A105 Stars and Galaxies

News Quiz Today Review Exam 1 Homework 6 (the Sun) due Thursday

Today’s APOD

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

• 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 wood

• The 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 Sun– 2 x 1033 grams of hydrogen– 6 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 energy– the Sun can burn half its hydrogen– FOUR 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 bulbsWhat is a joule?

Each second a 100-wattlight 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 regionshot enough for hydrogen fusion reactionsto occur.

We’ll learn more about the futureevolution of the Sun as we look more closely at other stars in the Galaxy.

• 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.

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

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

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

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%

What happens to sunlight that falls on the Earth?

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