One usually gets the impression that stars are fixed in the night sky and permanent. The star patterns that human civilizations have been observing over thousands of years led them to believe that heavens were unchangeable and eternal. The contemporary astronomy has shown a radically different picture. Stars are active, changing bodies that are subject to complicated existence cycles between millions or billions of years. They are born, they evolve and finally they never exist as they were before. The study of the formation and evolution of stars gives an idea of the mechanism that shapes the universe in general, and why matter in the universe is always recycled.

The process of the formation of a star starts within giant clouds of dust and gas called nebulae. Hydrogen is the main component of these clouds as well as helium and minor elements of heavier elements. Nebulae can seem serene at first glance since they are observed to be like in the distance, but they are dynamic spaces influenced by gravity, radiation, and shockwave of other stellar phenomenon nearby. With the lapse of time, areas inside these clouds attain a little more density than the other areas. Gravity starts drawing material to these accumulated dense parts making them larger and denser. The greater the amount of material, the greater the gravitational pull and so the gas and dust are drawn there.

The more and more this mass falls inward, the hotter and more concentrated is the center. Both the temperature and pressure increase gradually. The initial forming object is referred to as a protostar. At this phase, it is not yet a true star as the nuclear fusion has not begun. Rather, the protostar retains multiple material around itself and heats up. Hundreds of thousands of years may be required in this process. After some time, the core becomes hot enough to several million degrees. Here hydrogen nuclei start colliding with each other to form helium. This type of nuclear fusion emits massives of energy.

The beginning of the nuclear fusion is the birth of a real star. Fusion produces energy which exerts outward force that counteracts the pull of gravity which is inward. This balance is critical. It is the main sequence stage and the stage that most of the stars spend most of their lives.

The process of converting hydrogen to helium occurs continuously in the star at the main sequence stage. The hydrogen in the core is lost with time. Once this occurs the ratio between the force of gravity and outward pressure starts to alter. The nucleus begins to shrink making it hotter and more compact. Simultaneously, the outer layers of the star become wider. The star expands and cools on its surface to make a red giant.

Big stars have much more dramatic deaths. The temperature is so intense that heavier elements combine. Fusion ceases to be able to generate energy when the iron is accumulated in the core. In the absence of external forces to oppose gravity, the core falls in abruptly.

The fact that stars are born, live and die is thus not merely an astronomy thing. It is concerned with the knowledge of processes which preconditioned the existence in the first place.