Chapter 26: Exploring the
Universe
70 % Hydrogen
28% Helium
Section 1: The Sun
Energy From the Sun
ž  The
sun gives off tremendous amounts
of energy in the form of electromagnetic
radiation.
ž  Evidence from various branches of
science eventually led scientists to
conclude that the sun is about 4.6 billion
years old.
ž  It wasn’t until the 1900’s when we could
answer the question: How could the sun
produce energy for so long?
How Does the Sun Produce
Energy?
ž  The
real source of energy for the sun is
nuclear fusion in its central region.
ž  Here, the temperature and pressure are
high enough for fusion to take place.
ž  In the center of the sun less massive
nuclei combine into more massive
nuclei, releasing huge amounts of
energy.
ž  Hydrogen nuclei fuses into helium
nuclei.
Forces in Balance
ž  For
the sun to be stable, inward and
outward forces that act on the sun, like
temperature, pressure, and density,
must be in equilibrium.
ž  The sun remains stable because the
inward pull of gravity balances the
outward push of thermal pressure from
nuclear fusion.
ž  Astronomers estimate that the sun will
be stable for another 5 billion years.
The Sun’s Interior
ž  Astronomers
can use models and study
vibrations to infer what the inside of the
sun looks like.
ž  The sun’s interior consists of the core,
the radiation zone, and the convection
zone.
The Core
ž  The
sun’s core is its central region,
where nuclear fusion occurs.
ž  Diameter of 400,000 km.
The Radiation Zone
ž  As
energy moves outward from the
sun’s core, it first enters the radiation
zone.
ž  The radiation zone is a region of highly
compressed gas.
The Convection Zone
ž  Energy
from the radiation zone passes
into the convection zone, the outer layer
of sun’s interior.
ž  Energy is transferred outwardly by
convection currents.
The Sun’s Atmosphere
ž  Outside
the convection zone is the sun’s
atmosphere.
ž  The sun’s atmosphere consists of three
layers: the photosphere, the
chromosphere, and the corona.
The Photosphere
ž  The
innermost layer of the sun’s
atmosphere, which is the visible surface
of the sun.
ž  It is not solid, but is called the sun’s
surface.
The Chromosphere
ž  Just
outside the photosphere.
ž  High temperatures in the chromosphere
cause the hydrogen gas to emit a
reddish light.
ž  Normally visible only when the brighter
photosphere is blocked, such as during
a total solar eclipse.
The Corona
ž  The
chromosphere merges with the
corona, the outermost layer of the sun’s
atmosphere.
ž  Usually only seen during solar eclipses.
ž  The corona gradually thins into the solar
wind, which is a stream of electrically
charged particles that flow outward from
the sun into the solar system.
Features of the Sun’s
Atmosphere
ž  The
sun’s magnetic field produces
striking features on and above its
surface.
ž  Features of the sun’s atmosphere
include sunspots, prominences, and
solar flares.
Sunspots
ž  Sunspots
are areas of gas in the
photosphere that are cooler than the
surrounding gases.
ž  They can be huge!
ž  They give off less energy, so they
appear darker on the sun.
ž  Sunspots provide information on the
motions of the sun.
Prominences
ž  Spectacular
features of the sun’s
atmosphere that occur near sunspots
ž  Huge loops of gas that erupt from
sunspot regions.
ž  Travel along magnetic field lines that
connect sunspots.
Solar Flares
ž  A solar
flare is a sudden release of
energy.
ž  Usually occur near sunspots.
ž  Can cause magnetic storms in Earth’s
upper atmosphere which can disrupt
power transmission, radio, television,
and telephone signals.