A star is a large ball consisting of mostly hydrogen and helium at an enormously high temperature. The process of thermonuclear fusion combines hydrogen into helium and in the process releasing light and energy. It was originally believed that the stars were of a fixed amount, but medieval observers noted new stars did appear in the sky. In the 20th century, classifications of stars and advanced explanations of their interior led to models of the stellar life cycle that are still accepted today.
The most important factor determining the life cycle of a star is its mass. The amount of matter in a star will affect its brightness, called magnitude, its temperature, and its ultimate fate. If mass is not great enough to produce temperatures hot enough to begin thermonuclear fusion, the result is a brown dwarf, a failed star. Small stars eventually fizzle out and become inert. Large stars, on the other hand, become very hot and unstable and produce enormous explosions called supernovas, and can even collapse to form a black hole.
The major stages in the lifecycle of a star are protostar, main sequence, post-main sequence, and collapse. At each step, the star's mass will determine which phase it moves to next. Most stars become Red Giants in the post-main sequence period, with the largest stars becoming Red Supergiants. From there stars will become white dwarves and black dwarves if they are relatively small, and neutron stars or black holes if large.
Protostars are collections of hydrogen and helium gas held together by gravity. Over millions of years they contract, rising in temperature. Once fusion begins, hydrogen burns for hundreds of millions of years. Eventually, the surface expands and cools, producing a Red Giant. When temperatures in the core become hot enough to burn helium, the expansion process reverses, first creating a white dwarf. A smaller star will remain stable at this phase until no longer radiates heat or light, becoming a black dwarf. Stars with mass 1.5 times the sun or higher do not stabilize as white dwarfs, but continue to collapse until they reach temperatures so hot that electrons are stripped away from the lighter solids, which evaporate as streams of neutrons. This is the phase called a neutron star. Stars four times as massive as the sun or larger are so great they do not stabilize here, but collapse entirely, becoming a black hole.
Stars are classified by their temperature and their absolute magnitude, which is the brightness of the star not as it appears from Earth, but how it would appear if all stars were equally distant from the observer. The Hertzsprung-Russell diagram charts these two factors on a graph, producing clusters representing the various kinds of stars at different phases. Most stars currently fall along the "main sequence" line, with giants and white dwarfs clearly distinguished above and below
Each phase of a star's lifecycle lasts millions of years, so it's impossible to observe on Earth, even over many lifetimes. However, because stars at all stages of evolution currently exist, scientists are able to create theories explaining the progress through these phases. Stars were observed for thousands of years before many of the most basic features of stars could be deduced. Building on these earlier realizations, in combination with advanced technology, the study of stars has advanced much more rapidly.
Identifying the different phases of stars and how they are linked has led scientists to understand the origins of all heavier elements in the universe. From simple hydrogen, stars manufacture everything on the periodic table from helium to iron. When stars explode in novas or supernovas, the remaining elements are produced. Gold, silver and uranium are examples of heavy elements produced by the explosion of stars.