The Sun – A Typical Star
• The layers of the sun; core, radiative
zone, convective zone, photosphere,
chromosphere, and corona
• Sunspots and magnetic fields, the
sunspot cycle
• 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
upward
• 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.
Sunspots
• 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 off and darken.
• Vertical structure of a sunspot
Sunspots optical
HiRes sunspot
Sunspot in UV light. Hot Gas is following the
magnetic field lines
Earth w/ sunspot
Plages Are Hotter Areas
Near Sunspots, Where the
Energy Unable to Penetrate
a Sunspot Emerges Nearby
Sunspot cycle #vs time
Sunspot cycle 1760-1965
Solar Maxima have been of decreasing intensity for
~50 years
Helioseismology allows us to make rough predictions
of the solar cycle’s intensity for a few years in
advance
More Sunspots, And More
Plages Too, at Solar Max
How Does The Solar Cycle
Affect Earth?
• Two important ways…
• 1. The solar wind creates aurorae (more
later)
• 2. Solar luminosity changes during the
cycle. We have seen that lower solar
activity goes with lower average
temperatures on Earth
How Does Solar Activity Change
Solar Luminosity?
• Higher solar activity produces higher solar
luminosity.
• Mechanism – more surface magnetic field energy,
which thermalizes (i.e. becomes random kinetic
energy, as the 2nd Law of Thermodynamics
requires) and produces more net solar radiation.
• Some of the magnetic field energy directly
impacts Earth by high speed solar wind particles
hitting Earth.
• However, the luminosity changes are tiny; less
than 0.1%, as measured by satellites above the
atmosphere.
Satellite Data Shows TSI (Total Solar
Irradiance). Note the Decreasing
Luminosity of the Sun
Solar Luminosity: Past 400 Yrs, from Proxies
The Maunder Minimum and The
“Little Ice Age” (LIA); Connected?
• The Maunder Minimum in sunspot numbers in the late 1600’s
corresponds roughly to a cool period in climate on Earth.
• But detailed research finds several causes of the LIA…
• 1. Unusually intense volcanic ash/dust period
• 2. Disease killed ~20% of human population in the Americas,
reforestation took up extra CO2 out of the atmosphere.
• 3. Lower solar luminosity (relatively minor contributor, from
satellite correlations of sunspot number vs TSI)
• 4. Reduced thermohaline circulation failing to efficiently
distribute equatorial warmth to European and North American
latitudes
The Plague of the 1300’s is Considered a Starting
Point for the Little Ice Age. There’s Another Drop in
Human Population Just before Coal Discovery
Eclipse composite
Total eclipse corona
cme
solarwind
EarthOnionMagField
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
atoms
• These atoms then de-excite (electrons fall
back down through the energy levels) giving
off photons
• Exactly the same as how flourescent lights
work!
greenpurpleAurora
Aurora hoffman
Aurora ewoldt
Aurora tricolor
Aurora from space iss
Long Term Solar Evolution…
• 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 more luminous on the
long term
• During the 4.56 billion year life of the solar
system, the sun has increased in luminosity
by about 25%.
• This will continue, and gradually
accelerate…
Long term L,R,T
Key Points – Chap 14: The Sun and
Climate
• Sun driven by fusion of H into He in core, stable with
gravity balancing pressure
• Sunspots: places of strong magnetic field inhibiting
convection of heat from below, cool and darken
• Sunspot cycle 11 years, with ~100 year larger cycles
perhaps
• More spots = More solar activity = More solar luminosity,
but only 0.1% more at max vs min
• Long term solar luminosity; has risen 30% since Birth of
Earth, and will keep rising.
• Life on Earth will likely end in a billion years or so