1	Chapter 10 Our Star Chapter Outline Why Does the Sun Shine? Plunging to the Center of the Sun: An Imaginary Journey The Cosmic Crucible From Core to Corona Solar Weather and Climate
2	Sun - Physical Data
3	Why should we believe that we can understand the internal structure of the Sun when it is not directly observable? The fundamental behavior of the natural world that we uncover in Earth laboratories, such as expressed in conservation of energy and momentum, are assumed universally applicable and have not been shown to be flawed by any defensible study to date. By application of these fundamental laws in mathematical form, we can compute theoretical stellar models as a means of studying internal structure even though the deep interior is not directly accessible by observation.
4	Energy Loss by the Sun
5	Life Story of Stars Life story of star Gravity squeezes, pressure forces resist by pushing out Energy loss in luminosity decreases pressure Energy generation replaces losses When energy generation stops star is dead Pressure forces Kinetic pressure of hot gases Electron degeneracy pressure from exclusion principle Neutron degeneracy pressure from exclusion principle
6	Hydrostatic Equilibrium Perfect gas - solar gases almost totally ionized, behavior of nuclei and free electrons is such that pressure is proportional to temperature and density Equation: P µ r T Hydrostatic equilibrium - weight of overlaying layers balanced by pressure of hot gas pushing out Equation: weight of gas = pressure of hot gas Results: star neither expands nor contracts
7	Energy Generation Energy generation - energy source that replaces energy loss from luminosity If Sun radiated only because it is hot, Sun would cool at measurable rate Sun is not observed to be cooling Consequently, interior stays hot by releasing energy
8	Thermal Equilibrium Thermal equilibrium - amount of energy produced inside equals amount radiated away as luminosity Solar gases obey perfect gas law, P µ r T Self-regulating mechanism If too much energy produced, Sun heats up and expands If too little energy produced, Sun cools down and contracts
9	Thermonuclear Fusion Thermonuclear fusion - fusion of small mass nuclei, primarily hydrogen nuclei, to form more massive nuclei, primarily helium, with resulting direct conversion of mass into energy by Einstein’s mass-energy equivalence: E = mc² Equation: 4¹H₁ Þ ⁴He₂ + g + n + e⁺ where ¹H₁ = hydrogen nucleus, proton ⁴He₂ = helium nucleus, 2 protons, 2 neutrons g = gamma-ray photons n = neutrinos e⁺ = positron
10	Energy Transport Mechanisms Energy transport - various physical processes transport energy from deep interior to surface Radiative transport - energy moves outward by absorption and re-emission by matter Energy generated as a few high-energy gamma-ray photons Absorption and re-emission degrades high-energy gamma-ray photons to low-energy visible photons Opacity - resistance offered by matter to movement of radiation Convective transport - mass motions move energy
11	Solar Models - Evolution of the Sun Solar model - mathematical model for study of structure and evolution of the Sun and other stars Solar structure equations - describe variation of mass, pressure, temperature, and luminosity from center to surface Mass conservation - total mass equals sum of masses for each layer Hydrostatic equilibrium - weight of overlying layer balanced by outward pressure of hot gases Energy conservation - luminosity equals sum of energy generation in each layer Energy transport - energy moves from hot (high-energy density) region to cool (low-energy density) region
12	Importance of Solar Models Equations solved for about 200 points along radius and for particular time in the Sun’s life Result - solar model Confirms importance of various physical processes inside real stars Sequence of models calculated for different times in star’s life detail evolutionary tracks in H-R diagram
13	Solar Interior Model
14	Solar Neutrino Problem Number of neutrinos created in hydrogen burning at center of Sun depends on temperature About 10¹⁵ solar neutrinos flow through our bodies every second. Neutrinos almost never interact with nuclei Right energy neutrinos can transform ³⁷Cl₁₇ nucleus to radioactive ³⁷Ar₁₈^* Late 1960s experiment in 1-mile-deep South Dakota gold mine captured solar neutrinos Capture signaled by radioactive decay of argon ³⁷Ar₁₈^*
15	Solar Neutrino Experiment
16	Solar Neutrino Experimental Results Experiment ran over two decades Measured about 1/3 number of solar neutrinos predicated from solar model calculations Other experiments, but different methodology, started in late 1980s and early 1990s confirm deficiency of solar neutrinos Two kinds of solutions proposed Theory of solar structure is inaccurate Neutrino particle theory needs revising (change types)
17	Superkamiokande Solar Neutrino Detector
18	Rotation of the Sun 1947 sunspot group 1.5 solar rotations White-light images, Mt. Wilson Observatory Spectroscopy East limb, photospheric spectrum is blue-shifted West limb, photospheric spectrum is red-shifted Equatorial rotation velocity = 2 km/s
19	Differential Rotation Equatorial period of rotation = 25 days Polar latitude period of rotation = 36 days Computer generated image of depth and latitude variations Red - faster rotation Blue - slower rotation
20	Atmosphere - Photosphere Photosphere = “light sphere” - layer responsible for solar luminosity Limb darkening - brightness of Sun fades from center toward limb Geometrical path lengthens from center toward limb Radiation comes from higher layers in photosphere toward limb Stefan-Boltzmann -temperature declines outward through photosphere
21	Limb Darkening - Temperature
22	Solar Photospheric Spectrum Photospheric absorption spectrum - several tens of thousands of absorption lines Strongest - H and K lines, due to singly ionized calcium (Ca II) Hydrogen (H) Balmer series fairly strong Majority of lines due to neutral and singly ionized iron (Fe I and Fe II) About 70 of 92 naturally occurring elements observed in photospheric spectrum About 20 molecules are also observed
23	How do we identify which chemical elements and what quantities are present in the Sun’s atmosphere? Analysis of the light coming from the Sun or stars is the only diagnostic tool available for determining chemical composition. Spectroscopy of solar and stellar radiation followed by computer analysis can yield chemical composition.
24	Identifying Elements Stellar spectra are absorption spectra for which star’s photosphere is intervening cool gas in Kirchhoff’s third law Stars and gaseous nebulae contain mixtures of chemical elements Each element emits or absorbs its own particular set of wavelengths Emission spectrum of known gas (neon or vaporized iron) provides comparison lines of known wavelength against which wavelengths of unknown solar or stellar lines can be determined
25	Abundances of the Chemical Elements Measured wavelengths identify chemical elements present in the Sun or a star Presence of absorption lines of particular element in sun’s photospheric spectrum clearly indicate element is present Absence of absorption lines of particular element in sun’s photospheric spectrum does not say element is not present Physical environment, temperature and density, determines whether or not an element will absorbed radiation Chemical composition can be determined from spectrum and knowledge of temperature and density of photosphere
26	Solar Photospheric Composition Element Atomic Number % by Number % by Mass Hydrogen 1 90.9 70.7 Helium 2 8.9 27.4 Carbon 6 0.033 0.31 Nitrogen 7 0.010 0.11 Oxygen 8 0.077 0.95 Neon 10 0.011 0.17 Magnesium 12 0.003 0.06 Silicon 14 0.003 0.07 Sulfur 16 0.001 0.04 Iron 26 0.003 0.14
27	Photospheric Granulation Photospheric granulation - “rice-grained” pattern of convection cells Possess magnetic fields of several gauss averaged over large areas
28	Photospheric Sunspots Sunspots - cooler structures than normal photosphere Defined by magnetic field Properties of sunspots Observed in white light Average life about 6 days Size up to 10,000s km Dark umbra surrounded by striated penumbra Temperature about 1800^o K cooler Magnetic fields up to several thousand gauss
29	Large Sunspot Structure Average large sunspot, but not a typical sunspot Scale-size is about 10,000 km
30	Sunspot Magnetic Fields
31	Sunspot Cycle
32	Magnetic Dynamo
33	Sunspot Frequency and Magnetic Cycles Sunspot cycle - roughly 11 year cycle Successive maxima are unequal Interval between successive peaks is not constant, 7 to 15 years Polarity (north or south seeking) of magnetic field reverses each cycle 22-year magnetic cycle far more repeatable than 11-year frequency cycle
34	Chromosphere Thin pinkish fringe extending beyond photosphere Taken during an eclipse Visible image Inset shows structure of chromosphere Spicules
35	Chromospheric Spectrum Chromospheric emission spectrum Emission lines with some matching wavelengths of photospheric absorption lines Bright yellow line produced by helium (He) Chromospheric temperature up to 30,000 K at highest level Gas density is lower than photosphere From this, one concludes that temperature must rise rapidly up through chromosphere
36	Corona Corona - halo of pale white glowing gas extending several solar radii (several million kilometers) out from photosphere Coronal emission spectrum 30 emission lines in visible spectrum Hundreds of emission lines in ultraviolet and X-ray regions Emission lines originate in highly excited ions of familiar elements Temperature must be millions of degrees K to produce high degree of ionization Densities must be quite low compared to photosphere to produce emission spectrum from gas that is transparent to photospheric radiation
37	Coronal Heating Question - if temperature rises from photosphere, through chromosphere, into corona, how is energy deposited in corona, since it is transparent to photospheric radiation? Answer - energy is deposited from two sources Mechanical (acoustic) waves rise from photosphere into corona and dissipate their energy Magnetic fields extending from photosphere into corona transfer energy which is dissipated in corona
38	Center of Activity H_a Spectroheliogram of Active Sun Faculae – white light version of plage Plages - hotter denser regions than normal chromosphere Lifetimes of about 40 days Magnetic fields up to several hundred gauss Size about 50,000 km Sunspot group - cooler structures than normal photosphere, defined by magnetic field Flares - brief brightening in plages Lifetimes of about 20 minutes Size about 30,000 km Enhances particle density in solar wind and solar cosmic rays Filaments – prominence seen against disk Prominences - chromospheric material extending upward into corona, magnetic structures
39	Spectroheliograms May 19, 1998
40	Chromospheric Flares Flares - brief burst of X-rays and particle Observed in monochromatic light Lifetimes of about 20 minutes Size about 30,000 km Enhances particle density in solar wind and solar cosmic rays
41	Coronal Prominences Prominences - Chromospheric material extending upward into corona Seen against photospheric or chromospheric disk known as filaments Properties Much cooler than surrounding corona Sizes, if quiescent, height 30,000 km, length 200,000 km, thickness 5000 km Exhibit motions associated with magnetic fields up to several hundred gauss Lifetimes up to 90 days
42	Soft X-Ray July 7, 1998 Holes - lower temperature and much lower density regions Sizes up to hundreds of thousands of km Magnetic field lines open out to interplanetary space Source of solar wind particles Changeable in periods of days to weeks Active regions - relatively hot and dense regions consisting of magnetic loop structures Sizes up to hundreds of thousands of km Magnetic field lines form large loop structures Occur over chromospheric plages Quiet regions - between coronal holes and coronal active regions Magnetic fields weak and roughly in loop structures
43	Coronal Mass Ejections A. Shows relatively quiet corona Black disk blocks photospheric and chromospheric radiation B. 16 minutes later, huge balloon-shaped volume of high-energy gas is ejected from corona C. Ejected material expands at typical velocities of 400 km/s Ejection lasts several hours and contains trillions of tons of matter Often associated with solar flares, but not always
44	Space Weather Space weather – study of variable emission of high-energy photons, particles, and magnetic fields and their interaction with the geosphere Earth influences Van Allen radiation belts Spacecraft and crews High-altitude aircraft Electric power grid Communications, land and satellite Major source of natural variability in terrestrial climate
45	The Big Picture The Sun continues to shine, while it radiates away as its luminosity, by generating energy through thermonuclear fusion of hydrogen into helium. Gravitational and thermal equilibrium determine the Sun’s internal structure and its rate of energy generation. The Sun’s atmosphere displays its own version of weather and climate, governed by solar magnetic fields. Solar weather has important influences on the Earth. The Sun is important not only as our source of light and heat, but also because it is the only star near enough for us to study in great detail. In the coming chapters, we will use what we’ve learned about the Sun to help us understand other stars.