Transcript Magnetic Activity

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