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?
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
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From the Surface up into the atmosphere(corona)
Magnetic Loops in the Corona
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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
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
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)
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
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?