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The Sun – Our Star
Solar Cycles
Active Sun
General Properties
 Not a large star, but larger than most
 Spectral type G2
 It appears bright because it is so close
 Absolute visual magnitude = 4.83
 Sun’s radius is 109 times Earth’s
radius (Rʘ = 1.4 x 106 km)
 Sun’s mass is 333,000 times Earth’s
mass (mʘ = 2 x 1030 kg)
 Consists entirely of ionized gas
(av. density = 1.4 g/cm3)
 Central temperature = 1.5 x 107 K
 Surface temperature = 5800 K
Physical Properties of the Sun
Interior structure of
the Sun:
Outer layers are
not to scale
The core is where
nuclear fusion
takes place
Physical Properties of the Sun
Luminosity – the total power radiated by a
star – it can be calculated from the fraction
of that energy that reaches Earth.
The luminosity of the Sun is about
4 × 1026 W – the equivalent of 10 billion
1-megaton nuclear bombs per second.
Solar constant – amount of the Sun's
energy reaching Earth = 1400 W/m2. It is
the Sun’s energy flux (energy/unit area) at
1 AU.
Determining Luminosity
• Measure k, the solar
constant, the energy
flux reaching the
• Measure a, the
distance from the
Earth to the Sun
• Determine the
surface area A of a
sphere with the
radius a: A = 4π a2
• Luminosity:
L = kA
The Energy of the Sun
Nuclear fusion is the energy producing mechanism of the
Sun. In general, nuclear fusion works like this:
nucleus 1 + nucleus 2 → nucleus 3 + energy
But where does the energy come from?
 It comes from the loss of mass ∆m: if you add up the
masses of the initial nuclei, you will find that it is more
than the mass of the final nucleus. The difference in
mass ∆m has been changed into energy.
 The relationship between mass and energy is found in
Einstein’s famous equation:
∆ E = ∆m c2
In this equation, c is the speed of light which is a very
large number.
Energy Production
 Nuclear
energy up to
the formation
of iron.
 Nuclear
energy when
heavier than
iron split .
Binding energy is a result of
the strong force. It has a very
short range. It is the
strongest of the 4 known
forces: electromagnetic,
weak, strong, gravitational
Energy Generation in the Sun:
The Proton-Proton Chain
 Basic reaction:
4 1H → 4He + energy
 4 protons have 0.048 x
10-27 kg (= 0.7 %) more
mass than 4He
⇒ Energy gain =
Δm c2 = 0.43 x 10-11 J
per reaction
 ∴ the Sun needs 1038
reactions, transforming
5 million tons of mass
into energy every
second so that its
pressure balances its
own gravity to keep it
from collapsing.
Need large proton speed (⇒ high
temperature) to overcome Coulomb
barrier (electromagnetic repulsion
between protons).
T ≥ 107 K =
10 million K
The Solar Interior
The balance between gravitational forces pulling
toward the center with the heat pressure pushing
out is called hydrostatic equilibrium.
A differential
equilibrium as a
function of radius
is one of the
basic equations
used to model
stellar interiors.
The Solar Interior
Doppler shifts of solar spectral lines indicate a
complex pattern of standing waves inside the
The Solar Interior
These graphs plot solar
density and
temperature as a
function of distance
from the Sun’s center,
according to the
standard solar model
Heat Transport
Heat is transported by 3 physical mechanisms:
The Solar Interior
 Energy transport in stars is either by radiation or by convection
 Interior regions or zones are dominated by one transport mechanism or
the other
 A differential equation expressing energy transport as a function of
radius is another of the basic equations used to model stellar interiors.
 The radiation zone is relatively transparent; the cooler convection zone
is opaque
The Layers of the Solar
Coronal activity,
seen in visible
The Solar Atmosphere
is the apparent
surface of the
Heat Flow
The corona
and the
are only
visible during
solar eclipses
Solar interior
increases inward
The Solar Atmosphere
Spectral analysis can tell us what elements are
lines are used
for the
and the
lines are used
for the corona.
The Solar Atmosphere
Spectral lines are
formed when light is
absorbed before
escaping from the
Sun; this happens
when its energy is
close to an atomic
transition, so it is
absorbed. This
spectrum shows
absorption lines for
some of the
elements in the Sun.
The Photosphere
 Apparent surface layer of the Sun
 Depth ~500 km
 Temperature ~5800 K
 Highly opaque (H- ions)
 Absorbs and re-emits radiation produced in the solar interior
Energy Transport in the
Energy generated in the Sun’s center must be transported outward.
In the photosphere, this happens through
Cool gas
sinking down
~1000 km
Bubbles of hot
gas rising up
Bubbles last
for ~10-20 min
… is the visible consequence of convection
The Chromosphere
 Region of Sun’s atmosphere
just above the photosphere.
 Visible, UV, and
• X-ray lines from
highly ionized gases
 Temperature increases
gradually from ~4500 K to
~10,000 K, then jumps to
~1 x 106 K
Transition region from Chromosphere to Corona
Chromospheric structures visible in
Hα emission (filtergram)
The Chromosphere
Spicules: Spikes
of cooler gas from
the photosphere,
rising up into the
Visible in Hα
Each one lasts
~ 5-15 min.
Green River, WY
June 8, 1918
As seen during solar eclipses
Phillipines March 1988
The Solar Wind
The solar corona is blown
into interplanetary space
and is called the solar
The solar wind
 Is spiral shaped
because of the Sun’s
 Carries the Sun’s
magnetic field into
interplanetary space
 Interacts with planetary
magnetic fields
 Ionizes gas around
comets and forms the
comet’s ion tail
The Sun’s Magnetic Cycle
1. The Sun’s magnetic field is generated in the convection zone, not
in the core
2. The Sun rotates faster at the equator than near the poles.
3. This differential rotation pulls north-south magnetic field lines in
the direction of rotation at the equator.
The Sun’s Magnetic Cycle
4. After 11 years, the magnetic field pattern becomes so
complex that the field structure is re-arranged.
5. The new magnetic field structure is similar to the original
one, but reversed!
Magnetic Field Reversal
6. A new 11-year cycle begins with a reversed magnetic-field
Sunspots and Magnetic Fields
Magnetic North Poles
Ultraviolet filtergram
Magnetic image
South Poles
 Magnetic fields in sunspots are ~1000 times stronger
than average.
 In sunspots, magnetic field lines emerge out of the
 Cooler regions of the
photosphere (T ≈ 4240 K).
 Appear dark against the
bright Sun. They would still
be much brighter than the
full Moon when placed on
the night sky!
Sunspots and Magnetic Loops
 Sunspots are in
pairs of different
 A magnetic field
loops out of the
Sun from one
spot to the other
in the pair
 The leading spot
has the same
magnetic polarity
as the overall
polarity of the
Magnetic field lines
Magnetic Fields in Sunspots
Magnetic fields on the photosphere can be
measured using the Zeeman effect
⇒ Sunspots are related to magnetic
activity on the photosphere
The Solar Cycle
11-year cycle sunspot cycle
After 11 years, the
North/South order of
sunspots is reversed
⇒ Total solar cycle =
22 years
Reversal of magnetic polarity
 The sunspot cycle starts
out with a small number
of spots at higher
 It evolves to a larger
number of sunspots at
lower latitudes (towards
the equator) as the cycle
The Maunder butterfly diagram
The Maunder Minimum
 The period from ~1645 to 1715 had very few sunspots, which
coincides with a period of colder-than-usual winters in
 Is there a cause and effect relationship?
The Active Sun
 Areas around sunspots are active
 Large eruptions may occur in the photosphere.
 A solar prominence is a large sheet of ejected
The Active Sun
A solar flare is a large explosion on the Sun’s surface,
emitting a similar amount of energy as a prominence, but in
seconds or minutes rather than days or weeks.
Solar flares can
significantly influence
the Earth’s magnetic
field structure and
cause northern lights
(aurora borealis).
The Active Sun
Coronal mass ejection occurs when a large “bubble”
detaches from the Sun and escapes into space
The Active Sun
The solar wind escapes the Sun mostly through
coronal holes, which can be seen in X-ray
The Solar Neutrino Problem
 The solar interior cannot be observed
directly with electromagnetic radiation
because it is highly opaque.
 Neutrinos from fusion reactions can
escape the Sun without being absorbed.
 However, solar neutrino experiments
have detected a much lower flux of
neutrinos than expected (the “solar
neutrino problem”).
 Recent laboratory experiments have
shown that neutrinos oscillate between
different “flavors”
 They oscillate during their passage
from the Sun and only one “flavor” can
be detected thereby reducing the flux
 This solved the solar neutrino problem.
Davis solar neutrino