Lecture 19: The Sun Our Star

Some Facts about the Sun  distance from Earth: 1.5 x 10 8 km  luminosity: 3.86 x W  mass: 1.98 x kg (3.33 x 10 5 Earth masses)  radius: 696,000 km (10 9 times the radius of the Earth)  surface temperature: 5,800 K

What makes the Sun Shine?  chemical burning? –not enough atoms to sustain for lifetime of Earth  gravitational contraction? –Sun would have been much larger in recent past; could not shine for long enough  nuclear fusion? –should allow Sun to shine for 10 billion years

Nuclear Fusion and Fission

Nuclear Fusion in the Sun: the proton-proton chain

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Net effect of the p-p chain:  in: –6 protons ( 1 H nuclei)  out: –one 4 He nucleus –two protons –two neutrinos –two positrons  photons –two photons –kinetic energy

How much energy is released? mass of proton: x kg mass of four protons: x kg mass of Helium-4 nucleus: x kg mass difference: 4.7 x kg (about 0.7 percent of original mass) E = m c 2 = (4.7 x kg)(3 x 10 8 m/s) 2 = 4.3 x J

How much mass is converted to energy each second in the Sun? Luminosity of the Sun: 3.8 x J/s E = m c 2 m = E/c x J/s x 1 s (3.0 x 10 8 m/s) 2 m = = 4.2 x 10 9 kg

Structure of the Sun  core (15 million K)  radiation zone (10 million K)  convection zone (2 million K)  photosphere (6000 K)  chromosphere (10,000 K)  corona (1 million K)  solar wind

Radiative Diffusion

Granulation of the Sun’s surface

“Observing” the Solar Interior helioseismology

Gravitational Equilibrium

The Sun’s Built-in Thermostat  the rate of nuclear fusion is very sensitive to temperature  core temperature increase  increase in fusion rate  increase in pressure  core expands and cools down

The Sun is getting hotter…  4 H particles  1 He particle –causes solar core to shrink in size  fusion rate must increase to maintain gravitational equilibrium  solar core gets gradually hotter  the Sun is about 30 percent hotter now than it was 4.6 billion years ago  temperature on Earth has remained constant – Earth has its own thermostat?

Solar Neutrinos Homestake mine, South Dakota

Super- Kamiokande experiment

The Solar Neutrino Problem  models of the Sun predict the number of neutrinos that we should see with these detectors – but actual number seen is only about half as large as the prediction.  either the models are wrong or something funny is going on…

Neutrino Oscillations?  remember there are three different kinds of neutrinos, which go with the three kinds of leptons (electron, muon, and tauon)  Fusion produces only electron neutrinos, and detectors are only sensitive to electron neutrinos  could some of the electron neutrinos change into muon or tauon neutrinos during their journey out of the Solar core?

Sunspots  Sunspots are regions where the surface of the Sun is much cooler than the rest (4000 K instead of 5800 K)  what keeps these spots cool?

Magnetic Fields

The presence of a Magnetic field changes the spectral lines. Some lines split into two or more lines.

Magnetic fields trap the hot gas, and suppress convection.

Solar Prominence Solar Flare

Solar Activity and the Earth

Aurora Borealis/Australis