat) transfer Heat transfers from a hot body to a cooler body. The transfer can never be stopped, only slowed down. Evaporation Convection: heat exchange due to material flow Conduction: heat exchange due to direct contact Radiation: heat exchange due to photons (light) Phase change: heat exchange due to substance changing phase (for example, evaporation or melting)

44. The solar interior • Starting Point: We can’t actually get to the interior of the Sun. We have to model it, based on observations. • Model Basis: We do know (some!) solar physics and we have some boundary conditions that help us. 1) The Sun’s COMPOSITION 2) The Sun’s MASS 3) The Sun’s TEMPERATURE 4) The Sun’s AGE

45. Summary of the solar interior The Sun’s interior can be broken down into a few regions based on how energy (heat) transfers 1) Where it is hot and dense enough for fusion 2) Where it is hot and dense enough to prevent electrons from staying with atomic nuclei 3) Where it is not hot and not dense enough to permit electrons to combine with atomic nuclei These conditions then tell us: 1) how energy escapes from the Sun. 2) how material moves inside the Sun.

46. The standard model for the Sun’s properties Lead Water Deepest Ocean Trench

47. The solar core • Extends out to 0.25 Rsun • Contains ~50% of the Sun’s mass. • Contains ~2% of the Sun’s volume. • Is bounded (loosely) by the point where temperature and density are too low to support P-P fusion.

48. Structure of the solar core Core Boundary Core Boundary

49. The division between the radiative and convective zones Radiative convective Radiative convective

50. Energy transfer in the core and radiative zone Radiative diffusion, modeled as a “random walk” • Occurs in the core and in the radiative zone. • Light scatters randomly off free electrons and nuclei. This means a photon can bounce back towards the core, as well as go outwards. • This is a SLOWprocess. • A photon produced by fusion in the core today will take 105 – 106YEARS to exit the radiative zone!