Interactive · 3 questions
If you've ever heard of the Sun before, you probably know that it is very bright (and hot).
But what makes the Sun, and other stars, so bright?
That's right! Nuclear fusion is what happens when two atoms get so close to each other, and at such high speeds, that their nuclei crash into each other, making a whole new atmoic nucleus! The reaction is so spectacular that some of the matter is converted to energy in the form of photons (light), which gives the stars the abiltiy to shine so brightly.
Fusion usually happens with elements that are less massive than iron, which means they have a lower mass or atomic number. This is because lighter elements tend to release energy when they fuse, while heavier elements absorb energy. (Because of this, heavier elements are more suitable for nuclear fission - the opposite of fusion - because they release energy while lighter elements absorb it.)
Before we go on, do you remember the 4 fundamental forces of the universe?
That's right! Normally, atoms are kept separate because the electron shells repel each other. The nucleus of an atom is held together due to a by-product of the strong force called the nuclear force. Typically these forces are enough to keep fusion at bay, so that we aren't turning into stars all the time.
Wait, what are stars made of again?
Oh, yeah. Well there are some other stuff in stars, but Hydrogen (H) and Helium (He) make up the vast majority of them. Since these are the lightest elements, the conditions are ideal for fusion to occur. Stars are huge (like really huge), so the pressure inside of them is likewise huge because of how strong the gravity is. This internal pressure is necessary to overcome the electromagnetic and nuclear forces that keep atoms apart and stable. The star can then undergo fusion, using up its hydrogen and helium to make heavier and heavier elements, while also emitting copious amounts of energy. The process of making heavier elements from lighter ones is called stellar nucleosynthesis.