The Sun

Astronomy 311Professor Lee

CarknerLecture 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

With a solar filter in place we can use a telescope to get a high spatial resolution optical image of the Sun

Today’s Sun

The Sun From the Inside Out

We will examine the Sun from the core to the diffuse outer layers

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

What could power the Sun for this length of time?

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?

In general energy is transported in 3 ways: Conduction -- Radiation -- Convection --

The Inner Structure of the Sun

Radiative and Convective Zones Radiative Zone (0.25-0.71 Rsun)

Convective Zone (0.71-1.00 Rsun) Hot material rises causing convection

The Photosphere It takes about 170,000 years for the energy to

reach the surface of the Sun

The photosphere has an average temperature of 5800 K The top layer will absorb some of the light from the

bottom layers producing absorption lines

Granulation in the Photosphere

The photosphere is at the top of the convective zone

The photosphere is covered with

granules (each about 1000 km across)

Solar Granulation

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

The sunspots move towards the equator over the course of the cycle

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

The sunspots are caused by the magnetic field “kinks”

The Twisted Magnetic Field of the Sun

The Sun’s Magnetic Field

The Sun’s magnetic field extends far out beyond the surface

These loops can break and reconnect

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

The Structure of the Chromosphere

The chromosphere is composed of many rising filaments of hot gas called spicules

Between the chromosphere and the corona is the transition region where the temperature rises from 20,000 K to 1 million K over a small region

Spicules in the Chromosphere

Temperature in the Sun’s Atmosphere

The Corona The corona is the outer layer of the Sun’s

atmosphere

It is thinner and hotter than the chromosphere

The high temperatures produce a hot ionized gas called a plasma

Flares and Magnetic Activity

The material in the corona is constantly changing

Flare activity is linked to sunspot

activity

Changes in magnetic activity seem to effect climate

Magnetic Activity Cycle

The Structure of the Corona The high temperatures and

irregular structure of the corona are due to magnetic fields

The tangled, shifting magnetic loops gives the corona its structure

Structure of the Sun

Core

Radiative Zone

Convective

Zone

Photosphere

Chromosphere

Corona

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