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THE SUN The star we see by day 5 Goals • • • • Summarize the overall properties of the Sun. What are the different parts of the Sun? Where does the light we see come from? The scientific method: solar neutrinos. 5 The Sun, Our Star • The Sun is an average star. • From the Sun, we base our understanding of all stars in the Universe. • No solid surface. 5 Vital Statistics • • • • • • Radius = 100 x Earth (696,000 km) Mass = 300,000 x Earth (1.99 x 1030 kg) Surface temp = 5,800 K Core temp = 15,000,000 K Luminosity = 4 x 1026 Watts Solar “Day” = – 24.9 Earth days (equator) – 29.8 Earth days (poles) 5 Structure • ‘Surface’ – Photosphere • ‘Atmosphere’ – – – – Chromosphere Transistion zone Corona Solar wind • ‘Interior’ – Convection zone – Radiation zone – Core 5 Interior Properties • Core = 20 x density of iron • Surface = 10,000 x less dense than air • Average density = Jupiter • Core = 15,000,000 K • Surface = 5800 K 5 Do you see the light? • Everything in the solar system reflects light. • Everything also absorbs light and heats up producing blackbody radiation. • Q: Where does this light come from? • A: The Sun. • But where does the Sun’s light come from? 5 In The Core • Density = 20 x density of Iron • Temperature = 15,000,000 K • Hydrogen atoms fuse together. • Create Helium atoms. 5 Nuclear Fusion • 4H He • The mass of 4 H nuclei (4 protons): 4 x (1.6726 x10-27 kg) = 6.690 x 10-27 kg • The mass of He nuclei: = 6.643 x 10-27 kg • Where does the extra 4.7 x 10-29 kg go? • ENERGY! E = mc2 • E = (4.7 x 10-29 kg ) x (3.0 x 108 m/s)2 • E = hc/l l = 4.6 x 10-14 m (gamma rays) • So: 4H He + light! 5 The Radiation Zone • This region is transparent to light. • Why? – At the temperatures near the core all atoms are ionized. – Electrons float freely from nuclei – If light wave hits atom, no electron to absorb it. • So: Light and atoms don’t interact. • Energy is passed from core, through this region, and towards surface by radiation. 5 The Convection Zone • This region is totally opaque to light. • Why? – Closer to surface, the temperature is cooler. – Atoms are no longer ionized. – Electrons around nuclei can absorb light from below. • No light from core ever reaches the surface! • But where does the energy in the light go? • Energy instead makes it to the surface by convection. 5 Convection • • • • A pot of boiling water: Hot material rises. Cooler material sinks. The energy from the pot’s hot bottom is physically carried by the convection cells in the water to the surface. • Same for the Sun. 5 Solar Cross-Section • Progressively smaller convection cells carry the energy towards surface. • See tops of these cells as granules. 5 The Photosphere • This is the origin of the 5,800 K blackbody radiation we see. • Why? – At the photosphere, the density is so low that the gas is again transparent to light. – The hot convection cell tops radiate energy as a function of their temperature (5800 K). l = k/T = k/(5800 K) l = 480 nm (visible light) • This is the light we see. • That’s why we see this as the surface. 5 The Solar Atmosphere HOT You COOL • Above the photosphere: – Thin cool gas – Mostly transparent to light again • Unlike radiative zone, here atoms not totally ionized. • Therefore, there are electrons in atoms able to absorb light. • Absorption lines in solar spectrum are from these layers in the atmosphere. 5 The Chromosphere • Hydrogen most common. • Brightest hydrogen line – Ha. • Chromosphere = color 5 Ha Sun Photo by Big Bear Solar Observatory 5 Prominences 5 5 Corona • Magnetic activity carry energy up to the Transition Zone. • 10,000 km above photosphere. – Temperature climbs to 1,000,000 K – Remember photosphere is only 5800 K • The hot, low density, gas at this altitude emits the radiation we see as the Corona. – But corona very faint compared to photosphere. 5 5 Solar Wind • • • • At and above the corona: Gas is very hot Very energetic Like steam above our boiling pot of water, the gas ‘evaporates’. • Wind passes out through Coronal Holes • Solar Wind carries away a million tons of Sun’s mass each second! • Only 0.1% of total Sun’s mass in last 4.6 billion years. 5 Aurorae • The solar wind passes out through the Solar System. • Consists of electrons, protons and other charged particles stripped from the Sun’s surface. • When charged particles and magnetic fields interact: light! 5 5 Solar Cycle • Increase in solar wind activity - Coronal Mass Ejections • Increase in Auroral displays on Earth • Increase in disruptions on and around Earth. Courtesy of SOHO/LASCO/EIT consortium. 5 Magnetic fields and Sunspots • At kinks, disruption in convection cells. • Sunspots form. 5 • • • • 11-year sunspot cycle. Center – Umbra: 4500 K Edge – Penumbra: 5500 K Photosphere: 5800 K Sunspots 5 Magnetic fields and Sunspots • Where magnetic fields “pop out” of Sun, form sunspots. • Sunspots come in pairs. 5 5 Solar Neutrino Problem • We observe: – Sun’s luminosity (total light radiated). • We hypothesize: – 4H He + light + neutrinos • We can test: – Observe number neutrinos reaching Earth • Does or test agree with hypothesis? • No 5 What to Do? • For 30 years: – Theorists certain of nuclear reaction. – Observers positive of observations. – Detected only 1/3 the hypothesized neutrinos. • What to do? 5 Neutrino Flavors • 3 types of neutrinos – Electron neutrino – Tau neutrino – Muon neutrino • Nuclear reactions produce only electron neutrino. • Previous detectors only detected electron neutrinos. 5 Neutrino Fluctuations • New detector (2002) gives number of all three flavors. • Total number agrees with number predicted in core of Sun. • Conclusion: – Nuclear hypothesis is correct. – Neutrinos change flavor. – Neutrinos have mass (used to be thought massless). • Problem solved new science discovered. 5