Stellar evolution involves the formation, burning, and death of stars, which can be observed through various stages on a graph. Stars form in molecular clouds and evolve based on their chemical composition, eventually dying as white dwarfs, neutron stars, or black holes. The Sun is a main-sequence star that will become a red giant in five billion years.
Stellar evolution describes the process in which stars form, burn for an extended period of time, and finally die. Along the way, the star struggles with gravity and fuel consumption in an attempt to maintain balance. Much of stellar evolution is theory, as humans have not been around long enough to observe the entire life cycle of a single star, but thanks to the large number of stars in various stages scattered throughout the universe, it has been possible to make educated guesses about how stars evolve over time, and the process appears to be relatively predictable.
The process of stellar evolution begins with the formation of a star. Stars form in what are known as “giant molecular clouds,” swirling masses of gas and particles. Over time, these clouds undergo gravitational collapse, generating protostars, celestial bodies that have the potential to become stars. The type of star a protostar will evolve into depends on its chemical composition. As the star gradually approaches, a series of nuclear reactions begin to occur, causing the star to increase in brightness.
Eventually, a star will consume its fuel, typically collapsing and setting off another chain reaction of events that causes 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 occurs constantly throughout the universe as stars come and go.
Each stage in a star’s life can be plotted on a graph such as the Hertzsprung-Russel diagram. The graph compares the color, size, brightness, and temperature of the star. Stars of a certain class tend to cluster on such diagrams, demonstrating clear relationships between specific types of stars that can be used to learn more about stars in general. Using this graph, scientists can take observations of various stars and convert them into distances from Earth and other useful stellar evolution data.
Many stars fall into a classification known as the main sequence, referring to a characteristic star cluster that can be seen on many graphs used to plot stars by characteristic. The Sun is an example of a main-sequence star and will remain stable for at least another five billion 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 runs out of fuel and gobbling up several planets, including Earth, along the way.
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