 Read Chapter 11
 Homework and Quiz 9 this week on Friday
 Last Solar Lab on TOMORROW at NOON
Today’s APOD The Sun Today

• radius
•
7 x 10 5 km about 100 x Earth’s radius
•
• mass
•
= about 300,000 x Earth’s mass distance
•
1 AU, 8 light minutes
1.5 x 10 8 km
•
•
• about 100 x Sun’s diameter
Temperature
• about 6000 Kelvin (10,000 F) at about 15 million Kelvin inside the surface
•
Composition
•
90% of atoms are hydrogen
10% of atoms are helium

Helioseismology
With helioseismology, we can measure temperature, pressure and motion inside the Sun from sound waves that traverse the Sun’s interior.

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

Temperature peaks in the core and drops off at the outside edge of the Sun

 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 = mc
- Einstein, 1905
Nuclear Potential Energy (core)
Luminosity
~ 10 billion years

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:
4 He 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
 How much fuel does the Sun have?
 How fast is that fuel being consumed?
 When will it run out?

 The Sun “burns” hydrogen to helium in a nuclear reaction E = mc 2
 How many hydrogen atoms in the Sun
 2 x 10 33 grams of hydrogen
 6 x 10 23 atoms in each gram
12 x 10 56 hydrogen atoms

How Much Energy Can the Sun Produce?
E = 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
2
½ x ¼ x 12x10 56 x 5x10 -12
= 7 x 10 44 joules

The Solar
Constant
 How Much Energy Each 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?
1400 watts =
14 100-watt light bulbs
What is a joule?
Each second a 100-watt light bulb produces 100 joules of energy
1 meter

Surface area of a sphere
= 4 p r 2
The Sun produces
4 x 10 26 watts
How many square meters on a sphere with radius one AU?
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… E = mc 2
Total energy available = 7 x 10 44 joules
Radiating energy at 4 x 10 26 joules per second
Lifetime = 7 x 10 44 joules
4 x 10 26 joules per second
= 2 x 10 18 seconds = 6 x 10 10 years

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.
E = mc 2

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.
As the energy propagates outward, the electromagnetic energy shifts from gamma rays and X-rays to ultraviolet and optical light

In the outer regions of the
Sun’s interior, energy flows outward by convection; heat is carried upward by bubbling hot gas.
time lapse movie

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

The 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

 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.

 Ch 11 – The Sun
 Quiz, Homework on Friday
 Solar Lab tomorrow at NOON