The Sun as Our Star

1
The Sun as Our Star

• Last class we talked about how the Sun compares
  to other stars in the sky
• Today's lecture will concentrate on the different
  layers of the Sun's interior and its atmosphere
• We will also talk about the behavior of the Sun
  over time and some of the cycles it goes through

2
Properties of the Sun
3
Solar Composition
4
The Solar Interior

• We already know that the core of the Sun is where
  fusion takes place
• There are two other main levels in the solar
  interior, the radiation zone and the convection
  zone
• _

5
The Solar Core

• The central core of the Sun is about 200,000 km
  in size
• _
• All fusion of hydrogen into helium takes place
  inside the core

Core
6
The Radiation Zone

• The next layer out from the core is the radiation
  zone (about 300,000 km thick)
• _
• In this layer, the energy produced in the core is
  transported by radiation (light)
• Gamma rays scatter repeatedly off atoms in this
  layer, losing energy and making their way to the
  surface

Radiation Zone
7
The Convection Zone

• The convection layer is the final interior layer
  and is about 200,000 km thick
• _
• Here, the energy of the Sun is transported by the
  swirling motion of the heated gas
• Hot cells of gas rise to the surface, release
  their energy, cool, and drop down toward the
  interior

Convection Zone
8
The Solar Atmosphere

• The outer layers of the Sun are comprised of the
  material we can actually see
• The 'surface' of the Sun is the point at which
  light is free to escape, without bumping into
  more and more atoms
• _

9
The Photosphere

• The photosphere is the first layer in the solar
  atmosphere and is usually thought of as the
  'surface of the Sun' (only 500 km thick)
• _
• The photosphere can be viewed using a filter
  which blocks out the majority of the light from
  the Sun
• Images of the photosphere will show sunspots on
  the surface of the Sun

10
Features of the Photosphere

• Sunspots are features in the photosphere created
  by interactions with the Sun's magnetic field
• Just like a huge magnet, some bodies in the Solar
  System (like the Sun, Earth) will have a magnetic
  field
• Sunspots usually come in pairs, each member of
  the pair can be thought of a north and south
  component of a magnetic field line
• _

Umbra
Penumbra
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Features of the Photosphere
Features of the Photosphere

• Close up pictures of the Sun's photosphere reveal
  granulation - the Sun's surface appear to be
  broken into small cells
• _
• Each cell represents a convective 'bubble' that
  has risen to the surface
• The bright interior is where hot material is
  coming to the surface, the darker exterior is
  where cooler material is sinking below

Solar Granulation
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The Chromosphere

• The next layer of the solar atmosphere is the
  chromosphere, about 1,500 km thick
• _
• This region is not easily visible since the light
  of the photosphere is so much greater

The Sun using an H-alpha filter
13
Features of the Chromosphere

• Close up images of the chromosphere reveal small
  jets of gas shooting up from the surface, called
  spicules
• On much larger scales, long stringy filaments of
  gas can also be seen in the chromosphere
• _

Spicules
Filaments
14
The Corona

• The outermost layer of the Sun's atmosphere is
  called the corona, and extends 100,000's of km
  away from the Sun's surface
• Surprisingly, the corona is much hotter than the
  other layers of the atmosphere, in the millions
  of Kelvin
• _

The corona become visible during a total solar
eclipse
15
The Solar Wind

• _
• The solar wind is most dense near the Sun, and
  dissipates as you move further and further away
• The solar wind is easily detectable at the Earth,
  and is still strong at Jupiter and beyond

16
Solar Activity

• Very frequently, large amounts of gas will erupt
  from the surface of the Sun
• _
• A stream of material coming off the Sun is
  usually called a flare, while a loop of material
  that falls back to the Sun is called a prominence

Solar flare
Solar Prominence
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Coronal Mass Ejection

• Occasionally (a couple times a week), very large
  eruptions will explode off the Sun's surface into
  space
• These coronal mass ejections (CME's) travel very
  fast outward through the solar system
• A large CME directed toward the Earth can affect
  satellites, the space station, and electrical
  grids on the Earth's surface

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SOHO
19
Solar Sunspot Cycle

• _
• During the 1600's, there was a lack of activity
  now known as the Maunder Minimum
• After that, it became quite apparent that sunspot
  activity follows an 11-year period

20
What Causes the Sunspot Cycle?

• Observations of the Sun's rotation have revealed
  that it does not rotate at the same speed in all
  places
• _
• At its equator, the Sun rotates significantly
  faster than at the poles
• The magnetic field of the Sun is tied to the
  rotating gas

21
Twisted Magnetic Fields

• Differential rotation causes the magnetic field
  lines of the Sun to become twisted
• The more twisted they become, the higher the
  amount of activity on the Sun's surface

22
The Sunspot Cycle

• Eventually, the magnetic field lines become so
  disrupted that the field disappears and then
  reforms
• Each time the magnetic field is reformed, the
  field lines are straight and solar activity
  decreases
• Also, the polarity of the Sun's poles change (the
  north magnetic pole becomes the south and vice
  versa)