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Our Star The Sun The Sun is a fairly average star in terms of mass, luminosity and temperature. It’s only special because it’s so close to us. The next nearest star is about 270,000 AU away. Key facts Distance from Earth 1AU = 149,600,000 km Variation from nearest to furthest distance ~3% Light time travel to Earth ~ 8 minutes Radius 696,000 km = 109 X Earth’s radius Mass 1.99 x 1030 kg = 3.3 x 105 Earth’s mass Average Density 1410 kg/m3 (Earth ~5,500 kg/m3) Composition by mass: H – 74%; He - 25% All others 1% Luminosity 3.8 x 1026 W Why was the Sun’s energy source a major mystery? Is it on FIRE? Chemical Energy Content Luminosity ~ 10,000 years Is it on FIRE? ……..NO Chemical Energy Content Luminosity ~ 10,000 years Is it CONTRACTING? Gravitational Potential Energy Luminosity ~ 25 million years Is it CONTRACTING? …………….NO Gravitational Potential Energy Luminosity ~ 25 million years Why does the Sun shine? E = mc2 It can be powered by NUCLEAR ENERGY! Nuclear Potential Energy (core) Luminosity ~ 10 billion years Theoretical Model of the Sun Explains pressure, temperature, and density as a function of the distance from the Sun’s centre. Key observational factors: The Sun is not changing quickly, so it must be in equilibrium: - Hydrostatic pressure at any point is exactly enough to support the weight of the overlying gas, so the pressure must increase greatly with depth. - Thermal as energy is radiated away from surface, it must be re-supplied from below. Weight of upper layers compresses lower layers Mechanical , gravitational or hydrostatic equilibrium: Energy provided by fusion maintains the pressure Gravitational contraction: Provided energy that heated core as Sun was forming Contraction stopped when fusion began What is the Sun’s structure? Radius: 6.9 x 108 m (109 times Earth) Mass: 2 x 1030 kg (300,000 Earths) Luminosity: 3.8 x 1026 watts Solar wind: A flow of charged particles from the surface of the Sun Corona: Outermost layer of solar atmosphere ~1 million K Chromosphere: Middle layer of solar atmosphere ~ 104 - 105 K Photosphere: Visible surface of Sun ~ 6,000 K Convection Zone: Energy transported upward by rising hot gas Radiation Zone: Energy transported upward by photons Core: Energy generated by nuclear fusion ~ 15 million K Key concepts • Why was the Sun’s energy source a major mystery? – Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years • Why does the Sun shine? – The Sun shines because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion. Key concepts • What is the Sun’s structure? – From inside out, the layers are: • Core • Radiation Zone • Convection Zone • Photosphere • Chromosphere • Corona How does nuclear fusion occur in the Sun? Fission Fusion Big nucleus splits into smaller pieces Small nuclei stick together to make a bigger one (Nuclear power plants) (Sun, stars) High temperature enables nuclear fusion to happen in the core Sun releases energy by fusing four hydrogen nuclei into one helium nucleus Proton-proton chain is how hydrogen fuses into helium in Sun IN 4 protons OUT 4He nucleus 2 gamma rays 2 positrons 2 neutrinos Total mass is 0.7% lower 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 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 Solar thermostat keeps burning rate steady 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 How does the energy from fusion get out of the Sun? Energy gradually leaks out of radiation zone in form of randomly bouncing photons Convection (rising hot gas) takes energy to surface Bright blobs on photosphere are where hot gas is reaching surface How is energy transported from the center to surface? • conduction: not efficient for hot gases • radiation: diffusion of photons – the primary mechanism for the first ~ 71% of the Sun’s radius • convection: circulation of hot gases – the primary mechanism for the outer ~ 29% of Sun’s radius. How we know what is happening inside the Sun? We learn about inside of Sun by … • Making mathematical models • Observing solar vibrations • Observing solar neutrinos Patterns of vibration on surface tell us about what Sun is like inside Data on solar vibrations agree very well with mathematical models of solar interior Neutrinos created during fusion fly directly through the Sun Observations of these solar neutrinos can tell us what’s happening in core Solar neutrino problem: Early searches for solar neutrinos failed to find the predicted number Solar neutrino problem: More recent observations find the right number of neutrinos, but some have changed form Key concepts •How does nuclear fusion occur in the Sun? The core’s extreme temperature and density are just right for nuclear fusion of hydrogen to helium through the proton-proton chain Gravitational equilibrium acts as a thermostat to regulate the core temperature because fusion rate is very sensitive to temperature •How does the energy from fusion get out of the Sun? Randomly bouncing photons carry it through the radiation zone Rising of hot plasma carries energy through the convection zone to photosphere •How do we know what is happening inside the Sun? Mathematical models agree with observations of solar vibrations and solar neutrinos Model of the Sun’s Interior: • Temperature at the center ~ 15,000,000 K • Pressure at the center ~ 2 x 1011 atm • Density at the center ~ 160,000 kg/m3 E = mc2 If we could convert 1 kg of matter, completely to energy yield we would get c2 = 9 x 1016 J of energy! Energy required to boil a litre of cold water ~4 x 105 J Thermonuclear Fusion Proton-Proton cycle; 4 Hydrogen (H) nuclei (i.e protons) combine, in a 3-step process, to yield one Helium (He) nucleus. The He has less mass than the 4 protons; the deficit is accounted for as energy. 1 kg of H becomes 993 g of He, so 0.7% of the mass is lost, and becomes 6.3 x 1014 J. So how fast is the Sun consuming its Hydrogen? Power output of Sun = Luminosity From Stefan-Boltzmann Equation: L= 4πR2σT4 L = 3.8 x 1026 W But, 1 kg of “burning” H yields 6.3 x 1014 J, so rate of consumption: 3.8 x 1026 6.3 x 1014 = 6 x 1011 kg/s! Sun is so massive it would take ~ 80 billion years to burn up all the H at that rate. Note: This is far beyond the predicted lifetime of the Sun. It is estimated that the Sun will “only” last another 5-6 billion years. What causes solar activity? Solar activity is like “weather” • Sunspots • Solar Flares • Solar Prominences • All are related to magnetic fields Sunspots Are cooler than other parts of the Sun’s surface (4000 K) Are regions with strong magnetic fields Zeeman Effect We can measure magnetic fields in sunspots by observing the splitting of spectral lines Charged particles spiral along magnetic field lines Loops of bright gas often connect sunspot pairs Magnetic activity causes solar flares that send bursts of X-rays and charged particles into space Magnetic activity also causes solar prominences that erupt high above the Sun’s surface Corona appears bright in X-ray photos in places where magnetic fields trap hot gas How does solar activity affect humans? Coronal mass ejections send bursts of energetic charged particles out through the solar system Charged particles streaming from Sun can disrupt electrical power grids and can disable communications satellites How does solar activity vary with time? Number of sunspots rises and falls in 11-year cycle Sunspot cycle has something to do with winding and twisting of Sun’s magnetic field Key concepts •What causes solar activity? Stretching and twisting of magnetic field lines near the Sun’s surface causes solar activity •How does solar activity affect humans? Bursts of charged particles from the Sun can disrupt communications, satellites, and electrical power generation •How does solar activity vary with time? Activity rises and falls with an 11-year period