The Life Cycles of Stars
Imaged Above: Stellar evolution by Chandra X-Ray Observatory
Introduction
Initial Note: There’s a spilled set of jewels and treasures sparkling out there in the night sky most people are in the habit of ignoring whilst having their strolls to wherever their destinations may be. While some of us may already be aware of the little fact that we’re made of the same chemical elements as these cosmic gems, some still perceive them as merely bright little dots of light emitting faint photons into our eyes whenever the Sun clocks out for the day. However, you’d be surprised at just how active and diverse these dots of concentrated matter truly are. In the following set of posts courtesy of NASA, we’ll be taking a comprehensive look into the life of Stars.
What Is A Star?
A star is a sphere of gas held together by its own gravity. The force of gravity is continually trying to cause the star to collapse, but this is counteracted by the pressure of hot gas and/or radiation in the star’s interior. This is called hydrostatic support. During most of the lifetime of a star, the interior heat and radiation is provided by nuclear reactions near the center, and this phase of the star’s life is called the main sequence.
Before and after the main sequence, the heat sources differ slightly. Before the main sequence, the star is contracting and is not yet hot enough or dense enough in its interior for the nuclear reactions to begin. During this phase, hydrostatic support is provided by the heat generated during contraction.
After the main sequence, most of the nuclear fuel in the core has been used up. The star now requires a series of less-efficient nuclear reactions for internal heat. Eventually, when these reactions no longer generate sufficient heat to support the star against its own gravity, the star will collapse.
The Cycle
A star’s life cycle is determined by its mass. The larger the mass, the shorter the life cycle. A star’s mass is determined by the amount of matter that is available in its nebula, the giant cloud of gas and dust in which it is born. Over time, gravity pulls the hydrogen gas in the nebula together and it begins to spin.
As the gas spins faster, it heats up and is known as a protostar. Eventually the temperature reaches 15,000,000 °C and nuclear fusion occurs in the cloud’s core. The cloud begins to glow brightly. At this temperature, it contracts a little and becomes stable. It is now called a main sequence star and will remain in this stage, shining for millions or billions of years to come.
As the main sequence star glows, hydrogen in the core is converted into helium by nuclear fusion. When the hydrogen supply in the core begins to run out, the core becomes unstable and contracts. The outer shell of the star, which is still mostly hydrogen, starts to expand. As it expands, it cools and glows red.
The star has now reached the red giant phase. It is red because it is cooler than it was in the main sequence star stage and it is a giant because the outer shell has expanded outward. All stars evolve the same way up to the red giant phase. The amount of mass a star has determines which of the following life cycle paths it will take after the red giant phase.
