The first installment of this series may be found here, and it gives the basics as to how nuclear energy works. The way that the stars generate their energy is interesting, and we shall consider it in greater detail this time.
Young stars almost always fuse hydrogen into helium. There are several reasons for this, amongst them 1) hydrogen (protium, see the previous installment) is the most common nucleide in the cosmos, 2) more energy is released by fusing protium into helium nuclei than any other known process, and 3) there are multiple processes to accomplish it.