The study and observation of our Universe is, ultimately, a study of origins. Human cultures and religions around the world have always been imbued with a desire to understand that most fundamental of all questions about origins: "from whence did we come?" Mythologies and religions of societies from the distant past all share a common theme in answering this question of origins: that the answer is "out there" with the gods in the realm of the stars. It seems only natural, then, that throughout human history we have always looked to the skies above for inspiration and as a source of clues in our efforts to answer this profound question. The results of our sky watching have led to our present understanding of much of the workings of our Universe: from Kepler's laws of planetary motion, Newton's universal law of gravity and the Calculus, Einstein's General Relativity, to the existence of the chemical elements, the afterglow of the Big Bang and, most recently, indirect observations of dark matter and the effects of dark energy. Our understanding of these phenomena has arose from observations of stars and galaxies. Today, we know that in the first seconds of the Big Bang were forged the five lightest stable nuclei: protons, deuterium, alpha particles, 3He and 6Li. These five nuclei, along with a bath of high energy photons, were the essential ingredients that our Universe was seeded with shortly after its birth. The chemistry of such a Universe would have been incredibly uninteresting. Both variants of helium are chemically inert, resulting in an early Universe "chemistry" consisting only of: diatomic hydrogen gas, hydrogen-deuteride (chemically the same as diatomic hydrogen gas), diatomic lithium gas, lithium-hydride and lithium-deuteride. In addition to being uninteresting, this chemist is also wholly insufficient for the creation of life. And yet, here you are reading this. What happened along the way to the present epoch?
From its fiery beginning, our Universe has been continuously expanding and, subsequently, cooling as a result. The hydrogen produced in the Big Bang, which comprises the vast majority of all matter in the early Universe, has, in some locations, been able to cool and condense into immense gaseous nebulae. Within these nebulae, the first stars were born as pockets of gas within the nebulae were able to cool and collapse under the influence of the gravitational force. From the collapse, immense spherical bodies (for example, our Sun has a radius of about 696,000 km) comprised of hydrogen, were formed and resulted in the first generation of stars in an otherwise dull Universe devoid of any structures. Today, we still observe this ongoing process of stellar birth with scientific instruments such as the Hubble Space Telescope.
Stars, like people, are born; they live; and then they die. During their lives, they process the hydrogen from which they are made into heavier elements, such as helium, through nuclear reactions. The helium can then be processed into carbon. And carbon can then undergo a nuclear reaction with some of the helium still present in the star, to produce oxygen. In this way, a star acts as a giant "cauldron" in which the initial ingredient -- hydrogen -- is converted into heavier elements along with a prodigious emission of light energy. Through the nuclear reactions taking place inside of stars, that dull chemistry of the early Universe can be improved. However, it would not do to simply have these new elements produced within the interior of stars and have them remained locked there forever. These newly synthesized elements must, somehow, find a means by which they can be released into the space of the expanding early Universe; else, the chemical elements, so necessary for life, would have remained forever trapped in the interior of stars. That you are here reading this is proof that, by some mechanism(s), these elements were liberated from the early stellar interiors and were dispersed into the space the early Universe. Before we consider what these mechanisms are, let us first look with a bit more detail into what a star actually is so that we can then understand how they release their elements into the space of the Universe.