Announcements

• Star Assignment 2, due Monday March 15– READ chapter 15, do Angel quiz

• Global Warming Project, due Wednesday March 17– Sample 4 web sites taking different positions on

whether Global Warming is occurring & whether people are responsible.

– For each site, evaluate the science - Is it good or poor?

– Criteria (list from class discussion)

You already know a lot about the Sun

• The Sun heats the Earth. What is the source of the Sun’s energy?

• The color of the Sun is yellowish -> the Sun is hot. What makes the surface hot?

• The Sun does not seem to change much. It is very stable. What keeps it stable?

Tour of the Sun

Basic Properties of the Sun

Distance: 1.5 x 108 km = 1 A.U.

Mass: 2 x 1030 kg

Radius: 7 x 105 km

Density: 1.4 g/cm3

Luminosity: 3.8 x 1026 watts

What does the Sun look

like?

Convection =Granulation

Sunspots:Cooler ->darker

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From the Surface up into the atmosphere(corona)

The Sun’s Magnetic Field

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Magnetic Loops in the Corona

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TheSolar

Coronain

X-Rays

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TheSolarWind

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Stability: Pressure & Gravity

• Gravity pulls in -- keeps Sun from dispersing into space, holds stars together

• Pressure pushes out -- keeps Sun from collapsing

Gravitational equilibrium:

The outward push of pressure balances the inward pull of gravity

Pressure

• Higher density --> particles closer together --> more collisions --> higher pressure

• Higher temperature --> particles move faster --> more & harder collisions --> higher pressure

Pressure is force exerted by colliding particles

Equilibrium

Pressure balances Gravity

•Pressure = weight of overlying material•Pressure increases toward center to balance larger gravity toward center

Weight of upper layers compresses lower layers

What is the source of the Sun’s Energy?

• How do we get energy here on Earth– Chemical reactions (burning)– Gravitation Potential Energy (hydroelectric)– Nuclear (fission reactors)

Chemical Energy: 108 J/kg

Estimated life expectancy ~ 15,000 years

Earth is older

Gravitational Energy: 2 x 1011 J/kg

Estimated life expectancy ~ 30,000,000 years

Earth is older

Nuclear Energy: 5 x 1014 J/kg

Estimated life expectancy ~ 1011 years~ 100,000,000,000 years

E = mc2

- Einstein, 1905

What is the source of the Sun’s Energy?

• Sun gets its energy by nuclear FUSION– Have not succeeded in containing nuclear

fusion on Earth– Sun is a perfect nuclear fusion reactor

• Contained• Safe • Slow

Fission

Big nucleus splits into smaller pieces

(Nuclear power plants)

Fusion

Small nuclei stick together to make a bigger one

(Sun, stars)

Why does fusion occur ONLY in the Sun’s CORE ?

• Nuclear fusion• a reaction where heavier nuclei are

created by combining (fusing) lighter nuclei.

• all nuclei are positively charged

• Electromagnetic force causes nuclei to repel each other.• for fusion to occur, nuclei must be

moving fast enough to overcome E-M repulsion

• this requires high temperatures

• When nuclei touch, the nuclear force binds them together

Electric Barrier

Hydrogen Fusion (pp chain)

Overall Reaction:4 1H --> 1 4He + Energy

Why does the Sun Shine ?

mass of He = 99.3% of 4 x mass of Hwhere did the .007 (4 mH) go?

--> energy!!!

E = mc2

The Solar Thermostat • Temperature increase -->increased fusion--> energy production greater than energy loss --> core heats up --> pressure increases --> pressure greater than gravity --> core expands --> cools (work against gravity)--> energy generation decreases

The Solar Luminosity• The Sun’s luminosity is stable over the short-term.

• However, as more Hydrogen fuses into Helium:• four H nuclei convert into one He nucleus

• the number of particles in Sun’s core decreases with time

• the Sun’s core will contract, causing it to heat up

• the fusion rate will increase to balance higher gravity

• a new equilibrium is reached for stability at a higher energy output

• the Sun’s luminosity increases with time over the long-term

• Models indicate the Sun’s luminosity has increased 30% since it formed 4.6 billion years ago.• it has gone from 2.9 x 1026 watts to today’s 3.8 x 1026 watts

Energy Loss

• Heat is produced in the core

• Heat is lost (radiated away to space) from the surface

• How is heat transported from the hot core to the (relatively) cool surface?

Inner 2/3 of Sun Heat is transported by RADIATION

Energy gradually leaks out of radiation zone in form of randomly bouncing photons

Outer 1/3 of Sun Energy is transported by CONVECTION

Convection zone: Hot gas rises and cool gas sinks