Lesson Objectives:- How nuclear fusion in the Sun works
- The proton-proton chain
- How the energy gets out of the Sun
- How we know what is happening inside the Sun
On Earth, nuclear reactors use nuclear *fission* to split large nuclei, such as those found in uranium or plutonium -- into smaller nuclei. The Sun does the opposite.
It uses nuclear *fusion* to combine two small nuclei into a larger one.
As you recall, the nucleus of an atom is positively charged, which means the nuclei of two different atoms will be repelled from each other by their electromagnetic forces. There is only one force that is strong enough to overcome this electromagnetic repulsion between two positively charged nuclei, and that is called the "strong force."
The strong force is what binds protons and neutrons together in atomic nuclei. Basically, the strong force is like velcro -- when two nuclei get so close together they are practically touching, the strong force takes over and they stick together.
At 15 million Kelvin, the Sun's core is hot enough to generate the high-speed collisions necessary for the nuclei to come so close together that they fuse.
To better understand the fusion process in the Sun, we first need to know that the nucleus of a Hydrogen atom is just an individual proton, while the most common form of Helium consists of two protons and two neutrons.
There are several steps in the fusion process, but the basic idea is that individual Hydrogen protons fuse together, so that four hydrogen atoms eventually become one helium atom.
Helium has a slightly lower mass than the combined hydrogen nuclei. Where did that mass go? According to Einstein's formula of relativity, that mass was converted into energy. The Sun converts about 600 million tons of hydrogen into 596 million tons of helium every second, meaning 4 million tons of matter is turned into energy each second.
Because the rate of nuclear fusion is so sensitive to temperature, gravitational equilibrium and energy balance act together to act as a thermostat and keep energy production steady.
Energy can take over a hundred thousand years to get from the core to the photosphere. The energy is first released from the core as photons of light, but due to the high density of the plasma in the radiation zone, the photons do not take a straight path out. They are constantly colliding with electrons and take a long time to work their way up to the convection zone.
Once the photons make it to the convection zone, the cooler temperature allows the plasma to absorb the photons instead of just bouncing them around. Through convection, the hot plasma rises and the cool plasma sinks, transporting the energy outward.
Once the photons reach the photosphere, the density of the gas is low enough that they can escape into space, emerging as thermal radiation.
The primary way we learn about what is happening inside the Sun is by creating mathematical models using the known laws of physics to predict internal conditions. These models are checked against observations of the Sun's size, surface temperature, energy output, and other observable properties of the Sun. Since current models match up to the observed properties accurately, the models of the Sun's interior appear to be correct.
Scientists also study vibrations of the Sun's surface kind of like how they study vibrations caused by earthquakes on Earth. This type of study is known as helioseismology.
Finally, in recent years, the development of neutrino detectors have allowed scientists to study solar neutrinos, a type of subatomic particle produced by nuclear fusion. This has provided further proof that the Sun's energy comes from nuclear fusion.