The Sun – Our Star Properties Interior Atmosphere Photosphere Chromosphere Corona Magnetism Sunspots Solar Cycles Active Sun General Properties . Not a large star, but larger than most . Spectral type G2 . It appears bright because it is so close . Absolute visual magnitude = 4.83 . Sun’s radius is 109 times Earth’s 6 radius (Rʘ = 1.4 x 10 km) . Sun’s mass is 333,000 times Earth’s 30 mass (mʘ = 2 x 10 kg) . Consists entirely of ionized gas (av. density = 1.4 g/cm3) . Central temperature = 1.5 x 107 K . Surface temperature = 5800 K Physical Properties of the Sun Interior structure of the Sun: Outer layers are not to scale The core is where nuclear fusion takes place Physical Properties of the Sun Luminosity – the total power radiated by a star – it can be calculated from the fraction of that energy that reaches Earth. The luminosity of the Sun is about 4 × 1026 W – the equivalent of 10 billion 1-megaton nuclear bombs per second. Solar constant – amount of the Sun's energy reaching Earth = 1400 W/m2. It is the Sun’s energy flux (energy/unit area) at 1 AU. Determining Luminosity • Measure k, the solar constant, the energy flux reaching the Earth • Measure a, the distance from the Earth to the Sun • Determine the surface area A of a sphere with the radius a: A = 4π a2 • Luminosity: L = kA The Energy of the Sun Nuclear fusion is the energy producing mechanism of the Sun. In general, nuclear fusion works like this: nucleus 1 + nucleus 2 → nucleus 3 + energy But where does the energy come from? . It comes from the loss of mass ∆m: if you add up the masses of the initial nuclei, you will find that it is more than the mass of the final nucleus. The difference in mass ∆m has been changed into energy. The relationship between mass and energy is found in Einstein’s famous equation: ∆ E = ∆m c2 In this equation, c is the speed of light which is a very large number. Energy Production . Nuclear Binding energy is a result of fusion the strong force. It has a very produces short range. It is the energy up to strongest of the 4 known the formation forces: electromagnetic, of iron. weak, strong, gravitational . Nuclear fission produces energy when elements heavier than iron split . Energy Generation in the Sun: The Proton-Proton Chain . Basic reaction: Need large proton speed (⇒ high 4 1H → 4He + energy temperature) to overcome Coulomb barrier (electromagnetic repulsion . 4 protons have 0.048 x between protons). -27 10 kg (= 0.7 %) more T ≥ 107 K = 4 mass than He 10 million K ⇒ Energy gain = Δm c2 = 0.43 x 10-11 J per reaction . ∴ the Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second so that its pressure balances its own gravity to keep it from collapsing. The Solar Interior The balance between gravitational forces pulling toward the center with the heat pressure pushing out is called hydrostatic equilibrium. A differential equation expressing hydrostatic equilibrium as a function of radius is one of the basic equations used to model stellar interiors. The Solar Interior Doppler shifts of solar spectral lines indicate a complex pattern of standing waves inside the Sun. The Solar Interior These graphs plot solar density and temperature as a function of distance from the Sun’s center, according to the standard solar model Heat Transport Heat is transported by 3 physical mechanisms: • Conduction • Radiation • Convection The Solar Interior . Energy transport in stars is either by radiation or by convection . Interior regions or zones are dominated by one transport mechanism or the other . A differential equation expressing energy transport as a function of radius is another of the basic equations used to model stellar interiors. The radiation zone is relatively transparent; the cooler convection zone is opaque The Layers of the Solar Atmosphere Sunspot Ultraviolet Visible Regions Photosphere Corona Chromosphere Coronal activity, seen in visible light The Solar Atmosphere The corona and the chromosphere are only visible during solar eclipses The photosphere is the apparent surface of the Sun Heat Flow Temperature Solar interior increases inward The Solar Atmosphere Spectral analysis can tell us what elements are present. Absorption lines are used for the photosphere and the chromosphere. Emission lines are used for the corona. The Solar Atmosphere Spectral lines are formed when light is absorbed before escaping from the Sun; this happens when its energy is close to an atomic transition, so it is absorbed. This absorption spectrum shows absorption lines for some of the elements in the Sun. The Photosphere . Apparent surface layer of the Sun . Depth ~500 km . Temperature ~5800 K . Highly opaque (H- ions) . Absorbs and re-emits radiation produced in the solar interior Corona Energy Transport in the Photosphere Energy generated in the Sun’s center must be transported outward. In the photosphere, this happens through Convection Cool gas Bubbles of hot sinking down gas rising up Bubbles last ~1000 km for ~10-20 min Granulation … is the visible consequence of convection The Chromosphere . Region of Sun’s atmosphere just above the photosphere. Visible, UV, and• X-ray lines from highly ionized gases . Temperature increases gradually from ~4500 K to ~10,000 K, then jumps to Filaments ~1 x 106 K Transition region from Chromosphere to Corona Chromospheric structures visible in Hα emission (filtergram) The Chromosphere Spicules: Spikes of cooler gas from the photosphere, rising up into the chromosphere. Visible in Hα emission. Each one lasts ~ 5-15 min. Green River, WY June 8, 1918 As seen during solar eclipses Phillipines March 1988 The Solar Wind The solar corona is blown into interplanetary space and is called the solar wind The solar wind . Is spiral shaped because of the Sun’s rotation . Carries the Sun’s magnetic field into interplanetary space . Interacts with planetary magnetic fields . Ionizes gas around comets and forms the comet’s ion tail The Sun’s Magnetic Cycle 1. The Sun’s magnetic field is generated in the convection zone, not in the core 2. The Sun rotates faster at the equator than near the poles. 3. This differential rotation pulls north-south magnetic field lines in the direction of rotation at the equator. The Sun’s Magnetic Cycle 4. After 11 years, the magnetic field pattern becomes so complex that the field structure is re-arranged. 5. The new magnetic field structure is similar to the original one, but reversed! Magnetic Field Reversal 6. A new 11-year cycle begins with a reversed magnetic-field orientation Sunspots and Magnetic Fields Magnetic North Poles Magnetic Ultraviolet filtergram Magnetic image South Poles . Magnetic fields in sunspots are ~1000 times stronger than average. In sunspots, magnetic field lines emerge out of the photosphere. Sunspots . Cooler regions of the photosphere (T ≈ 4240 K). Appear dark against the bright Sun. They would still be much brighter than the full Moon when placed on the night sky! Sunspots and Magnetic Loops . Sunspots are in pairs of different polarity . A magnetic field loops out of the Sun from one Magnetic field lines spot to the other in the pair . The leading spot has the same magnetic polarity as the overall polarity of the hemisphere Magnetic Fields in Sunspots Magnetic fields on the photosphere can be measured using the Zeeman effect ⇒ Sunspots are related to magnetic activity on the photosphere The Solar Cycle 11-year cycle sunspot cycle After 11 years, the North/South order of leading/trailing sunspots is reversed ⇒ Total solar cycle = 22 years Reversal of magnetic polarity . The sunspot cycle starts out with a small number of spots at higher latitudes . It evolves to a larger number of sunspots at lower latitudes (towards the equator) as the cycle The Maunder butterfly diagram progresses. The Maunder Minimum . The period from ~1645 to 1715 had very few sunspots, which coincides with a period of colder-than-usual winters in Europe. Is there a cause and effect relationship? The Active Sun . Areas around sunspots are active . Large eruptions may occur in the photosphere. A solar prominence is a large sheet of ejected gas The Active Sun A solar flare is a large explosion on the Sun’s surface, emitting a similar amount of energy as a prominence, but in seconds or minutes rather than days or weeks. Solar flares can significantly influence the Earth’s magnetic field structure and cause northern lights (aurora borealis). The Active Sun Coronal mass ejection occurs when a large “bubble” detaches from the Sun and escapes into space The Active Sun The solar wind escapes the Sun mostly through coronal holes, which can be seen in X-ray images The Solar Neutrino Problem . The solar interior cannot be observed directly with electromagnetic radiation because it is highly opaque. Neutrinos from fusion reactions can escape the Sun without being absorbed. However, solar neutrino experiments have detected a much lower flux of neutrinos than expected (the “solar neutrino problem”). Recent laboratory experiments have shown that neutrinos oscillate between different “flavors” . They oscillate during their passage from the Sun and only one “flavor” can be detected thereby reducing the flux Davis solar neutrino . This solved the solar neutrino problem. experiment .