When I think about the sun, I like to think of the chorus from a song titled Why Does The Sun Shine? This lyrics for this song were written way back in 1951, but they're still true.

The sun is a mass of incandescent gas
A gigantic nuclear furnace
Where hydrogen is built into helium
At a temperature of millions of degrees

Technically speaking, nuclear fusion doesn't fuel our sun. Two elements comprise almost all of the fuel that our sun burns via nuclear reactions; those elements are hydrogen and helium, the two lightest and most abundant elements in the Universe. A hydrogen atom is made of one proton and one electron. A helium atom is made of two protons, two neutrons, and two electrons.

Let's talk nuclear fusion. What happens when you fuse two things together? They become one different thing. Nuclei are the centers of atoms, made of particles called neutrons and protons around which the atoms' electrons blanket. Nuclear fusion is the process of fusing two nuclei together and creating one nucleus. The nuclei we start with may or may not be of the same element, but the nucleus we create cannot be of the same element. Remember that elements are unique in that the atoms of a given element have a distinct number of protons, neutrons, and electrons compared to other elements.

We know the sun has a bunch of fuel in the form of hydrogen and helium. We also know that nuclear fusion consumes this fuel and gives off tremendous amounts of energy. What's specifically happening? The Sun is so massive that the lightest elements in the Universe cannot escape it's gravitational pull. Hydrogen and helium are trapped at the center of the Sun, and the immensity of the Sun's gravity forces four hydrogen atoms to fuse together into one helium atom and release the energy that keeps the Sun burning and our skin tanned.

This difference in mass isn't all good, though. Because the Sun is gradually getting lighter, scientists predict that in 5.5 billion years it will run out of fuel and burn out. So enjoy it while it lasts!

The sun undergoes what is known as proton-proton fusion. The gas that comprises the sun is very hot, which means that hydrogen atoms are moving around very quickly. When two protons collide at such a high speed, they are able to fuse to become a helium atom. There are actually a few intermediate steps in the process, but overall, the products that result are a helium atom and a photon, which carries a lot of energy. The energy of the photon is absorbed by the gas in the sun, which results in the gas being heated and further propagating the process.

Nuclear fusion in the sun is a continuous reaction that can happen because of the immense temperature and pressure (mostly temperature) in the sun's interior. The process of nuclear fusion releases immense amounts of energy in the form of heat, and it is this heat that slowly bleeds out of the sun's core and into the upper layers and finally to the surface, where it is emitted as light because the temperature is still hot enough to shine at visible wavelengths even on the coldest, outside edge.

My name is Mike. Thank you for your wonderful question "How does nuclear fusion fuel our sun?" The sun is an absolutely ginormous mass of interstellar gasses-mostly hydrogen and helium-that has been burning for about 4 or 5 billion years. The sun has so much mass, that it has incredibly strong gravity and magnetic fields that cause nuclear fusion to occur in the sun´s core. Overall, nuclear fusion in our sun combines two hydrogen atoms (the smallest and lightest atoms) together to form one helium atom (a slightly larger atom). When this combination occurs, energy is released in the form of gamma rays, which are very high energy rays of radiation. These gamma rays heat up the surrounding molecules, and the gamma rays eventually decay and are converted into the visible light, UV light, Infrared light, and X-ray light that finally makes its way to earth. Back inside the sun, the intense heat caused by the gamma rays, and the byproducts of the nuclear fusion reaction, heat the sun to incredible temperatures, and help stabilize the continual reaction inside the core of the sun.

I have also included a link to a webpage that explains the chemical chain reaction that occurs in nuclear fusion. I encourage you to take a look through it if you are interested!