Transcript 23sun4s

The Sun
Astronomy 311
Professor Lee Carkner
Lecture 23
Helios -- The God of the Sun

The Sun was often
worshiped by
ancient people
WARNING !
NEVER LOOK DIRECTLY AT THE
SUN
NEVER LOOK AT THE SUN WITH
BINOCULARS OR A TELESCOPE
PERMANENT EYE DAMAGE CAN
RESULT
Observing the Sun

 The solar surface is nearly featureless

Much of our information about the Sun
comes from multiwavelength observations,
indirect measurements, and modeling
The Sun From the Inside Out

The solar interior is the source of solar
energy which is transported to the
surface
Nuclear fusion and magnetic fields
play key roles in the energetics and
structure of the Sun
Why Does the Sun Shine?
From radioisotope dating we know that
the solar system is 4.5 billion years old

 Chemical energy (burning) -Kelvin-Helmholtz contraction
(gravitational energy) -Nuclear Fusion Reactions --
The Core
At the core of the Sun the temperature and
pressure are very high due to the weight of
the outer layers

At these conditions the hydrogen atoms are
moving so fast and are packed together so
tightly that they can fuse together to make
helium:

Hydrogen Fusion
How Does the Energy at the
Core Get Out?
Energy in the Sun is transported in two ways:
Radiation -Like a heat lamp

Gas in this area is very hot and ionized and so
doesn’t absorb the radiation very well
Convection --
Like baseboard heating

Cooler outer layers are less ionized and so absorb
the radiation and heat up and rise
The Inner Structure of the Sun
The Photosphere
It takes about 170,000 years for the energy to
reach the surface of the Sun

The visible surface of the Sun is called the
photosphere

 The photosphere is the top of the convection
zone

Each about 1000 km across
Granules
Sunspots 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
Sunspot Cycles
Sunspots exist for a maximum of a few months

There is a sunspot cycle of 11 years



 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

 The magnetic field prevents warmer material from
flowing into the sunspots, so the sunspots are cooler than
the rest of the photosphere
The Twisted Magnetic Field of
the Sun
The Sun’s Magnetic Field

 It is twisted and tangled producing loops of
magnetic force

 The Sun’s magnetic field is responsible for
the structure and heating of the outer layers
of the atmosphere
The Solar Wind
The magnetic fields in the outer layers of the
Sun can accelerate material outward

Interacts with the Earth’s magnetic field

Very strong solar wind bursts can cause
power and communication failures
The Chromosphere
The chromosphere is a diffuse layer of the solar
atmosphere extending from the photosphere to
about 2000 km

It has a temperature of about 20000 K

It is too thin for the atoms to collide and lose energy
The Corona
The corona is the outer layer of the Sun’s
atmosphere

It is thinner and hotter than the chromosphere


 This plasma emits strongly at ultraviolet and
X-ray wavelengths
Flares and Magnetic Activity
The material in the corona is constantly
changing


 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)
Core
Radiative Zone
Convective Zone
Photosphere
Chromosphere
Corona
Structure of the Sun
Summary: Structure of the
Sun
Core
fusion converts H to
He and power the
Sun
Radiative Layer
transports energy
from the core
Convective Layer
transports energy to
the photosphere
Photosphere
visible surface of the
Sun
Chromosphere
hot middle
atmospheric layer
Corona
very hot outer layer
Summary: Solar Energetics
Thermonuclear Fusion
Energy is produced at the core of the Sun
by converting hydrogen to helium
Energy Transport
Energy is transported via radiation where
the opacity is low and via convection
where the opacity is high
Magnetic Fields
The outer layers of the Sun are composed
of hot plasma in magnetic loops