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The Sun – A Typical Star
In Astro 3, we’ll do a very brief chapter on
the Sun. More detail… take Astro 4.
Here’s the Story we’ll Unfold for you…
• The layers of the sun; core, radiative zone,
convective zone, photosphere,
chromosphere, and corona
• Sunspots and magnetic fields, the sunspot
• Solar activity and how it influences the Earth
Gravity vs. pressure
A Star: A Balance between
Gravity and Pressure
• Self-regulating…
• Higher fusion rate would expand
star, lowering core’s self-gravity
and thence density, pressure,
temperature and thus lowering
fusion rate. And vice versa
Sun layers
Layers of the sun
• Core = where temperature exceeds fusion point
(10 million Kelvin)
• Radiative Zone = nothing much goes on here. It
just acts as an obstacle course for the photons
created in the core and random-walking their way
• Convection Zone = temperature gradient is so
steep that photon diffusion can’t carry the heat
outward fast enough. The rising temperature
expands the gas, lowering density and causing it to
rise (helium-balloon-like) to the surface, where it
cools, gets denser, and falls back down to get
reheated and start all over again. Think – soup
cooking on a stove.
• Photosphere = visible surface. This is where the
mean free path now gets so long the material is
transparent above here.
• Places where the sun’s magnetic field is concentrated and
inhibits the normal convective flow of hot material from
below. So the material sits on the surface and cools off as it
radiates to the sky.
• Charged particles in a magnetic field feel a force
sideways to their motion, binding the gas to the field.
• Sunspots are like “magnetic scabs” of gas unable to
be recirculated to lower, hotter levels. They are
bound to the magnetic fields in the photosphere,
cooling as they radiate to the cold universe, and
hence cool and darken.
• Vertical structure of a sunspot
Sunspots optical
Sunspot group optical
Marching sunspots
Sun pic at max,min
Sunspot cycle 1760-1965
Sunspot cycle #vs time
How Does The Solar Cycle
Affect Earth?
• Two important ways…
• 1. The solar wind creates aurorae (more
• 2. Climate. We have seen that lower solar
activity goes with lower average
temperatures on Earth
How Does Solar Activity
Change Earth Temperatures?
• Not well understood – clearly a very complex
mechanism is involved. However,
• Lower solar activity -> Lower solar luminosity,
so that’s one way. But probably inadequate to fully
explain historical climate record, since the effect is
far less than 1%
• Could solar activity explain current climate
change? No. In the last 10 years, climate change
has become so dramatic and so accelerated that it
far exceeds any historical correlation between
sunspot numbers and average temperatures.
Chromosphere all around
Diamond ring
Total eclipse corona
Eclipse composite
Total eclipse corona
Powerful solar flare
Sun Halpha + big prominence
Sun halpha + big prom; later
Earth magnetosphere
SOHO wide angle
Aurorae – GiNormous
Flourescent Lights!
• Caused when high speed solar wind particles
impact the Earth’s atmosphere
• Collisionally excites the nitrogen and oxygen
• These atoms then de-excite (electrons fall
back down through the energy levels) giving
off photons
• Exactly the same as how flourescent lights
Aurora westlake
Aurora lindersen
Aurora hoffman
Aurora c/IkeyaZhang
Aurora ewoldt
Aurora tricolor
Aurora from space iss
Comet NEAT kinky tail
Long Term Change…
• As the sun ages, its core collapses as
hydrogen converts to helium, and this
increases the gravity and pressure and
fusion rate in the core
• So, the sun is getting brighter long term
• During the life of the solar system, the sun
has increased in luminosity by 25%.