Students know the Sun, and average star, is the central and largest body in the solar system and is composed primarily of hydrogen and helium.

Hydrogen is far and away the most abundant element in the universe. Helium, which hardly exists naturally on our planet, is the second most abundant element. The reason for this is that these were essentially the only two elements made in the Big Bang, which scientists believe occurred at the beginning of our universe about 15 billion years ago.

At the very earliest stages of the universe, there were no stars. The hydrogen and helium were spread uniformly throughout space. After about a billion years, local collections of gas started to accumulate and attract more and more hydrogen and helium until they formed huge bodies of gas. When enough hydrogen gathers in one place, the atoms in the very center are forced so close to together that they fuse forming helium and releasing a huge amount of energy. This nuclear fusion is what makes a star shine and enables it to release energy in prodigious quantities.

In terms of size, the sun is an average sized star. It looks different to us because it is so much closer than the nearest other star. There are stars that are ten times bigger and stars that are ten times smaller than the sun. When informed that the sun is a star, a student once wrote on a test, “The sun is a star. It is a good thing that it remembers to become the sun again during the day.” Besides being humorous, this remark reminds us to probe our own and student thinking, to make sure that we really understand a concept, and are not simply repeating a phrase such as “the sun is a star.”

You may wonder if the universe originally contained only hydrogen and helium, where did the other elements came from? We did not know the answer to that question until the 1950’s. It turns out that essentially all the elements heavier than helium were made in stars through the processes of nuclear fusion – atoms being forced so close to together that they combine to form bigger atoms. Even today our sun is making huge amounts of helium by combining hydrogen atoms. When it runs out of hydrogen, the sun will begin to make bigger elements by combining the helium atoms together. We will have to wait a long time before that happens, maybe another four or five billion years.