At its simplest, a star is nothing more than a ball of gas, mostly hydrogen and helium, contracting under the force of gravity and releasing gravitational and fusion energy into space. At the beginning of its life, a star releases only gravitational potential energy. Later, when its core shrinks to a sufficiently high density, the star predominately radiates energy released through the conversion of hydrogen and helium into heavier elements. During this nuclear phase the shrinkage of the star's core slows dramatically. At the end of its luminous life, once all of its nuclear energy is exhausted, the star begins again to shrink rapidly until the star reaches its most compact form: a degenerate dwarf, a neutron star, or a black hole.
What separates a star from a brown dwarf or a gaseous planet is that the star passes through a phase of hydrogen fusion. Before this phase, a gaseous planet, a brown dwarf, and a protostar (a star before nuclear fusion takes place) look the same, emitting energy released through gravitational collapse. The gaseous planet, however, stays on this track it whole life, shrinking until the pressure of its cold core balances the gravitational forces. The brown dwarf, despite undergoing a period of deuterium fusion, never approaches a star in brightness, and never changes it composition significantly from the composition of a Jupiter; once it depletes its deuterium, it behaves as a Jupiter.
The determining factor for which type of object a hydrogen sphere becomes is the mass of the sphere. The dividing line is about 0.075 times the mass of the sun.1 Above this line, steady hydrogen fusion is possible; below this line, hydrogen fusion does not occur, and the sphere becomes either a brown dwarf or a Jupiter (it is a brown dwarf if the mass is greater than 0.012 solar masses).
The parameter with the greatest influence on stellar evolution is a star's mass. Large stars, those with masses 100 times that of the Sun, evolve rapidly, glowing bright and blue over most of their lives, with life spans as short as two million years. Small stars, those with a mass around one-tenth the mass of the Sun, burn a dim red for several tens of billions of years. A more massive star burns its nuclear fuel faster than a smaller star because the more massive star has a higher internal density and pressure, which forces the nuclear reactions to occur at a much higher rate than in the smaller star. The higher fusion rate more than compensates for the larger mass.
A star's maximum mass is set by the tendency of large stars to drive strong winds from their surface. As the size of a star increases, the luminosity of the star increases. The radiation from a star exerts pressure on the atmosphere of the star, and if the pressure is great enough, a substantial wind is created that drives off a large fraction of the star's mass. Estimates place the maximum size of a star at around 350 times the mass of the sun.
There are very few parameters other than mass that determine how a star changes during its existence. One is the star's composition. A star by mass is three parts hydrogen for every part helium. A tiny fraction of a star is composed of other elements, such as carbon, nitrogen, and oxygen, all collectively termed as metals in the astronomer's jargon. Metals influence the rate at which hydrogen is converted into helium, they affect the escape of energy from the center of the star, and they control the stellar wind coming off the surface of the largest stars. The final and least important parameter that determines the life of a star is its angular momentum.
The appearance of a star as it ages can change dramatically, because, while the core always shrinks as a star ages, its outer layers can either shrink or expand, depending on the amount of energy released by the core. When the rate at which energy is generated by the core increases, the outer layers of the star puff-up until the energy can leak through these layers at a rate equal to the rate at which it is created. The temperature at the surface generally drops, but because of the star's larger surface area, the total power radiated by the star's surface remains equal to the power generated as thermal energy at the star's core. The converse occurs if the energy generation decreases, with the surface of the star becoming smaller and bluer. As a star ages and burns through its different elements, the power generated within the star varies, so the size of the star varies.
A star begins the fusion stage of its life by converting hydrogen into helium. A star in this state is called a main sequence star, because on a chart of surface temperature versus luminosity, a hydrogen-burning star falls on a line called the main sequence. Low luminosity stars on the main sequence, which are low-mass stars, have a low surface temperature, and are therefore red; on the other hand, high luminosity stars on the main sequence, which are high-mass stars, have a have a high temperature, and are therefore blue.
Once a star exhausts its supply of hydrogen at its core, the core collapses until nuclear reactions begin that convert helium into carbon and oxygen. Outer layers of these stars continue to convert hydrogen into helium. A star in this stage produces more power than when it was on the main sequence, so its outer layers expand, making the star many times larger than previously. Such stars are in their red-giant phase, and while they produce more power, their larger surface area means that they are cooler and redder than before.
After a star exhausts its supply of nuclear fuel, its core collapses until either the star achieves a stable configuration, with its internal pressure counteracting gravity as it cools to zero temperature, or the star collapses to a black hole. This core collapse is generally accompanied with the expulsion of the outer layers of the star, because the amount of gravitational energy released in the collapse provides a pressure that more than counteracts the gravitational forces on the outer layers. A moderately-sized star, for instance, a star the size of the Sun, collapses to a stable star, called a degenerate dwarf, that is roughly the radius of Earth. The core of a larger star collapse to a radius of about 15km; this releases tremendous amounts of energy, leading to a supernova explosion that drives the remainder of the star way. The remnant star left behind is a neutron star. If a star is large enough, its core collapses to a black hole. What happens when this occurs is very speculative.
Degenerate dwarf stars have two possible fates. If the star is alone, it will cool gradually to invisibility. If the star is in a binary system, then the star may undergo a thermonuclear detonation to produce a type Ia supernova. Two different theories for this event exist; in one, the companion is a main sequence star, and in the other, it is a degenerate dwarf. If the companion star is a main sequence star entering the red giant phase, then, as it gradually expands, it will dump some of its atmosphere onto the degenerate dwarf. If enough material is transferred, the degenerate dwarf becomes unstable to collapse. When the collapse commences, the higher pressure at the star's center causes the carbon and oxygen to fuse into iron and other heavier elements, resulting in a thermonuclear explosion. If the companion star is a degenerate dwarf, then the two stars eventually merge when the system loses enough energy through gravitational radiation. As before, this produces a stellar collapse, which leads to a thermonuclear explosion. Either way, a type Ia supernova is produced, and the degenerate star is totally destroyed.
The neutron star has only one fate, and that is to cool to invisibility.
The most complex behavior seen among stars is stellar pulsation. While most stars maintain a stable configuration that permits the steady transport of nuclear energy to the surface of the star, some stars never find this configuration, and instead they oscillate in size. When one of these stars is at its smallest, energy within the star builds up, building up the pressure within the star, and driving the outer layers of the star to greater radii. At some point, depending upon the details of the physics driving the pulsation, the transport of energy becomes more efficient, and the energy within the star leaves the star faster than it is produced. This drives the star back to its smallest size. Stellar pulsation is therefore driven by the physics of the radiative transport.