A100 The Sun
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Transcript of A100 The Sun
Today’s APOD
Read Chapter 11Homework and Quiz 9 this
week on FridayLast Solar Lab on
TOMORROW at NOON
The Sun Today
A100 The Sun
Basic Facts• radius
• 7 x 105 km• about 100 x Earth’s radius
• mass = about 300,000 x Earth’s mass
• distance • 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
The Sun
Helioseismology
With helioseismology, we can measure temperature, pressure and motion inside the Sunfrom sound waves thattraverse the Sun’s interior.
Listen to the Sun
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.
Helioseismology
blue = inward motion
red = outward motion
Basic Structure
INSIDECore
Radiative ZoneConvection Zone
OUTSIDEPhotosphere
ChromosphereCorona
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 woodThe Sun is contracting due to gravity
NONE OF THESE PRODUCES ENOUGH ENERGY
NUCLEAR REACTIONS produce enough energy
Luminosity~ 10 billion years
Nuclear Potential Energy (core)
E = mc2
- Einstein, 1905
Fission:Big nucleus splits into smaller pieces
Fusion:Small nuclei stick together to make a bigger one
What produces nuclear energy?
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 rays2 positrons2 neutrinos
Total mass is 0.7% lower
•The missing mass is converted to energy•Rate of nuclear fusion depends on temperature
•Neutrinos created during fusion fly directly out of the Sun•These neutrinos can be detected on Earth
How do we know nuclear reactions are going on in the Sun?
Estimating 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’sLifetime
How much fuel?
The Sun “burns” hydrogen to helium in a nuclear reaction
How many hydrogen atoms in the Sun2 x 1033 grams of hydrogen6 x 1023 atoms in each gram
E = mc2
12 x 10 56 hydrogen atoms
How Much Energy Can the Sun
Produce?For each helium nucleus created, the Sun produces 5 x 10-12 joules of energythe Sun can burn half its hydrogenFOUR hydrogen atoms are needed to make
each helium atom
E = mc2
½ x ¼ x 12x1056 x 5x10-12
= 7 x 10 44 joules
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?
The Solar Constant
1 meter
1400 watts =14 100-watt light bulbs
What is a joule?
Each second a 100-watt light bulb produces 100 joules of energy
Sphere with radius of 1 AU
Surface area of a sphere
= 4r2
How many square meters on a sphere with radius one AU?
Area times energy per square meter per second equals total energy outputper second
The Sun produces4 x 1026 watts
Solar Energy
From the conversion of hydrogen into helium by nuclear reactions… E = mc2
How long will the Sun shine???
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
Solar Energy
In fact, the Sun will only shinefor about 10 billion years, twiceits present age.
E = mc2
How long will the Sun shine???
Not all the Sun’s hydrogen is in regions hot enough for hydrogen fusion reactions to occur.
• Radiation
• Convection
How does the energy from fusion get out of the Sun?
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
Energy TransportIn 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
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
Balancing Gravity
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
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.
Ch 11 – The Sun Quiz, Homework on FridaySolar Lab tomorrow at NOON
Dates to Remember