Video Transcript
Which of the following elements can only be formed in a supernova? A) Hydrogen, B) Helium, C) Carbon, D) Iron, or E) Uranium.
Let’s start by recalling that when a star begins its life, it’s mostly made out of hydrogen. The temperature and pressure inside a star is so great that hydrogen atoms can collide together with enough energy that they fuse, forming helium. The fusion of hydrogen atoms into the heavier element helium gives off a large amount of energy. In fact, this process is responsible for all of the heat and light that’s given off by our sun. Eventually, the hydrogen in the star will begin to run out. At this point, an average-sized star will become what’s known as a red giant and will begin fusing helium together to produce even heavier elements such as carbon and oxygen.
As a rule of thumb, the larger the star is at this point, the heavier the elements it will be able to produce in these fusion processes. In fact, the largest red supergiants are capable of producing much more massive elements, including fluorine, calcium, and titanium. However, there is a limit to the largest element that can be produced by a star in this way. A red giant or a red supergiant will only carry out fusion reactions that give out energy. But as the fusion products get larger and larger, they give off less energy. So, there’s actually a maximum mass of element that can be produced during this phase of a star’s life. And the most massive element that can be produced by a star in this way is iron, which has atomic symbol Fe.
Now, if we look at iron in the periodic table, we’ll see that it has an atomic number of 26. Remember that the atomic number describes the number of protons in an element’s nucleus. Because iron is the heaviest element that can be produced by fusion processes in a red giant or a red supergiant, that means that any elements with an atomic number below 26 will not be produced in this phase of a star’s life.
So, one thing that can help us answer this question is if we look up each of the elements given in the periodic table. We can see that the elements hydrogen, helium, carbon, and iron will have an atomic number of 26 or less. This means that these elements have low enough mass to be formed by fusion processes during the red giant or red supergiant phase of a star’s life. With the exception of hydrogen, of course, which we know is present at the beginning of any star’s life. This means the correct answer to this question can’t be A, B, C, or D because we know that these elements exist in a star without a supernova happening.
But let’s talk a bit about what happens in a supernova as well, and how this might cause the creation of heavier elements such as uranium. A supernova occurs when a star with above-average mass reaches the end of its red supergiant phase. Once the star has finished carrying out all the fusion reactions it can, it explodes in a process known as a supernova. This explosion provides sufficient energy for the formation of elements heavier than iron.
And in fact, supernovas are the only natural process we know of that produces elements heavier than iron. So, we know that things like gold, bromine, and uranium, and in fact, most of the periodic table were all formed in supernovas. As the only option with an atomic number greater than 26, uranium is the only element given which can only be formed in a supernova. So, the correct answer is E. The only element here, which can only be formed in a supernova is uranium.
