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