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Magnetic Activity Astronomy 315 Professor Lee Carkner Lecture 11 Exercise 10 -- Stellar Interiors a) (3X1029 kg in core) / (1.67X10-27 kg per H atom) a) = b) (1.8X1056 H atoms) / (4 H atoms per He atom) b) = c) (4.5X1055 He atoms)(0.048X10-27 kg liberated per He atom) c) = d) E = mc2 = (2.2X1027)(3X108)2 d) = e) (1.9X1044 Joules) / (3.8X1026 Joules per second) e) = f) (5.1X1017 seconds) / (3.1536X107 seconds per year) f) = The sun has enough hydrogen in its core to shine for 16 billion years. The Outer Limits This is the part of the star that we see Is there anything above it? What happens during an eclipse when the photosphere of the sun is blocked out? Magnetic Activity Spectra of the corona reveal a temperature of 1-10 million K Where is this energy coming from? Answer: Magnetic fields are generated by motions inside stars and greatly affect the movement and heating of the outer regions of stars Magnetism What is a magnetic field? Ions or electrons Since they represent the potential to do work Magnetic Field Generation Much of the sun is ionized and is a good conductor of electricity Known as the dynamo effect Manifestations of Magnetic Activity We see the results of stellar magnetic fields in two ways: Starspots Coronal activity We will use the sun as our example since it is the only star for which we can resolve magnetic effects Spots in the Photosphere The photosphere sometimes has small dark regions called sunspots Sunspots are regions where the Sun’s magnetic field inhibits the flow of warmer material Sunspots Starspots on IL Hydrae Sunspot Cycles The average number of sunspots changes with time 11 years between one sunspot maximum and the next Just after sunspot minimum the spots appear at about 30 degrees north or south Sunspot Maximum and Minimum The Sunspot Cycle Sunspot Cycles and Differential Rotation The Sun rotates differentially The magnetic field gets “wrapped-up” around the equator Eventually the magnetic field is squeezed so tight it reconnects and cancels itself out 11 year sunspot cycle caused by 11 cycle of winding The Twisted Magnetic Field of the Sun My Corona The corona is the outer layer of the Sun’s atmosphere T ~ 1-10 million degrees Why is it so hot? The corona is very thin, so the particles don’t collide very often to lose their kinetic energy Temperature in the Sun’s Atmosphere The Structure of the Corona The high temperatures and irregular structure of the corona are due to magnetic fields These loops contain charged particles The tangled, shifting magnetic loops heat the corona and give it its structure Flares and Magnetic Activity Sometimes a large outburst of material is seen, called a solar flare or coronal mass ejection They are both examples of magnetic activity During the Maunder Minimum in sunspot activity (1645-1715) it was very cold in Europe (The Little Ice Age) Magnetic Activity and Other Stars There are many stars that exhibit magnetic activity, some are much stronger than the sun Usually because they are rotating faster May be “spun up” by interaction with a binary companion Next Time Read Chapter 19.1-19.4