The Life Cycle of a Star
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- It all beginsfrom the collapse of material in a giant molecular cloud. These clouds are clouds that form between the stars and consist primarily of molecular gas and dust. Turbulence within the cloud causes knots to form which can then collapse under its own gravitational attraction. As the knot collapses, the material at the center begins to heat up. That hot core is called a protostar and will eventually become a star (the temperature of the core of the prostar is not yet hot enough to be a star).
- As the protostar gains mass, its core gets hotter and more dense. At some point, it will be hot enough and dense enough for hydrogen to start fusing into helium. It needs to be 15 million Kelvin in the core for fusion to begin. When the protostar starts fusing hydrogen, it enters the "main sequence" phase of its life. (It is shown on the HR diagram and it will become one of the main sequence stars.)
- Stars on the main sequence are those that are fusing hydrogen into helium in their cores. The radiation and heat from this reaction keep the force of gravity from collapsing the star during this phase of the star's life. This is also the longest phase of a star's life.Eventually the core of the star runs out of hydrogen. When that happens, the star can no longer hold up against gravity. Its inner layers start to collapse, which squishes the core, increasing the pressure and temperature in the core of the star. While the core collapses, the outer layers of material in the star to expand outward. The star expands to larger than it has ever been – a few hundred times bigger! At this point the star is called a red giant.What happens next depends on the mass of the star.
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