We are star stuff

    If I say that the universe only contained hydrogen and helium in the beginning. most of you will definitely think that I am talking some nonsense. You may ask me to take high school chemistry again or throw the periodic table on my face. Considering how many different kinds of elements we have on Earth, the idea that universe started with nothing but hydrogen and helium seems not to be true. If universe only contained hydrogen and helium long time ago, where did other elements come from? To answer this question, we must look into the Sun and other stars.

    Our sun and other stars generate energy by combining lighter elements into heavier ones. Stars spend most of their lives generating energy by fusing hydrogen into helium.  However, stars like our sun can fuse helium into carbon toward the end of their lives. The cores of heavier stars can create many other heavier elements. For example, the can fuse carbon into oxygen and silicon, oxygen into neon and sulfur. If a star is massive enough, it can even fuse other elements into iron. All those manufactured elements were released into space after stars died. The elements dispersed by the dead star may combine with each other and form new stars or planets. For instance, our Sun contains about 2% of elements other than hydrogen and helium. This composition of the Sun shows that our Sun is relatively young. Similar to our Sun, our solar system, contains about 2% of elements that is not hydrogen and helium. That's why we have water, rocks, iron, and many other elements in our solar system.

    There are two pieces of evidence that can support the idea that universe only contained hydrogen and helium in the beginning. First, stars of different ages have a different composition. While the oldest stars are made of nearly pure hydrogen and helium, younger stars contain a higher proportion of elements heavier than hydrogen and helium. This pattern shows that those heavier elements were created by older stars and were released into space after those stars died. If the universe was started with not only hydrogen and helium but also other elements, the composition of all of the stars should be about the same. Second, the abundance of the elements also gives us some clues. If the prediction that elements heavier than hydrogen and helium is true, the elements carbon and oxygen should be more abundant than nitrogen because while most of the stars can manufacture carbon and oxygen, only heavier stars can manufacture nitrogen. When we observe the abundance of the elements, we do see that carbon and oxygen are more abundant than nitrogen.

    The idea that most of the elements we have discovered came from the stars is fascinating. If it were not those stars, we would no be here. All of the elements of rock, iron, and living creatures came from stars. Rocks, irons, and every kind of creatures are made of "star stuff".