Solar
system: SOL - sun and energy supplying starSolar
system: SOL - sun and energy supplying starThe main sequence
The main sequence is marked by Hydrogen Core Fusion. All main sequence stars derive energy in this way. Since Hydrogen is the simplest element, consisting only of one proton, then it fuses at the lowest possible temperature. Because the fusion rate, the rate at which energy is used up in a star, is a strong function of the temperature, then the main-sequence of hydrogen core burning phase of stellar evolution is the longest one. All subsequent phases of stellar evolution are marked by higher core temperatures and shorter lifetimes.
The Gravity
Gravity comes about by the virtue of the star having mass. Gravity is directed inwards at all times and attempts to decrease the radius of the star. The radius of the star will continue to shrink until enough internal gas pressure is built up (that is until the core gets hot enough) for this pressure to stabilize the star.
The Pressure
The pressure in a star comes from its hot central core. This pressure is an outward force that serves to stabilize the star against further gravitational collapse. When pressure and gravity are in balance, the star is stable, meaning its radius does not change. Each stellar mass requires a unique core temperature for stabilization. Higher masses require higher core temperatures.
The Red Giant Stage
The red giant stage of stellar evolution is marked by Helium core fusion. The core energy source now stabilizes the star. When this stage is reached the luminosity of the star is about 100 times higher than when the star was on the main sequence, fusing hydrogen in its core. The radius of the star has increased by about a factor of 40 as well. When the sun becomes a Red Giant, its radius will then be about 30 million kilometers.
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