Overview: The Sun Properties of the Sun Sun s outer layers Photosphere Chromosphere Corona Solar Activity Sunspots & the sunspot cycle Flares, prominences, CMEs, aurora Sun s Interior The Sun as an energy source Fusion in the Sun Nearest Distance ~ _ AU Temperature ~ K From peak intensity Sun s Properties Radius ~ 7 x 10 5 km About x Earth s radius Mass ~ 2 x 10 30 kg About x Earth s mass Sun s Properties Composition of the Sun Sun is gaseous Violent, bubbling up close Too hot to be solid or liquid 73% 25% 2% other stuff (near surface) The outer layers Photosphere: Surface Innermost visible layer, only 400 km thick (0.06% of radius) Temperature: 6,000 K Defines of the Sun lines 1
Photosphere isn t actually smooth! Features called are about 1000 km across Convection causes granules : transfer of energy through currents Dark-colored areas => gas Light-colored areas => gas Granules Up-Close: like boiling water Chromosphere: sphere of Middle layer, temperature 10 4-10 5 K Chromosphere: Hot hydrogen atoms form a red streak when viewed during a solar eclipse Corona or of the Sun Very low density 10 million to 10 billion x less dense than Earth s atmosphere However, corona is extremely hot! About K Known from spectrum 2
Solar activity is like weather Sunspots Solar Flares Solar Prominences Coronal Mass Ejections (CMEs) All are related to magnetic fields, vary on an -year cycle Sunspots Dark spots on the photosphere than other parts of the Sun s surface (4000K vs. 5800K) Sunspot Number Number & locations change daily Can use observations to determine how fast the Sun Sunspots One of Galileo s major observations More than 30 sketches from summer of 1612 Solar Solar Sunspot number about every 11 years: the sunspot cycle Sunspot Cycle Sunspots usually appear within 30º of equator Where they form changes during a cycle Data goes back to the 1600 s Butterfly Diagram 3
Sunspots often come in We think most solar activity is related to changing magnetic fields One is positive, the other negative, like a magnet Connected by loops of bright gas Fig. 7-14a, p. 135 Fig. 7-14b, p. 135 Fig. 7-7c, p. 135 Fig. 7-14d, p. 135 4
Other Solar Activity: Solar Prominences Solar Flares Coronal Mass Ejections All occur in associated with strong fields Fig. 7-14e, p. 135 Solar Flares Bursts of X- rays and charged particles 5-10 minutes Solar Prominences Erupt high above the Sun s surface Can be quite stable (hoursdays) Usually form near! Coronal Mass Ejections Rare bursts, more energetic than flares or prominences Speeds up to 1,000 km/s 50,000 km above Sun s surface Solar Wind Not moving air, like on Earth Stream of from coronal holes 900,000 mph, reaches Earth in about 4 days 5
Solar Wind Charged particles get trapped in Earth s magnetic field and cause auroras The Sun Today www.spaceweather.com Structure of the Sun Core Corona Core: Energy generated by nuclear fusion Inner 25% ~ K Very dense! Radiation Zone Radiation Zone: Energy transported upward by photons Convection Zone Convection Zone: Energy transported upward by rising hot gas K Outer 30% Photons spend a long time here K 6
causes granules Convection (rising hot gas) takes energy to surface What is the Sun s structure? From inside out, the layers are: Core Corona How does the Sun shine? The Sun has its own energy source Main difference between a star and a planet Not well understood until 1940 s Need to explain & Lifetime of the Sun Need a vast, constant source of energy Sun is at least years old (from fossils) Most ideas could not sustain the energy rate needed Luminosity of the Sun Energy output: 3.9 x 10 24 Joules/sec A 100-Watt light bulb emits 100 Joules per second 7
Possible Solar Heating Mechanisms Gravitational Collapse Gives lifetime of about years Nuclear Power on Earth Nuclear Fission: large nuclei This is what happens in nuclear power plants. Nuclear Fusion: small nuclei This has only been used for weapons. Combustion (burning/fire) Gives lifetime of about years Need 4.6 billion years! San Onofre Fission Plant, CA Hydrogen (fusion) bomb, Pacific Ocean, 1962 How does nuclear fusion occur in the Sun? Fission Big nucleus into smaller pieces (Nuclear power plants) Fusion Small nuclei to make a bigger one (Sun, stars) Energy from the Sun: The Sun has hydrogen at its core, at ~15 million K Basic Particles: Protons (in nucleus), positive charge Neutrons (in nucleus), no charge Electrons, negative charge Particles in nucleus have binding energy Core is so hot, the hydrogen is in plasma form: the electrons are free 8
Speed Interaction High temperature enables nuclear fusion to happen in the core: Overcome repulsion Nuclear Fusion 4 hydrogen nuclei (protons) must collide Not very likely Helium nucleus is built up in steps This sequence of steps is called the - Speed Interaction Where Stars Get Their Energy End product of fusion process is a nucleus 1 He is massive than 4 H by a factor of 0.007 (0.7%) Where did that mass go? E =mc 2 c is the speed of light c = 3 x 10 8 m/s so c 2 = 9 x 10 16!!!!! IN 4 protons OUT 4 He nucleus 2 gamma rays 2 positrons 2 neutrinos Total mass is 0.7% lower A small amount of mass can produce a lot of energy Proton-proton chain has 3 steps Fusion in the Sun P-P chain 3 steps: 2 1 H -> 2 H + e + + ν (14 billion ) 2 H + 1 H -> 3 He + γ (6 ) 3 He + 3 He -> 4 He + 2 1 H (1 million ) Most of the stuff in the Sun has not yet undergone fusion! (and that s a good thing ) 9
How does the energy from fusion get out of the Sun? 10