A105
Stars and Galaxies
Today’s APOD
 News Quiz Today
 Review Exam 1
 Homework 6 (the Sun) due Thursday
Announcements…
• Solar Lab today at 11, tomorrow at 2
PM
• Kirkwood Obs. Moon Festival, Oct. 4,
8:30-10:30
• Rooftop Session, Oct. 4, 9:00
• Orionid meteor shower, Oct. 21-22, just
before midnight
Exam Review
• Range of score: 24-76 of 80 questions
• Average score: 54/80
NovaSearch I Homework
• If you picked a year with just a few observations
available, you will need to select another year with
more for NS II and NS III!
• If the year you selected did not seem to show any
blemishes, view some other years to see what
they look like.
News Quiz
• On a piece of paper, list three
important ideas from this week’s
audio selection
• Print your name carefully – if we can’t
read your writing, we can post the
points to the gradebook!
The Sun Today
• Image credit:
Solar Orbiting
Heliospheric
Observatory/MDI
Basic Facts
•
radius
•
•
distance
•
•
The Sun
7 x 105 km
about 100 x Earth’s radius
mass = about 300,000 x Earth’s mass
•
•
•
1 AU, 8 light minutes
1.5 x 108 km
about 100 x Sun’s diameter
• Temperature
• about 6000 Kelvin (10,000 F) at the surface
• about 15 million Kelvin inside
• Composition
• 90% of atoms are hydrogen
• 10% of atoms are helium
WHY WE STUDY THE SUN
Helioseismology
With helioseismology,
we can measure
temperature, pressure
and motion inside the Sun
from sound waves that
traverse the Sun’s interior.
Helioseismology
Listen to the Sun
blue = inward
motion
red = outward
motion
The Sun oscillates in complex patterns over the
whole interior and surface. The frequency and
location of the oscillations give us a detailed
picture of the inside of the Sun.
Basic
Structure
INSIDE
Core
Radiative Zone
Convection Zone
OUTSIDE
Photosphere
Chromosphere
Corona
Solar Wind
Conditions
inside the
Sun
Temperature peaks in
the core and drops off
at the outside edge of
the Sun
Why does
the Sun
Shine?
• The Sun is a “cooling ember”
• The Sun is burning like coal or wood
• The Sun is contracting due to gravity
NONE OF THESE PRODUCES ENOUGH ENERGY
NUCLEAR REACTIONS
produce enough energy
E =
2
mc
- Einstein, 1905
Nuclear Potential Energy (core)
Luminosity
~ 10 billion years
What produces nuclear energy?
Fission:
Big nucleus splits into
smaller pieces
Fusion:
Small nuclei stick
together to make a
bigger one
The Sun releases energy
by fusing four hydrogen
nuclei into one helium
nucleus
Nuclear fusion requires
high temperatures and
high density
The Sun’s Energy
Comes from Nuclear Fusion
The
Conversion
of hydrogen
into helium
Proton-proton chain fuses hydrogen into helium
IN: 4 protons
OUT:
4He
nucleus
2 gamma rays
2 positrons
2 neutrinos
Total mass is
0.7% lower
•The missing mass is converted to energy
•Rate of nuclear fusion depends on temperature
How do we know nuclear reactions are going
on in the Sun?
•Neutrinos created during
fusion fly directly out of
the Sun
•These neutrinos can be
detected on Earth
The Sun’s
Lifetime
Estimating the
Sun’s Lifetime
• How much fuel does the Sun have?
• How fast is that fuel being consumed?
• When will it run out?
How much fuel?
• The Sun “burns” hydrogen to helium in a
nuclear reaction
2
E = mc
• How many hydrogen atoms in the Sun
– 2 x 1033 grams of hydrogen
– 6 x 1023 atoms in each gram
12 x 10 56 hydrogen atoms
How Much Energy Can
the Sun Produce?
E=
2
mc
• For each helium nucleus created, the Sun
produces 5 x 10-12 joules of energy
– the Sun can burn half its hydrogen
– FOUR hydrogen atoms are needed to make
each helium atom
½ x ¼ x 12x1056 x 5x10-12
= 7 x 10
44
joules
The Solar
Constant
1400 watts =
14 100-watt light bulbs
• How Much Energy Each
What is a joule?
Second?
– At the distance of the Earth,
the Sun radiates 1400 watts
(1400 joules per second) in
each square meter on the
surface of a sphere with a
radius equal to one AU
– How many square meters
are on that sphere?
Each second a 100-watt
light bulb produces
100 joules of energy
1 meter
Sphere with radius of 1 AU
Surface area of
a sphere
= 4pr2
How many square
meters on a sphere
with radius one AU?
The Sun produces
4 x 1026 watts
Area times energy
per square meter
per second equals
total energy output
per second
Solar Energy
How long will the
Sun shine???
From the conversion of hydrogen
into helium by nuclear reactions…
Total energy available = 7 x 1044 joules
Radiating energy at 4 x 1026 joules per second
Lifetime =
7 x 1044 joules
4 x 1026 joules per second
= 2 x 1018 seconds = 6 x 1010 years
E = mc2
Solar Energy
How long will the
Sun shine???
In fact, the Sun will only shine
for about 10 billion years, twice
its present age.
Not all the Sun’s hydrogen is in regions
hot enough for hydrogen fusion reactions
to occur.
We’ll learn more about the future
evolution of the Sun as we look
more closely at other stars in the
Galaxy.
E = mc2
How does the energy from fusion
get out of the Sun?
• Radiation
• Convection
How does the energy from fusion
get out of the Sun?
Radiation: In the inner
regions of the solar interior,
energy gradually leaks
upward in form of randomly
bouncing electromagnetic
waves.
Further from the center,
the wavelengths of the
electromagnetic waves
become longer.
Energy Transport
In the outer regions of the
Sun’s interior, energy flows
outward by convection; heat
is carried upward by
bubbling hot gas.
The convection zone gives the surface the
appearance of boiling liquid
Balancing Gravity
Gravitational
contraction:
Provided energy that
heated core as Sun was
forming
Contraction stopped
when fusion began
Gravitational equilibrium:
Energy provided by fusion maintains the pressure
Solar Thermostat
Decline in core temperature
causes fusion rate to drop, so
core contracts and heats up
Rise in core temperature
causes fusion rate to rise, so
core expands and cools down
Summary:
• Why was the Sun’s energy source a
major mystery?
– Chemical and gravitational energy sources
could not explain how the Sun could
sustain its luminosity for more than about
25 million years
• Why does the Sun shine?
– The Sun shines because gravitational
equilibrium keeps its core hot and dense
enough to release energy through nuclear
fusion.
The Budget of Solar Radiation
What happens to sunlight that falls
on the Earth?
percent
Reflected back out to space
35%
Absorbed by atmosphere
18%
Scattered to the Earth from blue sky
10%
Scattered to the Earth from clouds
14%
Hits the Earth's surface directly
23%
100%
Recall the structure of the Sun…
Corona
The Sun’s Atmosphere: Photosphere, Chromosphere, Corona, Solar Wind
 The Sun! Units 49, 50, 52
 Homework 6 Due THURS.
SOLAR LAB TODAY AT 11:00,
TOMORROW AT 2:00