Guiding Questions
What is the source of the Sun’s energy?
What is the internal structure of the Sun?
How can we measure the properties of the Sun’s interior?
How can we be sure that thermonuclear reactions are
happening in the Sun’s core?
5. Does the Sun have a solid surface?
6. Since the Sun is so bright, how is it possible to see its dim
outer atmosphere?
7. Where does the solar wind come from?
8. What are sunspots? Why do they appear dark?
9. What is the connection between sunspots and the Sun’s
magnetic field?
10. What causes eruptions in the Sun’s atmosphere?
1.
2.
3.
4.
The Sun’s energy is generated by
thermonuclear reactions in its core.
• The Sun’s luminosity (power output) is
L = 3.9 x 1026 watts (or joules per second)
• The Sun is powered by thermonuclear fusion
reactions in the core, where hydrogen is
converted into helium, releasing energy in a
process called the proton-proton-chain.
• Einstein’s equation, E = mc2 describes how
much energy, E, can be created from an amount
of mass, m.
Thermonuclear reactions in the Sun’s
core turn mass into energy.
At extremely high temperatures and pressures,
4 Hydrogen atoms can combine to make 1 Helium atom
and release energy by E = mc2
4H  He + energy
HYDROGEN FUSION
A theoretical
model of the
Sun shows
how energy
gets from its
center to its
surface.
Thermonuclear fusion
can only occur at very
high temperatures and
pressures.
Solar energy
flows from the
(1) core

(2) radiative zone

(3) convective
zone
Astronomers probe the solar interior
using the Sun’s own vibrations.
Sections of the Sun’s surface quickly oscillate up on down.
The Sun’s surface vibrations reveal its
internal structure and motions.
• Exploring the Sun’s
interior by studying its
vibrations is called
HELIOSEISMOLOGY.
• Because we can not
actually “see” inside the
Sun, helioseismology
provides theoreticians
with a way to check to be
sure their models of the
solar interior are correct.
Neutrinos provide information about the Sun’s
core - and have surprises of their own.
• Current models of the solar interior
predict that 1038 neutrinos should be
released every second by solar fusion.
• Neutrino (n) detectors on Earth
measure captures by cleaning fluid:
Cl + n  Ar + en + n  p + e• MYSTERY: Only 1/3 of the expected
neutrinos from the Sun are being
detected.
• Solution to this “solar neutrino
problem”: neutrinos have mass, so
they change identity enroute!
Outer Layers of the Sun’s
Atmosphere
• Photosphere - the 5800 K “surface” we see.
• Chromosphere - the lower solar
atmosphere, which rises to 25,000 K
• Corona - the very thin outer atmosphere at
millions of degrees (T>106 K)
The
photosphere
is the lowest
of three main
layers in the
Sun’s
atmosphere.
Granulation caused by convection
The chromosphere is characterized by
spikes of rising gas.
• The chromosphere is the thin,
pinkish layer of SPICULES just
above the photosphere.
• Spectrum is dominated by Ha
emission lines, suggesting it is
quite tenuous.
• MYSTERY: The temperature is
rises with height about the solar
surface, though we would
expect it to cool with increasing
distance from the source!
• Solution = solar magnetic fields
The corona ejects mass into space to
form the solar wind.
• Most easily seen
during an eclipse.
• Thin gas at millions of
degrees
• The outflow of mass
from the Sun is called
the solar wind.
Magnetic fields cause structure, heating,
and energy outbursts from the Sun’s
atmosphere.
• Sunspots
• Hot atmosphere
• Flares and mass
ejections
• Solar cycle
• Temperature changes
on Earth
Sunspots are low-temperature regions in
the photosphere.
The daily
movement of
sunspots
reveals that
the Sun’s
rotation takes
about 4
weeks.
The cyclical change in the latitude of
sunspots also reveals that the Sun
experiences an 11-year solar cycle.
Extreme peaks or absences of
sunspots may change Earth’s climate
Maunder Minimum ~ 1650
Mideval Maximum ~ 1100
• Few sunspots
• Many sunspots
• Colder climate
• Hotter climate
• Famine in Europe
• Famine in N. America
• Thames froze
• Ancient Pueblo people
abandoned Chaco canyon
• Icemelt permitted Vikings
to reach N. America
Sunspots are produced by a 22-year
cycle in the Sun’s magnetic field.
• Charged particles,
such as electrons, will
move along magnetic
field lines.
• The Sun’s positive
pole is in the North for
11 years, then switches
to the South for 11
years
This X-ray image of the Sun shows bright regions where gas
follows magnetic field lines.
The sunspot cycle is partly due to the Sun’s
differential rotation. This helps solar
magnetic fields twist up, intensify, emerge,
cancel, then repeat the cycle.
The interior of the Sun rotates at slower than the equator and faster than the poles.
The radiative zone seems to rotate as a rigid sphere.
The Sun’s magnetic field also produces
other forms of solar activity.
• The highly charged
gases in the Sun’s
outer atmosphere,
follow loops in the
Sun’s magnetic field.
Solar magnetic fields create
plages and filaments / prominences.
Solar magnetic fields also create
coronal holes, solar wind, flares, …
… and Coronal Mass Ejections,
which can disrupt cell phone service
(among other things)
The Sun’s magnetic field affects all life on Earth,
not just cell phone users.
Guiding Questions
What is the source of the Sun’s energy?
What is the internal structure of the Sun?
How can we measure the properties of the Sun’s interior?
How can we be sure that thermonuclear reactions are
happening in the Sun’s core?
5. Does the Sun have a solid surface?
6. Since the Sun is so bright, how is it possible to see its dim
outer atmosphere?
7. Where does the solar wind come from?
8. What are sunspots? Why do they appear dark?
9. What is the connection between sunspots and the Sun’s
magnetic field?
10. What causes eruptions in the Sun’s atmosphere?
1.
2.
3.
4.