Briefly discuss stellar evolution. What forces are opposing one another throughout the life of a star? How do these forces influence the various stages of a star's life cycle
http://en.wikipedia.org/wiki/Stellar_evolution
Stellar evolution refers to the process by which a star changes over time, from its formation to its ultimate demise. Throughout a star's life, several forces are at play and opposing each other.
The two key opposing forces in stellar evolution are gravity and radiation pressure. Gravity acts to compress the star, attempting to collapse it, while radiation pressure acts to expand the star. These forces are in balance during most stages of a star's life, providing stability.
During the early stages of a star's life, gravity dominates. A star forms from a dense region of gas and dust, called a molecular cloud, under the influence of gravity. The force of gravity causes this cloud to collapse inward, bringing the gas and dust particles closer together. As the cloud contracts, it heats up and eventually reaches a temperature where nuclear fusion begins in its core.
Once nuclear fusion initiates, the energy release from the fusion reactions pushes outward, counteracting the force of gravity and maintaining the star's stability. At this point, the opposing forces are in equilibrium, and the star enters the main sequence phase. The balance between radiation pressure and gravity enables a stable star, like our Sun, to exist for millions to billions of years.
As a star evolves, its core undergoes changes. In low to medium mass stars, like the Sun, the core runs out of hydrogen fuel after several billion years. At this stage, gravity begins to overpower the weakened radiation pressure, causing the core to contract. The outer layers of the star expand, and the star transforms into a red giant or supergiant. During this phase, the star becomes much larger and cooler than before.
In more massive stars, after exhausting the hydrogen fuel, gravity's power is strong enough to cause the core to collapse under its own weight. This leads to a massive release of energy in a supernova explosion. The remaining core may collapse to form a dense object such as a neutron star or a black hole.
Throughout the star's life cycle, the balance between gravity and radiation pressure determines its stability, size, and temperature. Changes in this equilibrium result in the various stages of stellar evolution, ranging from protostar formation to the explosive death of massive stars. Understanding these opposing forces helps us analyze the life cycles and behavior of stars at different stages of their evolution.