Chapter 16: The Sun
16.1 The Sun in Bulk
The Sun is the SOLE source of light and heat for life on Earth. It is a star.
Star: a glowing ball of gas held together by its own gravity and powered by nuclear
fusion at its center
Physical Properties:
 Radius: 700,000 km
 Mass: 2.0 * 1030 kg
 Surface Temperature: 5,780 K
 Solar Rotation Period: found by timing sunspots and other surface features. These
observations indicate that the Sun rotates in about a month.
 Sun spins differentially: faster at the equator and slower at the poles (like Jupiter
and Saturn)
Overall Structure: (Page 355, Figure 16.2)
 Sun does not have a solid surface (Sun contains NO solid material)
 Photosphere: this “surface” is the part of the Sun that emits the radiation we see
 Chromosphere: just above the photosphere
 Transition Zone: just above the chromosphere
 Corona: thin, hot upper atmosphere, turns into the solar wind
 Convection Zone: below the photosphere, material in
constant convection motion
 Radiation Zone: Below the convection zone, solar energy
is transported toward the surface by radiation rather than by
convection
 Core: site of powerful nuclear reactions that generate the Sun’s enormous energy
output {FUSION – Hydrogen (H) into Helium (He)}
Table 16.1: The Standard Solar Model – fill in using table in your book
Region
Temperature (K)
Defining Properties
Core
Radiation Zone
Convection Zone
Photosphere
Chromosphere
Transition Zone
Corona
Solar Wind
Luminosity
The Sun RADIATES! Perhaps the most important property of the Sun.
 Radiates a great deal of energy
 Radiates uniformly
 Radiates in ALL directions
Solar Constant: The amount of solar energy reaching the surface of Earth each second
Luminosity: TOTAL energy radiated by the Sun each second, at all wavelengths
 Produces nrg equivalent to the detonation of about 100 billion 1-megaton nuclear
bombs
 Put another way, solar luminosity, is equivalent to 4 trillion 100-W light bulbs
shining simultaneously – about 1019 dollars’ worth of energy radiated every
second!
16.2 The Solar Interior
Modeling the Structure of the Sun
 NO direct measurements of the sun
 Created a model by combining all available observations with
theoretical insight into solar physics and created the Standard Solar
Model
Helioseismology: The study of conditions far below the Sun’s surface
through analysis of internal “sound” waves that repeatedly cross the solar
interior
Energy Transport
 Very hot solar interior ensures violent and frequent collisions among
gas particles
 Particles move in ALL directions at high speeds, bumping into one
another unceasingly
 As you move outward from the core, the temperature falls, atoms
collide less frequently and less violently
 Energy moves through the radiation zone, to the convection zone
 Energy finally nears the surface through convection, in convections
cells, within the convection zone
Solar Eclipse: celestial event during which the new Moon passes directly
between Earth and the Sun, temporarily blocking the Sun’s light
16.3 The Solar Atmosphere
The Composition
The following is the 10 most common elements in the Sun (following by the
elements abundance as a percentage of total mass).
Hydrogen (71%)
Silicon (0.099%)
Iron (0.14%)
The
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Helium (27%)
Nitrogen (0.096%)
Sulfur (0.040%)
Oxygen (<1%)
Carbon(0.4%)
Magnesium (.076%)Neon (.058%)
Chromosphere
The “inner” part of the solar atmosphere
Lies just above the photosphere
Cooler than the photosphere
Emits very little light of its own and cannot be observed visually under
normal conditions
 During a total solar eclipse the photosphere is completely obscured by
the moon – we SEE the chromosphere at this time
 Solar storms erupt every few minutes (expelling jets of hot matter
known as spicules {long thin spikes of matter} into the Sun’s upper
atmosphere)
The Transition Zone, Corona, and Solar Wind
 The corona’s gas is so hot that it is able to escape the Sun’s gravity
 The corona begins to flow outward as the Solar Wind.
 Solar Wind: An outward flow of fast-moving charged particles from
the Sun.
 The Sun is, in effect, “evaporating” – constantly shedding mass through
the solar wind
 Coronal Holes: “windows” where solar wind escapes
16.4 The Active Sun
Most of the Sun’s luminosity results from continuous emission from the
photosphere. This radiation arises from what we call the quiet Sun.
Quiet Sun: the underlying predictable star that blazes forth day after day
Active Sun: sporadic, unpredictable radiation.
o Unpredictable radiation, a much more irregular component of our star’s
energy output, characterized by explosive, unpredictable behavior.
Sunspots: An Earth-sized dark blemish found on the surface of the Sun.
The dark color of the sunspot indicates that it is a region of lower
temperature than its surroundings.
 Sunspots are NOT steady
 Most change their size and shape
 All come and go
Solar Magnetism
What causes a sunspot? Why is it cooler than the surrounding photosphere?
o The Sun’s magnetism causes both
o Magnetic field in a sunspot ~1000 times greater than the
neighboring field
o Cooler than their surroundings because these abnormally strong
fields tend to block (or redirect) the convective flow of hot gas,
which is normally toward the surface of the Sun
o More evidence? Sunspots come in pairs and they’re found to each
have opposite magnetic polarities
The Solar Cycle
Sunspot Cycle: The fairly regular pattern that the number and
distribution of sunspots follows, in which the average number of spots
reaches a maximum every 11 or so years then fall off to almost zero
o Sunspots come and go
o Numbers and distribution across face of the Sun also changes in
a fairly regular fashion
o Solar Minimum: Point in sunspot cycle, only a few sunspots are
seen
o Solar Maximum: Point in sunspot cycle during which many spots
are seen
Active Regions
Active Regions: Region of the photosphere of the Sun surrounding a
sunspot group, which can erupt violently and unpredictably. During a
sunspot maximum, the number of active regions is also a maximum.
o Prominences: Loop or sheet of glowing gas ejected from an active
region on the solar surface, which then moves through the inner
parts of the corona under the influence of the Sun’s magnetic
field (p. 369 fig. 16.22 a & b)
o Quiet prominences: persist for days or even weeks
o Active prominences: come and go erratically, changing
appearance in a matter of hours
o Flares: Explosive event occurring in or near an active region
o Flare much more violent than prominences
o A flare can release as much energy as the largest
prominences, but in a matter of minutes or hours rather
than days or weeks (p. 370 fig. 16.23)