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  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 ( R sun )    Convective Zone ( R sun )    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 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