Magnetic Activity

Astronomy 315Professor Lee

CarknerLecture 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