Proton-proton cycle 3 steps PHYS 162 1
4 Layers of the Sun CORE : center, where fusion occurs RADIATION: energy transfer by radiation CONVECTION: energy transfer by convection PHOTOSPHERE: what we see PHYS 162 2
Layers of the Sun Mostly Hydrogen with about 25% Helium. Small amounts of heavier elements Gas described by Temperature, Pressure, and Density with P= kdt (mostly) Larger temperature near Radius = 0 Inner radius is a PLASMA - gas where all atoms are ionized. T >100,000 degrees K and so free electrons H (48) He (4) electron (56) PHYS 162 3
PHYS 162 4
Equilibrium Temperature of the Sun is constant for any given radius. It doesn t change as heat flows out Gravitational Force pulling in BALANCES the gas pressure (Electric force) pushing out At center : highest gravitational pressure gives the highest temperature PHYS 162 5
Convection Zone T = 6,000 100,000 K Radiation Zone T = 100,000 5,000,000 K Temp is highest in the core where nuclear fusion occurs heat flows outward to surface, then radiated as light to (say) Earth PHYS 162 6
Core - Center of Sun High temperature ~15,000,000 degrees K high density ~ 100 g/cm 3 where fusion occurs H He and heat flows out source of neutrinos PHYS 162 7
Core - changes with time As heavier, Helium produced by fusion reaction tends to float to the center. For now, He isn t burning and there is a mini-core of (mostly) He with reduced fusion being built up Red=H green=he PHYS 162 8
Radiation Layer temperature 100,000 to 5,000,000 degrees (plasma) no fusion electrons are not in atoms very, very opaque Energy transferred by absorption and reradiation of light photon photon photon electron electron PHYS 162 9
Convection Layer temperature 6,000 to 100,000 degrees no fusion electrons in atoms less opaque Energy transferred through convection. Movement of gas to/from surface ( hot air rises) PHYS 162 10
Convection and Radiation layers differ on how heat is transferred PHYS 162 11
Photosphere Sun gas cloud no true surface Light we see comes from a 200 km fairly transparent region photosphere and top of convection region temperature 4,500-6,000 photosphere cooler than convection region dark line absorption spectrum photosphere Convection region PHYS 162 12
Outer Atmosphere Surface of the Sun hot, turbulent with electric/magnetic storms which throw out energetic particles CHROMOSPHERE low density, high T glows red (H atom) seen in eclipse CORONA even lower density and higher T (over 1,000,000 degrees) SOLAR WIND protons escaping Sun s gravity so large velocity. Can interact in Earth s atmosphere PHYS 162 13
Sunspots Intense magnetic fields which inhibited convection currents to the surface appear darker as at lower temperature Solar storms/flares often associated with sunspots Had been observed prior to Galileo s time (and without telescopes) Galileo gets credit as he had best explanation Sunspot activity varies with time. 11 year cycle plus variation over hundreds (thousands) of years change in Solar energy output PHYS 162 14
Outer Atmosphere Can see during eclipses. Interactions of solar wind with Earth s magnetic field and atmosphere causes Aurora Borealis PHYS 162 15
Aurora Borealis Northern Lights seen at high latitudes as magnetic fields are lower in the atmosphere. rarely seen in DeKalb. Photos are from Alaska and Maine PHYS 162 16
Solar Storms Large eruptions from Sun s surface are called flares or storms Will increase flow of charged particles to Earth, increase Northern Lights, and have (some) radiation impact (plane flights, on space station, radio signals) Large one in January 2012 PHYS 162 17
Test 1 Guide for short answer questions Motion of Sun, stars, planets through sky vs seasons Galileo s astronomical observations Kepler s Laws of planetary motion Newton s Laws of motion apply to Kepler s (mostly F=ma) how light is produced (accelerated charge) plus discrete vs. continuous nuclear reactions in the Sun : p-p cycle Layers in the Sun 4 forces with examples PHYS 162 18
The Nature of Stars Measure properties of Stars Distance Mass Absolute Brightness Surface Temperature Radius Find that some are related Large Mass Large Brightness Determine model of stellar formation and life cycle PHYS 162 19
Distances to Stars Important as determines actual brightness but hard to measure as stars are so far away Closest Alpha Centauri 4.3 light years = 4 x 10 13 km (1 AU = distance Earth to Sun = 8 light minutes) Close stars use stellar parallax (heliocentric parallax or triangulation same meaning) Can easily measure distance using parallax to a few 100 LY. Need telescope: first observed in 1838. Study close stars in detail. Other techniques for distant stars PHYS 162 20
Distances to Stars - Parallax PHYS 162 21
Shifting Star Positions The orbit of the earth is used as the base. Near stars appear to move more than far stars distance = (base length)/angle define: 1 parsec = 1/(angle of 1 second of arc) = 3.3 LY site A December angle Sun site B June PHYS 162 22
Stellar Parallax A photo of the stars will show the shift. July PHYS 162 23
Nearest Stars 61 Cygni first parallax in 1838 PHYS 162 24
Nearest Stars The larger the angle (T.Par. = trigonometric parallax) the closer the star many stars come in groups like the 2 stars in the Sirius binary cluster close together, within same solar system Alpha Centauri and Procyon are close binary systems PHYS 162 25
Parallax Data 200 BC, Hipparcos 850 stars, 1 degree; 1627, Brahe, 1000 stars, 1 arc-minute; 1725 (telescope) 3000 stars, 10 arc-seconds In 1900 only 60 stars had parallax measurements. photographic plates 0.01 arc-seconds 1997-2000 European satellite Hipparcos parallax measurements > 2,300,000 stars up to 500 LY distance 118,000 stars.001 arc-second resolution OLD(1990): 100 stars with distance known to 5%. NEW (2005): 7000 such stars ESA Gaia satellite: 2013 0.00001 arc-second. Goal: measure 1 billion stars (2016 still analyzing) PHYS 162 26