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The Sun: Our Star The Sun is an ordinary star and shines the same way other stars of its type do. The bright part normally seen is called the photosphere, which is about 500 km deep. It is an almost perfect black body with a temperature of 5800 K. Deviation from a black body are due to absorption lines by gas in the Sun’s atmosphere Assigned Reading • Chapter 8. What is the Sun? • The Sun is a star, a fairly common and typical star • Its spectral type is G2, which means Teff~5800 K • A star is a ball of gas held in equilibrium against its own self-gravity by the thermal pressure and outflow of energy from center to the surface • The Sun radiates away a lot of energy. Every second, the equivalent of a 60 Watt bulb is being emitted by each and every 2-mm2 of the Sun surface (there are 3x1024 of them). • IMPORTANT: it would cool off very quickly if energy were not replenished. Where does this energy come from? • The energy comes from nuclear fusion reactions at the center of the Sun The Sun’s General Properties • Average star, very very common • Spectral type G2, relatively modest luminosity • It appears so bright because it is so close. • Absolute visual magnitude = 4.83 (magnitude if it were at a distance of 32.6 light years) • 109 times Earth’s diameter • 333,000 times Earth’s mass • Consists entirely of gas (av. density = 1.4 g/cm3) • Central temperature = 15 million 0K • Surface temperature = 5800 0K Why is the Sun important for life? Provides virtually all of the energy necessary for life on Earth. That energy is required for 1) 2) 3) Producing the primary link of the food chain (plants) Making life possible (enabling tis chemical reactions) The weather, etc. Balance in the stars Thermal Pressure Gravitational Contraction Pressure and Temperature of a Gas How does a star hold itself? This balance between weight and pressure is called hydrostatic equilibrium. The Sun's core, for example, has a temperature of about 16 million K. It is hotter than the surface (T~5800 K), because it needs lots of pressure to sustain the immense weight of all the upper layers. That is why deeper layers have higher temperature: they need more pressure to establish the equilibrium. What is the Sun made of It is made of hot gas; there is no solid material in the sun! The gas is mostly hydrogen (~80%) and helium(~20%), as is in the whole universe The gas is a plasma, i.e. its atoms are ionized. This means that they have lost most or all of their electrons, and that the gas is a mixture of electrically charged electrons (negative) and nucleii (positive) A plasma is an ideal medium the realize a black body radiator: Electrically charged particles can easily be accelerated and decelerated Why does the Sun have such a sharp edge? Pretty much an optical illusion! Structure of the Sun The Atmosphere of the Sun Although the sun appears to have sharp edge, its “surface” actually has complicated structure. Its atmosphere, too, is complex “Surface” and atmosphere of the Sun consist of: " " Photosphere: 5,800 K, continuum + abs. Chromosphere: 4,500-500,000 K, emiss.+abs. Corona: 1-2 106 K, continuum+emiss.+abs. Each square millimeter of the Photosphere emits the same Energy as a ~60 Watt light bulb Corona (1-2,000,000 K) The Sun surface (photosphere) would cool off very quickly if Energy were not produced continuously from Below spicules It seems to have a sharp edge, because immediately above it the temperature and density drop atoms recombine, and there is no more thermodynamic equilibrium: no B.B. any more, no continuum emission (4500-500,000 K) (5,800 K) The light we see originates from the thin photosphere The Solar Atmosphere Apparent surface of the sun Heat Flow Only visible during solar eclipses Solar interior Temp. incr. inward What would you expect to see if you looked at the Sun’s photosphere now? • The photosphere: the region where the black body radiation that we see is made. Only ~500 km thick • Thin gas: 3,400 times thinner than air • It is the transition region from a perfect blackbody emitting plasma to a transparent gas • Its upper layers produces the absorption spectrum • Limb darkening: proof that the Photosphere has a thickness and that energy comes from below • How does the photosphere look like if we close up on it? The limb is slightly fainter A real effect: we see less into the depth of the photosphere at the edge Granulation of the Photosphere Each Granule is about the size of Texas and lasts for only 10-20 minutes before fading away! Energy Transport near the Photosphere Energy generated in the sun’s center must be transported outward. Near the photosphere, this happens through Convection: Cool gas sinking down Bubbles of hot gas rising up ≈ 1000 km Bubbles last for ≈ 10 – 20 min Granulation of the Photosphere Each Granule is about the size of Texas and lasts for only 10-20 minutes before fading away! Granules: convective cells • The granules are just densely packed convective cells • Convective cells are very similar to thunderstorms Sunspots Sunspots, about 1000 K cooler than the rest of the Sun's photosphere, appear as dark spots. Sunspots come and go with time. Big sunspots can live for several weeks. Chromosphere Reddish in color, which is the origin of its name (chromos meaning ``color'') 2000-3000 km thick Faint relative to the photosphere From 4,500 up to 500,000K: hotter than the photosphere and much less dense. Absorption + Emissionline spectrum 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 oK to ≈ 10,000 oK, then jumps to ≈ 1 million oK Transition region Chromospheric structures visible in Ha emission (filtergram) The Chromosphere Spicules: Filaments of cooler gas from the photosphere, rising up into the chromosphere Visible in Ha emission Each one lasting about 5 – 15 min The Layers of the Solar Atmosphere Visible Sun Spot Regions Ultraviolet Photosphere Corona Chromosphere Coronal activity, seen in visible light UltraViolet (UV) image of the Sun: looking at areas that are more energetic than average. It shows regions of the Photosphere above Sunspots are more energetic than elsewhere. It also shows magnetic link between Sunspots extending into the Cromosphere and Corona Corona Only visible during eclipses - the outermost layer - T~1 million K -made up of very diffuse (but extremely hot gas) - coronal emission is dominated by X-rays The Magnetic Carpet of the Corona • Corona contains very low-density, very hot (1 million oK) gas • Coronal gas is heated through motions of magnetic fields anchored in the photosphere below (“magnetic carpet”) Computer model of the magnetic carpet Solar Prominence Composed of hot gas trapped in magnetic fields extending from one sunspot to another. Prominence X-ray emission from the Sun's corona The corona is so hot that it emits X-ray radiation Rotation period: about a month The middle rotates faster than the north or south. Corona has an emission line spectrum and a continuum spectrum (dust scattering of photospheric black body light). Some of the continuum spectrum has no absorption lines, because of the dilution of the absorption lines by very hot electrons The heating of the corona The heating of the corona has puzzled scientists for a long time. The corona is heated by energy outflowing from the sun's interior not as heat but as magnetic energy. How can heat go from cooler regions to hotter ones? It doesn’t! Radiant heat penetrates the corona nearly Undisturbed Corona heated by something else: whipping by magnetic force lines The gas is heated by the motion through it of long lines of magnetic force, which act like whips. The gas is being “whipped”. Solar Flare A solar flare is a violent outburst that lasts in an hour or less. It radiates X-ray, ultraviolet, and visible radiation, plus streams of high-energy protons and electrons. A large flare can be a billion times more energetic than a large hydrogen bomb. Solar Wind Sun loosing mass, but only a small fraction: 10-14 Mo/yr Mass loss will intensify later, and will much larger (~1/3) Auroras: the Northern and Southern Lights November 24, 2001 • This was a Big One! • Caused by two fast moving Coronal Mass Ejections • Seen as far south as Texas and Arkansas, New Zealand and Australia also witnessed them. May 11, 2002 • Auroras seen as far south as New England • Caused by a gust of solar wind • These powerful gusts are guided by Earth’s magnetic field and can excite gases in the upper atmosphere, causing the air there to glow The key point here is that nearly all “solar weather” is a result of changes in the magnetic fields that penetrate the Photosphere. The Solar Wind Constant flow of particles from the sun Velocity ≈ 300 – 800 km/s The sun is constantly losing mass: 107 tons/year (≈ 10-14 of its mass per year) Helioseismology The solar interior is opaque (i.e. it absorbs light) out to the photosphere. The only way to investigate solar interior is through Helioseismology. = analysis of vibration patterns visible on the solar surface: Approx. 10 million wave patterns! The Sun continuously emits a lot of energy. It must replenish it, i.e. generate the energy it produces and emits or else it would cool off quickly (100,100 yr). How, Where? The Core is where all the action is: THE PRODUCTION OF THE SUN’S ENERGY • The core is the only place in the Sun where the temperature (10 million K) and density are high enough to support nuclear fusion (hydrogen bomb needs atomic bomb for ignition!) • What is nuclear fusion? Every second, about 600 million tons of Hydrogen are transformed (fused , really) into 596 million tons of helium. • The remaining mass (4 million tons) is converted into energy in line with Einstein’s formula E= 2 mc The Key to understand energy production by NUCLEAR FUSION • Take 4 free (=not part of a nucleus) protons. Their total weight is 4xWp • NOTE: protons do not want to come close to each other: same electrical charge repel each other • But if, using sufficient force, one can bring them close enough that the nuclear force makes them stick together, they become nuclearly bound • In this situation, the total weight of the 4 protons is no longer 4xWp, but 4xWp– “some missing mass” • This “some missing mass” has been converted into energy via E = mc2 41H --> 4He + energy ( E = mc2 ) mass(4He) = 0.993 x {mass(H)+mass(H)+mass(H)+mass(H)} mass loss is 0.007 x {mass(4H)} = 5 x 10-29 kg E=mc2=(5 x 10-29)(3 x 108)2 = 4 x 10-12 joules How many fusions per second? Solar Luminosity = 4 x 1026 joules/sec 4 x 1026 joules/sec ----------------------- = 1038 fusions/sec , which is 200 million tons/sec!!! 4 x 10-12 joules Actually, about 500 million tons/sec are needed! Nuclear Fusion in the Sun • Hydrogen is constantly being transformed into helium in the Sun’s core. Energy Generation in the Sun: The Proton-Proton Chain Basic reaction: 4 1H 4He + energy 4 protons have 0.048*10-27 kg (= 0.7 %) more mass than 4He. Need large proton speed ( high temperature) to overcome Coulomb barrier (electrostatic repulsion between protons) T ≥ 107 0K = 10 million 0K Energy gain = Dm*c2 = 0.43*10-11 J per reaction Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second, to resist its own gravity. Energy Production Energy generation in the sun (and all other stars): Nuclear Fusion = fusing together 2 or more lighter nuclei to produce heavier ones. Nuclear fusion can produce energy up to the production of iron; For elements heavier than iron, energy is gained by nuclear fission. Binding energy due to strong force = on short range, strongest of the 4 known forces: electromagnetic, weak, strong, gravitational When nucleons are packed together (bound), their energy status changes relative to when they are not bound. If becoming bound puts them in more energetically advantageous state, then they can be fused together. The quantity to look at is the binding energy of each individual nucleon (note that nucleons can transform each other from p to n and n to p as they need) The importance of Temperature •To make the fusion reaction possible we have to bring the protons close enough that they feel each other’s Strong Force (nuclear force) •Strong Force acts on a short-range: •it is zero at distances larger than a critical distance; •it is different from zero at distances shorter than a critical distance •At distances shorter than the critical distance, the strong force is ATTRACTIVE, and it is very, very, VERY STRONG. Much stronger than the repulsive electromagnetic force. •To get the protons close enough to give the Strong Force a chance to work, we have to smash them with sufficiently high speed: we need very high Temperature •The higher the temperature, the larger the number of fusion reactions each second. •Even small Temperature variations cause large variations in the number of fusion reactions Counting Solar Neutrinos The solar interior can not be observed directly because it is highly opaque to radiation. But, neutrinos can penetrate huge amounts of material without being absorbed. Early solar neutrino experiments detected a much lower flux of neutrinos than expected ( the “solar neutrino problem”). Recent results have proven that neutrinos change (“oscillate”) between different types (“flavors”), thus solving the solar neutrino problem. Davis solar neutrino experiment An aside about nuclear fusion as a viable energy source A few key points about the P-P Chain fusion reaction: • It is a “clean” form of energy production. • Put 4 protons (Hydrogen) in and get one Helium and some energy back out. • Given fuel (in the form of Hydrogen) it is a selfsustaining reaction. • It requires *very* high temperatures (107 K) • Where to put the reacting gas? • We are *very* hard at work trying to build a controllable fusion reactor here on Earth to produce unlimited, clean, dirt-cheap energy. Balance in the stars Thermal Pressure Gravitational Contraction Pressure and Temperature of a Gas How does a star hold itself? This balance between weight and pressure is called hydrostatic equilibrium. The Sun's core, for example, has a temperature of about 16 million K. The House’s Thermostat Air temperature inside drops below the set point of the thermostat Electrical signal triggers ignition of furnace Furnace shuts off Furnace burns oil/gas and generates heat Air warms thermostat until electrical signal is shut off Radiators transfer energy from water to air Heat is absorbed by water running near furnace causing water temperature to rise Hot water is circulated throughout house The Solar Thermostat Outward thermal pressure of core is larger than inward gravitational pressure Core expands Nuclear fusion rate rises dramatically Contracting core heats up Core contracts Expanding core cools Nuclear fusion rate drops dramatically Outward thermal pressure of core drops (and becomes smaller than inward grav. pressure) Balance • If the star contracts at all, the temperature goes up, the rate of fusion increases, the pressure increases, and the star re-expands. • If the star expands too much, the temperature drops, the rate of fusion drops, the pressure drops, and the star contracts. How is the energy transported from the center to the surface? • Radiative transfer • Photons are continually absorbed and re-emitted by particles like electron and nuclei (reprocessing of radiation), and when they are re-emitted from more external layers, which are colder, the loose energy (but total energy is conserved, because more photons are emitted) • Convection • Large scale motions of gas. Hot gas moves upward and carries energy there. Cold gas move downward to restore balance Radiative Transport • Photons are absorbed and re-emitted in random walk motion. • Each photon mean-free path is very, very short • It takes each γ photon produced by nuclear fusion (gamma ray, very energetic) million years to reach the surface, and when it does, it has been reprocessed into ~1,800 cooler visible-light photons (energy is conserved!!) Convection Activity Energy Transport in the Sun g-rays Radiative energy transport The energy emitted by the Sun is produced • in a small region at the very center of the Sun. • uniformly throughout the entire Sun. • throughout the entire Sun but more in the center than at the surface. • from radioactive elements created in the Big Bang. The bulk of the violent surface activity on the Sun is due to “seasonal” variations in the Sun’s _________. 1) energy output 2) radius 3) electric fields 4) magnetic fields What is burning in stars? • • • • Gasoline Nuclear fission Nuclear fusion Natural gas The Solar Constant • The S.C tells us the amount of energy the Sun produces: • It is 1360 Joules per second per square meter… • …or about the luminosity (=ENERGY PER SECOND) of 60 W bulb every 2 square millimeters. • A stable change of ~1% in the S.C. would change Earth’s temperature by 1-2 degrees • Random variation ~0.1% over days or weeks • Long-term, cyclical variation 0.018% per year (period longer than the 22 years of the sunspots) • Small random fluctuations do not affect Earth’s climate The Solar Constant The energy we receive from the sun is essential for all life on Earth. The amount of energy we receive from the sun can be expressed as the Solar Constant: Energy Flux F = 1360 J/m2/s F = Energy Flux = = Energy received in the form of radiation, per unit time and per unit surface area [J/s/m2] A variation of F of only ~1% would make the Earth’s temperature change by 1-2 C