Announcements
• Star Assignment 2, due Monday March 15
– READ chapter 15, do Angel quiz
• Global Warming Project, due Wednesday
March 17
– Sample 4 web sites taking different positions on
whether Global Warming is occurring &
whether people are responsible.
– For each site, evaluate the science - Is it good
or poor?
– Criteria (list from class discussion)
You already know a lot about the Sun
• The Sun heats the Earth. What is the source
of the Sun’s energy?
• The color of the Sun is yellowish -> the
Sun is hot. What makes the surface hot?
• The Sun does not seem to change much. It
is very stable. What keeps it stable?
Tour of the Sun
Basic Properties of the Sun
Distance: 1.5 x 108 km
= 1 A.U.
Mass: 2 x 1030 kg
Radius: 7 x 105 km
Density: 1.4 g/cm3
Luminosity: 3.8 x 1026 watts
What
does the
Sun look
like?
Convection =
Granulation
Sunspots:
Cooler ->
darker
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
From the
Surface up
into the
atmosphere
(corona)
The Sun’s Magnetic Field
QuickTime™ and a
Video decompressor
are needed to see this picture.
QuickTime™ and a
Photo decompressor
are needed to see this picture.
Magnetic Loops in the Corona
QuickTime™ and a
Motion JPEG A decompressor
are needed to see this picture.
QuickTime™ and a
Video decompressor
are needed to see this picture.
The
Solar
Corona
in
X-Rays
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
The
Solar
Wind
Stability: Pressure & Gravity
• Gravity pulls in -- keeps Sun from
dispersing into space, holds stars together
• Pressure pushes out -- keeps Sun from
collapsing
Gravitational
equilibrium:
The outward push
of pressure
balances the
inward pull of
gravity
Pressure
Pressure is force exerted by colliding particles
• Higher density --> particles closer together
--> more collisions --> higher pressure
• Higher temperature --> particles move
faster --> more & harder collisions -->
higher pressure
Equilibrium
Pressure balances Gravity
•Pressure = weight of overlying material
•Pressure increases toward center
to balance larger gravity toward center
Weight of upper layers
compresses lower layers
What is the source of the Sun’s Energy?
• How do we get energy here on Earth
– Chemical reactions (burning)
– Gravitation Potential Energy (hydroelectric)
– Nuclear (fission reactors)
Chemical Energy:
108 J/kg
Estimated life expectancy ~ 15,000 years
Earth is older
Gravitational Energy:
2 x 1011 J/kg
Estimated life expectancy ~ 30,000,000 years
Earth is older
E=
2
mc
- Einstein, 1905
Nuclear Energy:
5 x 1014 J/kg
Estimated life expectancy ~ 1011 years
~ 100,000,000,000 years
What is the source of the Sun’s Energy?
• Sun gets its energy by nuclear FUSION
– Have not succeeded in containing nuclear
fusion on Earth
– Sun is a perfect nuclear fusion reactor
• Contained
• Safe
• Slow
Fission
Fusion
Big nucleus splits into
smaller pieces
Small nuclei stick
together to make a
bigger one
(Nuclear power plants)
(Sun, stars)
Why does fusion occur ONLY in the
Sun’s CORE ?
• Nuclear fusion
• a reaction where heavier nuclei are
created by combining (fusing) lighter
nuclei.
• all nuclei are positively charged
• Electromagnetic force causes nuclei
to repel each other.
• for fusion to occur, nuclei must be
moving fast enough to overcome E-M
repulsion
• this requires high temperatures
• When nuclei touch, the nuclear
force binds them together
Electric Barrier
Hydrogen Fusion (pp chain)
Overall Reaction:
4 1H --> 1 4He + Energy
Why does the Sun Shine ?
mass of He = 99.3% of 4 x mass of H
where did the .007 (4 mH) go?
--> energy!!!
E=
2
mc
The Solar Thermostat
• Temperature
increase -->
increased fusion
--> energy
production
greater than
energy loss -->
core heats up -->
pressure
increases -->
pressure greater
than gravity -->
core expands -->
cools (work
against gravity)
--> energy
generation
decreases
The Solar Luminosity
• The Sun’s luminosity is stable over the short-term.
• However, as more Hydrogen fuses into Helium:
•
•
•
•
•
•
four H nuclei convert into one He nucleus
the number of particles in Sun’s core decreases with time
the Sun’s core will contract, causing it to heat up
the fusion rate will increase to balance higher gravity
a new equilibrium is reached for stability at a higher energy output
the Sun’s luminosity increases with time over the long-term
• Models indicate the Sun’s luminosity has increased 30% since it
formed 4.6 billion years ago.
• it has gone from 2.9 x 1026 watts to today’s 3.8 x 1026 watts
Energy Loss
• Heat is produced in the core
• Heat is lost (radiated away to space) from
the surface
• How is heat transported from the hot core to
the (relatively) cool surface?
Inner 2/3 of Sun Heat is transported by
RADIATION
Energy gradually leaks out of radiation zone in form of randomly
bouncing photons
Outer 1/3 of Sun Energy is transported
by CONVECTION
Convection zone: Hot gas rises and cool gas sinks