26.1 The Sun
The sun gives off
tremendous
amounts of
energy. Plants on
Earth use sunlight
directly in
photosynthesis.
26.1 The Sun
Energy From the Sun
•produced in core by fusion of hydrogen nuclei into
helium nuclei.
– Fusion: nuclei combine into more massive nuclei,
releasing enormous amounts of energy.
– The sun gives off tremendous amounts of energy
as electromagnetic radiation.
26.1 The Sun
Energy From the Sun
•Nuclear fusion within the sun’s central region converts
mass into energy.
•The green particles in the diagram are protons, and the
purple particles are neutrons.
Hydrogen nuclei
collide to form
helium-3
Gamma ray
Two helium-3
nuclei collide.
Helium-4 and
hydrogen nuclei form.
26.1 The Sun
Forces in Balance
Sun has been a fairly stable energy source for billions of years
Inward and outward forces within the sun
must be balanced so that temperature,
pressure, and density at any depth remain
fairly constant over time.
•The sun is stable because an inward pull of
gravity balances an outward push of thermal
pressure from nuclear fusion
– Ions move faster, exerting an outward
thermal pressure
– At the same time, gravity pulls the gas
inward.
–Astronomers estimate the sun will remain
stable for another 5 billion years.
26.1 The Sun
The Sun’s Interior
•The sun’s interior consists of the core, the
radiation zone, and the convection zone.
–Astronomers infer the structure of the sun’s interior
using:
•mathematical models
•observations of vibrations (similar to earthquakes)
26.1 The Sun
The Sun’s Interior
Core
•The sun’s core is its central region, where nuclear
fusion occurs.
•The core has a diameter of about 400,000
kilometers, a little more than a quarter of the diameter
of the entire sun.
26.1 The Sun
The Sun’s Interior
Radiation Zone
•The radiation zone is an area of highly compressed
gas.
Energy is transferred by
absorption and re-radiation of
electromagnetic waves.
This region is so dense, energy
can take more than 100,000 years
to pass through it.
26.1 The Sun
The Sun’s Interior
Convection Zone
•Energy from the radiation zone passes into the
convection zone, the outer layer of the sun’s interior.
– Energy is transferred outward mainly by
convection currents.
–Hot gases in the convection zone
rise toward the sun’s atmosphere
while cooler gases sink downward.
26.1 The Sun
The Sun’s Atmosphere
The sun’s atmosphere consists of three layers: the
photosphere, the chromosphere, and the corona.
Photosphere
The innermost layer of the sun’s atmosphere, the
visible surface of the sun, is the photosphere.
• The photosphere is about 500 kilometers thick and has
an average temperature of about 5800 K.
• The photosphere is called the sun’s surface.
• The photosphere’s surface has a bubbly appearance.
• The bubbles, called granules, are the tops of convection
currents.
26.1 The Sun
The Sun’s Atmosphere
Chromosphere
•In the chromosphere, the middle layer of the sun’s
atmosphere:
–Temperature is nearly 10,000 K
–At these temperatures, hydrogen gas emits a reddish light
–Normally visible only when the photosphere is blocked,
such as during a total solar eclipse.
26.1 The Sun
The Sun’s Atmosphere
Corona
The chromosphere merges with the corona, the
outermost layer of the sun’s atmosphere:
• Gases in the corona are very hot, about 1 million Kelvin.
• But the corona has such a low density that the total amount
of thermal energy in it is relatively small.
• The corona extends millions of kilometers above the
chromosphere.
• The corona gradually thins into the solar wind: a stream of
electrically charged particles that flows outward from the sun
through the solar system
26.1 The Sun
The Sun’s Atmosphere
The corona can only be
seen from Earth during a
total solar eclipse or
when viewed with a
special telescope.
26.1 The Sun
Features of the Sun’s Atmosphere
Features of the sun’s atmosphere include sunspots,
prominences, and solar flares.
Sunspots
Sunspots are areas of gas in the photosphere that
are cooler than the surrounding gases
–Some are larger than Earth.
–If sunspots could be seen apart from the rest of the sun,
they would shine brightly.
–Sunspots are often found in groups and are associated
with intense magnetic fields on the sun.
26.1 The Sun
Features of the Sun’s Atmosphere
• Sunspots last from a few hours to a few
months.
• The number of sunspots varies in cycles,
with about 11 years separating one sunspot
peak from the next.
• During peak periods, there may be a
hundred sunspots on the sun.
•Galileo Galilei discovered that the sun rotates by observing
the motion of sunspots.
•Sunspots near the sun’s equator move faster than sunspots
near the sun’s poles.
•This means that the sun rotates faster at its equator than
near its poles.
26.1 The Sun
Features of the Sun’s Atmosphere
Prominences
•Prominences are huge loops of gas that
erupt from sunspot regions
•They extend upward from the photosphere
into the chromosphere and sometimes into
the corona.
• Some prominences reach heights of more
than 100,000 kilometers above the sun’s
surface.
•Prominences travel along the magnetic
field lines that connect sunspots.
26.1 The Sun
Features of the Sun’s Atmosphere
Solar Flares
•The sun’s surface sometimes erupts
dramatically, producing X-rays and hurling
charged particles into space at speeds of
1000 km/s or more in a solar flare
–Solar flares usually occur near sunspots and
heat the corona to a temperature near 20
million K.
–Solar flares increase the solar wind, so they
can disrupt electric power transmission and
radio, television, and telephone signals.
26.1 The Sun
The Sun
The sun has an interior and an atmosphere.
Convection zone
(about 200,000 km thick)
Prominence
Chromosphere (about
Radiation zone
(about 300,000
km thick)
1500 km thick)
Photosphere
(visible surface)
Core
(about 400,000 km
in diameter)
Corona
(extending millions of
km into space)
Solar flare
Sunspot
Granulated surface
(cool region)
(caused by convection)
26.1 The Sun
Assessment Questions
1. What is the source of the sun’s energy?
a. The friction between its molecules gives off thermal
energy.
b. The conversion of potential energy into thermal energy
as gravity pulls the sun into its own core.
c. Combustion of hydrogen gas on the sun’s surface.
d. Fusion of hydrogen nuclei into helium nuclei.
ANS:
D
26.1 The Sun
Assessment Questions
2. The sun has remained stable for billions of years
because there is a balance between
a. gravity and the tendency of hydrogen and helium to
rise.
b. gravity and thermal pressure.
c. thermal pressure and the repulsion between nuclei.
d. the amount of energy it absorbs and the amount of
energy it produces.
ANS:
B
26.1 The Sun
Assessment Questions
3. Convection currents in the sun’s convection zone
carry energy from
a.
b.
c.
d.
the core to the photosphere.
the radiation zone to the photosphere.
the core to the radiation zone.
the photosphere to the corona.
ANS:
B