Magnetic Activity Astronomy 315 Professor Lee Carkner Lecture 11

Exercise Stellar Interiors a)(3X10 29 kg in core) / (1.67X kg per H atom) a)= b)(1.8X10 56 H atoms) / (4 H atoms per He atom) b)= c)(4.5X10 55 He atoms)(0.048X kg liberated per He atom) c)= d)E = mc 2 = (2.2X10 27 )(3X10 8 ) 2 d)= e)(1.9X10 44 Joules) / (3.8X10 26 Joules per second) e)= f)(5.1X10 17 seconds) / (3.1536X10 7 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 ( ) 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 

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