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 radius (R ʘ = 1.4 x 10 6 km) Sun s mass is 333,000 times Earth s mass (m ʘ = 2 x 10 30 kg) Consists entirely of ionized gas (av. density = 1.4 g/cm 3 ) Central temperature = 1.5 x 10 7 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 10 26 W the equivalent of 10 billion 1-megaton nuclear bombs per second. Solar constant amount of the Sun's energy reaching Earth = 1400 W/m 2. 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 a 2 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 c 2 In this equation, c is the speed of light which is a very large number.
Energy Production Nuclear fusion produces energy up to the formation of iron. Nuclear fission produces energy when elements heavier than iron split. Binding energy is a result of the strong force. It has a very short range. It is the strongest of the 4 known forces: electromagnetic, weak, strong, gravitational
Basic reaction: Energy Generation in the Sun: 4 1 H 4 He + energy 4 protons have 0.048 x 10-27 kg (= 0.7 %) more mass than 4 He Energy gain = Δm c 2 = 0.43 x 10-11 J per reaction the Sun needs 10 38 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 Proton-Proton Chain Need large proton speed ( high temperature) to overcome Coulomb barrier (electromagnetic repulsion between protons). T 10 7 K = 10 million K
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 Visible Sunspot Regions Ultraviolet 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 Solar interior Temperature 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.
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 Solar Atmosphere
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 Cool gas sinking down Convection Bubbles of hot gas rising up ~1000 km Bubbles last 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 ~1 x 10 6 K Filaments Transition region from Chromosphere to Corona Chromospheric structures visible in H α emission (filtergram)
The Chromosphere Spicules: Spikes of cooler gas from the photosphere, rising up into the chromosphere. Visible in H α 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 Ultraviolet filtergram Magnetic image Magnetic fields in sunspots are ~1000 times stronger than average. In sunspots, magnetic field lines emerge out of the photosphere. Magnetic South Poles
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 spot to the other in the pair The leading spot has the same magnetic polarity as the overall polarity of the hemisphere Magnetic field lines
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 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 progresses. Total solar cycle = 22 years The Maunder butterfly diagram
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 This solved the solar neutrino problem. Davis solar neutrino experiment