Stellar evolution refers to the processes and radical changes undergone by a star during its lifespan. It is the process through which pressure and forces of gravity change or alter a star. Depending on its mass, the lifespan of a star can take several million years or trillions of years.
Stars undergo several stages during their lifetime, with the first stage being Giant Molecular Cloud. During the Giant Molecular cloud, a star begins its life at this stage where it is a large, dust- filled cloud of gas which is very cold. Fragments of solar masses start collapsing due to a number of reasons, for instance, shock waves. The next stage is Protostar whereby the clump of gas starts collapsing and releasing heat from its centre as it collapses. Energy from gravity is converted to heat and it produces a lot of microwave and infrared radiation. Although it is hot enough to glow at this point, the gas and dust block its visibility. T- Tauri is the next stage whereby the strong winds around the star get rid of a lot of materials from the star, including the gas and dust cocoon. It then starts fusion.
Main Sequence is the next stage whereby the star becomes stable due to Hydrostatic equilibrium. It fuses hydrogen to helium in its core. It spends most of its lifetime (90%) at this stage. The next stage is the Subgiant, Red Giant, in which the star has run out of core fusion fuel and its hydrostatic equilibrium has been disturbed. The core reduces in size and there is rapid fusion around core stars’ shells. It increases its energy output (Luminosity) and the gas enveloping the core is puffed out. On the surface, the energy spreads out to cover a much larger area making it cooler and lighter red in color. It is smaller in size than the previous stage.
After the Main Sequence, Core fusion is the next stage where the star is much smaller in size and hot enough to begin helium fusion. It becomes bigger and more stable at the end of this stage. Red Giant, Supergiant is the next stage whereby the core fuel has run out once more. It may repeat stages 5 to 7, depending on mass. Here, stellar nucleosynthesis of the heavier elements takes place. There is interaction of nuclear fusion and gravity. After Red Giant, Supergiant, Planetary Nebula or Supernova is the next stage in which the outer layers are ejected as the core shrinks and becomes more compact. Low mass stars could go the way of the planetary nebula while high mass stars go the way of the explosive supernova.
The last stage in the lifecycle of a star is the remnant whereby the low mass core shrinks in size to form White Dwarf. It does not collapse further due to electrons present and is about the same size as the earth. The main forces responsible for the formation are stars are the force of gravity and pressure..
Reference
Kaufmann, W. (1994). Universe. New York, NY: Freeman and Company.
