Stellar evolution describes the process in which stars form, burn for an extended period of time, and eventually die. Along the way, the star struggles with gravity and fuel consumption in an attempt to maintain equilibrium. Much of stellar evolution is theory, since humans have not been around long enough to observe the entire life cycle of a single star, but thanks to the ample numbers of stars at various stages scattered around the universe, it has been possible to make educated and intelligent guesses about how stars evolve over time, and the process appears to be relatively predictable.
The process of stellar evolution starts with the formation of a star. Stars form in what are known as “giant molecular clouds,” swirling masses of gases and particles. Over time, these clouds undergo gravitational collapse, generating protostars, celestial bodies which have the potential to become stars. The type of star that a protostar will evolve into depends on its chemical composition. As the star gradually pulls together, a series of nuclear reactions start to occur, causing the star to gain luminosity.
Eventually, a star will consume its fuel, typically collapsing and setting off another chain reaction of events which cause the star to burn extremely hot and bright. Depending on the type of star, it can turn into a white dwarf, a neutron star, or a black hole when it dies, often preceding this event with a fantastic supernova, caused by a runaway chain reaction. Stellar evolution is constantly occurring all over the universe as stars come and go.
Each stage of a star's life can be plotted on a chart such as the Hertzsprung-Russel Diagram. The chart compares the star's color, size, luminosity, and temperature. Stars of a certain class tend to lump together on such diagrams, demonstrating clear relationships between specific types of stars which can be used to learn more about stars in general. Using such a chart, scientists can take observations about various stars and convert them into distance from Earth and other useful data about stellar evolution.
Many stars fall into a classification known as the main sequence, referring to a signature cluster of stars which can be seen on many plots used to chart stars by characteristic. The Sun is an example of a main sequence star, and it will remain stable for at least five billion more years, according to most estimates. At the end of the main sequence period, the sun will convert into a red giant, expanding considerably as it uses up its fuel and engulfing several planets, including Earth, along the way.