10.2 Nuclear Fusion in the Sun Our Goals for Learning How does nuclear fusion occur in the Sun? How does the energy from fusion get out of the Sun? How do we know what is happening inside the Sun?

How does nuclear fusion occur in the Sun?

High temperature enables nuclear fusion to happen in the core of the Sun; high density ensures it happens often enough to power the Sun

Sun releases energy by fusing four hydrogen nuclei into one helium nucleus, which has less mass than four hydrogen nuclei do. The difference in mass is released as energy, in the form of gamma-ray photons.

Hand-out Activity: “Fusion in the Sun: the proton-proton chain” We'll go over the front page. Then you'll turn the page over, fill in the blank circles, and answer the two questions.

The proton-proton chain is how hydrogen fuses into helium in Sun

IN 4 protons OUT one 4 He nucleus 2 neutrinos 2 gamma rays 2 positrons (which collide with 2 electrons, making more gamma rays) Total mass is 0.7% lower (4 million tons of matter converted to energy every second inside the Sun!)

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 Structure of the Sun

How does the energy from fusion get out of the Sun?

Energy starts out in the core of the Sun as gamma ray photons. What is the most common kind of photon that escapes from the visible surface of the Sun?

Energy gradually leaks out of radiation zone in form of randomly bouncing photons. It takes millions of years for this to happen.

Photosphere: Visible surface of Sun (not solid) ~ 5,800 Kelvin ~ 5,500 Celsius

How do we know what is happening inside the Sun?

We learn about the Sun's interior by … Making mathematical models Observing solar vibrations (`sun quakes'), just like we learn about the Earth's interior from earthquakes Observing solar neutrinos, a kind of particle that rarely interacts with other particles

Patterns of vibration on the Sun’s surface tell us about what the Sun is like inside Results agree very well with mathematical models of the solar interior

Neutrinos created during fusion fly directly through the Sun Observations of these solar neutrinos can tell us what’s happening in core

Solar neutrino problem: Early searches for solar neutrinos failed to find the predicted number

Solar neutrino problem: Early searches for solar neutrinos failed to find the predicted number More recent observations with the Sudbury Neutrino Observatory (pictured) in northern Ontario find the right number of neutrinos, but some have changed form, which allowed them to escape earlier searches.

What have we learned? How does nuclear fusion occur in the Sun? Fusion of hydrogen into helium, which occurs via the proton–proton chain. Gravitational equilibrium acts as a thermostat that keeps the fusion rate steady.

What have we learned? How do we know what is happening inside the Sun? theoretical models use known laws of physics and then check the models against observations and studies

What have we learned? How does the energy from fusion get out of the Sun? Energy moves through the deepest layers of the Sun—the core and the radiation zone—in the form of randomly bouncing photons. After energy emerges from the radiation zone, convection carries it the rest of the way to the photosphere, where it is radiated into space as sunlight.