Announcements Star Assignment 3, due Wednesday March 17 –READ chapter 16, –Do Angel quiz, –Do Astronomy Place tutorial “Measuring Cosmic Distances: Lesson 2, Stellar Parallax” 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) –Submit via

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

Energy Source: Nuclear Fusion 4 1 H --> 1 4 He Energy

Fusion occurs ONLY in the Sun’s CORE ? Nuclear fusion Lighter nuclei are fused into heavier 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

Answers to Explain the Thermostat Good - gives details Gravitational equilibrium acts as the Sun’s internal thermostat. If there is a small rise in the core temperature, the fusion rate will rise dramatically. The increased energy creates increased thermal pressure that can overcome gravity, causing the core to expand and cool which restores the fusion rate to normal. If there is a small decrease in the core temperature, there will be a dramatic decrease in the fusion rate. The decrease in energy will allow gravity to compress the core. This will heat up the core and in turn restore the fusion rate to normal. Bad - does not answer question: Nuclear fusion transforms hydrogen into helium and creates extra energy. This energy is so great it heats up to temperatures no spacecraft could survive if it ever attempted to journey to its core.

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

Radiative zone: Energy gradually diffuses outward (in about a million years) by randomly bouncing photons. Each hot core gamma ray photon becomes many visible cool surface photons.

Random Walk Activity Each person in center of classroom take a balloon. Blow it up and tie it. Tap it up. Everyone tap balloons UP when one comes to you Time how long it takes half of balloons to reach a wall Now compare with time it takes if balloon are tapped directly towards a wall.

Rate of energy loss Determined by bulk of star outside of core If star is good insulator -> rate of energy loss smaller If star is poor insulator -> rate of energy loss greater

Why does energy take so long to reach the surface? Good - gives details Gamma-ray photon collide with electrons often, sending them bounced in random directions. With each bounce the photon drifts farther from its initial location. As a result it has to go a much longer distance than a straight line and so takes a very long time to reach the surface Bad - no details Energy generated in the core takes about a million years to reach the surface, because the transportation process (via photons and convection) is so complex.

Outer 1/3 of Sun Energy is transported by CONVECTION

Granule ~ fountain Hot gas rises, spreads out, falls back

Convection zone: Hot gas rises, radiates away its heat at the surface, becomes cool gas and sinks, pulled down by gravity.

Convection pattern at solar surfact Hot gas rises (floats up) -> Brighter Cool gas sinks (pulled down by gravity) -> Darker ~ 1000 km

Convection pattern at the solar surface Sunspots: Magnetic fields inhibit convection -> cooler -> Darker (Fig )

Equilibrium: The Structure of a Star is determined by two balance conditions 1.Energy Production (by nuclear fusion in core) = Energy Loss (by radiation from surface) 2.Pressure (pushing out) = Gravity (pulling in)

Feedback - the Solar Thermostat (Fig. 15.8)

The Solar Thermostat Temperature increase Ü increased fusion  energy production greater than energy loss Ücore heats up Ü pressure increases Ü pressure greater than gravity Ü core expands  core cools (convert KE to PE)  energy generation decreases

Tests: Compare predictions of models with observations 1. Observations of resonant sound wave oscillations (helio-seismology) 2. Neutrino observations

Sound Waves in the Sun If wave comes back on itself, it is reinforced = resonant

Sound waves penetrate different distances probe the solar interior.

Global Oscillation Network Group

Observed as Doppler Shift at the solar surface

Test Solar Models: Compare frequencies of observed resonant oscillation modes with frequencies calculated from solar models Agreement Excellent

Model vs. Helioseismic Observations Density water = 1 g/cm 2 Fig

Sun in Neutrinos

Solar Neutrino Flux: Theory vs. Observation

Theory is biased by Experiment

Conclusion: either 1.Something wrong with models of the Sun 2.Something wrong with theory of neutrinos

Resolution (recent) 3 types of neutrinos - e, mu, tau Sun produces only e type SNO observing only e-type, sees 0.35 predicted number Super-Kamiokanda, sensitive to e + some mu & tau, sees 0.46 predicted number Conclusion: e-type mu & tau - types Hence, see only 1/3 predicted if observe only e-type