A star's life is a using up of its original energy by transforming the energy to light and radiating it out to space. When the energy is used up the star stops shining. It then remains as a dark mass with gravity but without luminosity forever.

The two energy sources of a star are gravitational contraction and nuclear fusion. Whenever gravitational energy is being used the star is getting smaller in diameter; half the energy goes into light and the other half into raising the temperature. Whenever nuclear fusion is being used the star maintains a steady size and temperature; all the energy goes into light. The star toggles back and forth between these two energy sources, spending more time with nuclear, of course, because it is so much more energy than gravitational contraction.

Stage 1. A cold gas cloud in interstellar space gets jostled somehow and begins to contract gravitationally. Clouds are huge (1-2 light years across) so the gravity is very weak. It takes a long time for the atoms (roughly 70% hydrogen, 25% helium, 5% all else) to accelerate and fall together toward the cloud's center. Most of 20 million years is spent in freefall (gravitational energy -> kinetic energy). Eventually the atoms and simple molecules begin to hit each other. Conservation of angular momentum allows more contraction along the spin axis, less contraction in the equatorial plane; the cloud becomes flatter as it shrinks in radius. Collisionally excited electronic and vibrational levels radiate some of the freefall energy as infrared light, so the cloud center begins to glow feebly. This is called a protostar. The infrared light in the inner 5% of the cloud breaks up molecules (especially H2, CO2, ice, ammonia, and methane) and pushes them gently outwards. This leaves the heavy elements (mostly metals) in this inner depleted region close around the protostar. This is where the terrestrial planets begin to form. With only a small supply of raw materials, they end up being small mass objects. The jovian planets form in the main part of the cloud which has a full supply of elements, especially hydrogen (70% of the original composition), so they end up being large mass planets.
This is called the formation stage.

Stage 2. This begins when the temperature in the center of the protostar exceeds 10 million K, which is sufficient to begin nuclear fusion of hydrogen nuclei (proton + proton -> deuterium nucleus + positron + light). This is called the proton proton chain reaction. This stops the contraction of the star's core (innermost 10% of radius). A blast of heat and light sweeps outward through the star, ejecting the outermost thin layer which clears out the solar system of gas and dust. Larger object such as planets, moons and asteroids remain in orbit. The other layers of the star then stop contracting also.

The fusion proceeds further to make helium nuclei, releasing a huge amount of energy, lasting about 90% of the star's total lifetime. Since this nuclear reaction is very temperature-sensitive it acts as a thermostat, maintaining stable conditions.
This is called the main sequence stage.

Stage 3. This begins when the hydrogen in the core is nearly all converted into helium nuclei. Then the fusion power does not produce enough energy to keep the star supported against its own gravity, so it begins to contract once again. This releases gravitational energy which raises the temperature throughout the star, but especially in the region just outside the core. In that region there is still 70% hydrogen composition, and it is already nearly hot enough to do fusion. Soon the temperature there exceeds 10 million K, so fusion begins in the outer core. A blast of heat and light sweeps outward through the star, ejecting the outermost thin layer of the star. This time it does not have quite escape velocity, so hangs at about 100x the star's main sequence radius.

In our solar system this filmy layer would engulf Mercury, which would then gradually spiral in towards the sun because of viscous drag. When it joined the sun it would burn up. Venus would also be engulfed and spiral in and be burned up. The Earth would still orbit outside the filmy layer, but would see a fuzzy red ball about 90 degrees wide. It would take the sun hours to rise fully! The temperature on Earth would get toasty, the oceans would boil, and the atmosphere would escape. (So we should move away before this happens! Where should we go?)
This is called the red giant stage.

Stage 4. This begins when the shell (outer core) has transformed most of its hydrogen into helium nuclei. The nuclear energy dwindles again, and cannot support the star against its own gravity, so the filmy layer falls back onto the star, and the whole star begins to contract once again. Before the next outer shell gets hot enough to do hydrogen fusion, something new happens in the innermost core. The temperature is hot enough there to begin helium- to-carbon fusion. This takes more temperature than the proton proton chain because more temperature is needed to overcome the electrostatic repulsion of helium nuclei (each twice the electric charge of a proton). This is called the triple alpha reaction. Since the energy is coming from the deep core, the star has the same structure as a main sequence star, but running at a higher temperature. This is the star's second main sequence stage.

The star also goes through a second red giant stage. In the second red giant stage the Earth will spiral in to the sun and burn up.
I have lumped these together and call this the replay of the main sequence and red giant stage.

Stage 5. This begins when the nuclear energy from the second shell source begins to dwindle. The star contracts again, but before the central temperature gets high enough to ignite carbon fusion, something new happens in the core. The electrons (remember them?) become as close together as they can be (the Pauli exclusion principle), so they stop the contraction abruptly. They are degenerate matter. The star stops contracting at about 1% of its main sequence size, quivers a while, then just sheds light without replenishing it. It cools and fades for a long time.
This is called the white dwarf stage.

Stars which are more massive than the sun:

Stars which are smaller in mass than the sun

This page is www.csam.montclair.edu/~west/starslife.html
Last modified in Nov, 2004.