The Sun

1
The Sun

• Astronomy 311
• Professor Lee Carkner
• Lecture 23

2
Helios -- The God of the Sun

• The Sun was often worshiped by ancient people

3
WARNING !

• NEVER LOOK DIRECTLY AT THE SUN
• NEVER LOOK AT THE SUN WITH BINOCULARS OR A
  TELESCOPE
• PERMANENT EYE DAMAGE CAN RESULT

4
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

5
Todays Sun
6
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

7
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?

8
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

9
Hydrogen Fusion
10
How Does the Energy at the Core Get Out?

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

11
The Inner Structure of the Sun
12
Radiative and Convective Zones

• Radiative Zone (0.25-0.71 Rsun)
• Convective Zone (0.71-1.00 Rsun)
• Hot material rises causing convection

13
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

14
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)

15
Solar Granulation
16
Granules
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Sunspots in the Photosphere

• The photosphere sometimes has small dark regions
  called sunspots
• Sunspots are regions where the Suns magnetic
  field inhibits the flow of warmer material

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Sunspots
19
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

20
Sunspot Maximum and Minimum
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The Sunspot Cycle
22
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

23
The Twisted Magnetic Field of the Sun
24
The Suns Magnetic Field

• The Suns magnetic field extends far out beyond
  the surface
• These loops can break and reconnect

25
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

26
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

27
Spicules in the Chromosphere
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Temperature in the Suns Atmosphere
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The Corona

• The corona is the outer layer of the Suns
  atmosphere
• It is thinner and hotter than the chromosphere
• The high temperatures produce a hot ionized gas
  called a plasma

30
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

31
Magnetic Activity Cycle
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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

33
Structure of the Sun
Core Radiative Zone Convective Zone Photosphere Ch
romosphere Corona
34
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

35
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